'mu_id'|'mu_desc' 1|'Lakes, Rivers, Ocean, etc..' 150| 151| 152| 153|'Including granite, rhyolite porphyry, dacite porphyry, and granodiorite' 154| 155|'Includes leucocratic granite border phase. See Kiilsgaard and others (2001) for additional information.' 156|'Member 4 (Wolfcampian to Leonardian) of the Grand Prize Formation consists of more than 450 m of thin- to medium-bedded carbonaceous siltstone, sandy to silty micrite, and minor micritic sandstone (fig. 6). Carbonaceous siltstone of member 4 gradationally overlies banded sandstone and siltstone of member 3. Member 4 weathers to a dark-gray regolith and forms slopes in the southern part of the White Cloud Peaks area.' 157|'Member 3 (Wolfcampian to Leonardian) of the Grand Prize Formation consists of 650-1,700 m of fine-grained sandstone and siltstone arranged in rhythmically interbedded couplets 30 cm-3 m thick. The distinctive banded appearance of the member is produced by these couplets of light-gray fine-grained micritic sandstone gradationally overlain by dark-gray carbonaceous siltstone. Interbedded with the couplets is thick-bedded light-brown micritic sandstone and light-gray sandy micritic limestone (fig. 6). Member 3 gradationally overlies member 2. Member 3 has an irregular weathering pattern; the sandier intervals are exposed in bold relief against the more easily weathered finer grained intervals. The member forms distinctly banded light-gray cliffs throughout the White Cloud Peaks area.' 158|'Member 2 (Desmoinesian to Wolfcampian) of the Grand Prize Formation consists of 500-1,100 m of thick-bedded to massive light-brown micritic sandstone and subordinate light-gray sandy micritic limestone and gray carbonaceous siltstone. Member 2 gradationally overlies member I; the contact is placed above the bioclastic limestone of member I. Member 2 forms discontinuous cliffs and ledges in the northern Smoky Mountains and in the White Cloud Peaks.' 160|'The upper member of the Dollarhide Formation (Wolfcampian and Leonardian) is approximately 900 m thick and is composed of thin-bedded to laminated dark-gray carbonaceous siltstone and light-gray silty micritic limestone and minor light-brown micritic sandstone and conglomerate (fig. 6). The upper member gradationally overlies the middle member, and the upper boundary is not recognized due to erosion. The upper member weathers to a dark-gray regolith, is poorly exposed, and forms slopes throughout the Smoky Mountains, which, as a result, contain large areas of sparsely vegetated "black-shale" regolith (Wavra and Hall, 1989) (fig. 1, plate 1).' 161|'The middle member of the Dollarhide Formation (lower Wolfcampian) consists of about 300 m of fine-grained light-brown micritic sandstone and light-gray sandy micritic limestone and subordinate dark-gray to black carbonaceous siltstone and lenticular conglomerate (fig. 6). The middle member forms prominent light-colored cliffs throughout the central and southern Smoky Mountains. North of Deer Creek (plate 1) it is interpreted to grade into thick sandstone beds of the Eagle Creek Member of the Wood River Formation.' 162|'The lower member of the Dollarhide Formation (Desmoinesian to lower Wolfcampian) is at least 800 m thick and consists of rhythmically interbedded dark-gray carbonaceous silty micritic limestone and very fine grained light-gray to light-brown micritic sandstone and subordinate medium- to thick-bedded light-brown micritic sandstone and light- to dark-gray lenticular conglomerate containing mainly intrabasinal clasts of siltstone and bioclastic limestone. Extrabasinal clasts (vitreous quartzite and fine-grained sandstone) are rare. The lower member of the Dollarhide Formation includes at the base a minimum of 175 m of quartzite, phyllite, and calc-silicate hornfels formerly assigned to the "Carrietown sequence." The lower member is exposed in the southern Smoky Mountains, where it forms dark-colored slopes punctuated by thin ridges of resistant micritic sandstone. The base of the lower member either is a fault or is masked by the Idaho batholith, and the upper contact is placed above a mappable dark-gray carbonaceous limestone and below thick light-brown micritic sandstone of the middle member (O''Brien, 1991) (fig. 6).' 163|'The Wilson Creek Member (Wolfcampian to Leonardian) consists of more than 800 m of dark-gray carbonaceous siltstone and sandstone, thin-bedded light-brown graded silty micritic limestone, light-brown silty micritic limestone, light- to dark-gray sandy micritic limestone, and subordinate light-brown medium-bedded micritic sandstone (fig. 6). The member forms slopes and is sparsely exposed east of Bellevue and Hailey, although it is well exposed in alpine ridges of the Boulder Mountains northeast of Ketchum (figs. 1, 9, plate 1). The fine-grained, thin-bedded silty micritic limestone of the Wilson Creek Member weathers to a characteristic reddish-brown regolith. The upper part of the member is dolomitic in part and contains diagenetic chert. The Wilson Creek Member contains a diverse assemblage of bathyal trace fossils that has been interpreted to reflect oxygenation levels controlled by turbidite depositional facies (Burton and Link, 1991). The upper contact of the Wilson Creek Member is eroded below an unconformity with the Eocene Challis Volcanic Group or the present-day topographic surface (fig. 8).' 164|'The Eagle Creek Member of the Wood River Formation (upper Desmoinesian to Wolfcampian) is 880-1,300 m thick and consists of 260 m of light-purple silty micritic limestone overlain by 620-1,140 m of light-brown micritic sandstone, light-gray sandy micritic limestone, and subordinate quartzarenite (fig. 6). The Eagle Creek Member forms the bulk of the outcrop area of the Wood River Formation both east and west of the Wood River. It is best exposed on the high ridges of the Boulder Mountains (fig. 9). About 250 m of anomalous carbonate-rich facies assigned to the Eagle Creek Member by Batatian (1991) crops out just east of the summit of Kent Peak. The rock is coarse-grained carbonaceous bioclastic limestone and resembles bioclastic beds in the Wilson Creek Member (David Seeland, U.S. Geological Survey, written commun., 1992). The unit yielded conodonts (collection 12, fig. 7) that indicate a Late Pennsylvanian to Early Permian age and macrofossils, including Chaetetes sp., that indicate a (Middle Pennsylvanian) Desmoinesian age (D.E. Fortsch, Idaho State University, oral commun., 1991). If the Desmoinesian age is correct, the beds may be the carbonaceous equivalent of limestone in the upper part of the Hailey Member. The facies resembles bioclastic parts of the Bloom Member of the Snaky Canyon Formation (Chesterian to Missourian) exposed 55 km to the east across several thrust faults in the Lost River Range (Skipp, Kuntz, and Morgan, 1979) (fig. 7).' 165|'The basal Hailey Member of the Wood River Formation, as defined by Mahoney and others (1991), consists of the Hailey Conglomerate Member and the bioclastic limestone of unit 2 of the Wood River Formation, as described by Hall and others (1974). Unit 2 is vertically and laterally gradational with the conglomerate (Burton, 1988; Mahoney and others, 1991). The Eagle Creek Member consists of units 3 through 6 of Hall and others (1974), and the Wilson Creek Member consists of units 7 and 8 of Hall and others (1974, 1978). The Hailey Member is 0-200 m thick and consists of 0-180 m of light-brown to light-gray conglomerate gradationally overlain by 15-30 m of bluish-gray bioclastic limestone (fig. 6). The member is primarily exposed east of the Wood River in the Pioneer and Boulder Mountains, although it is exposed in a few areas west of the Wood River near Hailey (plate I). To the east, near Fish Creek Reservoir in the Idaho Falls l°x2° quadrangle, the Hailey Member contains 180 m of calcareous sandstone and dispersed chert-pebble conglomerate and bioclastic beds (Link and others, 1988). The Hailey Member may have been deposited on an irregular topographic surface and is locally depositionally absent (Winsor, 1981). The basal contact of the Hailey Member is a locally sheared unconformity with the underlying Devonian Milligen Formation (Burton, 1988; Burton and others, 1989; Ratchford, 1989. in press; Mahoney and others, 1991; Burton and Link. this volume). In many areas interstratal slip along the unconformity has produced a shear zone in which the Hailey Member is present as kilometer-scale boudins, or is attenuated. particularly in fold limbs, or is thickened in fold hinges. Silver-lead-zinc vein deposits are present locally along this sheared unconformity (Burton and Link, this volume). The upper contact of the Hailey Member is placed at the first appearance of the distinctive light-purple silty micrite of the Eagle Creek Member (table 2, fig. 6). The age of the Hailey Member is Desmoinesian, based on the biostratigraphy of coral. fusulinid, and phylloid green algae occurrences (Bostwick, 1955; Hall and others, 1974) (fig. 9).' 166|'Includes rocks west of the Pioneer thrust fault. The Copper Basin Formation contains graded beds of cobble and pebble conglomerate, as well as sand- and silt-size siliciclastic turbidite and argillite. Clasts in conglomerate include dark-colored chert and argillite similar to rocks of the Devonian Milligen Formation, as well as a large proportion of light-colored quartzite clasts of uncertain provenance.' 167|'Includes the Scorpion Mountain Formation of Paull and others (1972) and rocks equivalent to the Green Lake Limestone Member. An upper unit (unit Mcu) of pebble and cobble conglomerate, sandstone, and argillite (more than 1,150 m thick) is mapped on plate I. This upper unit includes the Scorpion Mountain Formation of Paull and Gruber (1977), the lower part of the Muldoon Canyon Formation of Paull and Gruber (1977), and the beds mapped as Green Lake Limestone Member of Copper Basin Formation north of Dry Canyon along the East Fork of the Big Lost River by Dover (1981).' 168|'Includes the Little Copper Member and Drummond Mine Limestone Member of Paull and others (1972) and Dover (1981). The lower part of the Copper Basin Formation is of Early Mississippian age (Kinderhookian and Osagean) and is at least 3,000 m thick....the formation consists of a lower interval (unit Me!) that contains a basal of dark-gray argillite, siltite, and granule conglomerate turbidite (the Little Copper Member, from 0 to more than 660 m thick) overlain by a discontinuous, but generally eastward thickening wedge of fine-grained mixed carbonate-siliciclastic turbidite (Drummond Mine Limestone Member, 0-910 m thick).' 169|'The Lower to Upper Devonian Milligen Formation is the host for rich silver-lead-zinc ores in the historically productive Minnie Moore and Triumph mineralized areas near Hailey and Bellevue (fig. I). The Milligen Formation was named by Umpleby and others (1930) for carbonaceous and argillaceous rocks originally included in the lower part of the Wood River Formation by Lindgren (1900). As described by Sandberg and others (1975), Dover (1981), and Turner and Otto (1988, this volume), the Devonian Milligen Formation contains more than 1,200 m of unfossiliferous, fine-grained, locally carbonaceous strata that are poorly exposed on sagebrush- and grass-covered slopes in the Boulder and Pioneer Mountains east of the Wood River. The base of the Milligen Formation has not been observed in detailed mapping of the Triumph mine area or on the western side of the Boulder Mountains (Dover, 1983; Turner and Otto, this volume). The upper contact varies along strike and is locally an unconformity below the Middle Pennsylvanian Hailey Member of the Wood River Formation, a possibly conformable contact with strata mapped as Mississippian Copper Basin Formation, or a fault against Pennsylvanian and Permian strata of the Sun Valley Group. A stratigraphic division of the Milligen Formation in the area near the Triumph mine (Turner and Otto, 1988, this volume) (fig. 3) includes, as informal members, (1) a lower argillite member (130+ m thick), which locally contains diamictite or is chert- or sandstone-rich, (2) the limestone member of Lucky Coin (50-250 m thick), which contains limestone turbidite, black argillite, and diamictite, (3) the quartzite member of Cait (lenses of variable thickness, usually less than 10m), which contains carbonaceous coarse grained sandstone, diamictite, and black argillite, (4) the argillite member of Triumph (0-150 m thick), which contains black argillite, locally cherty, and interbedded sandstone, and (5) the sandstone member of Independence (150+ m thick), which contains interbedded sandstone and limestone turbidite. Diamictite in the lower argillite member, coarse-grained sandstone in the quartzite of Cait, and calciclastic turbidite of the limestone of Lucky Coin represent distinctive markers that allow correlation of the Milligen Formation across structurally complex areas.' 170|'Includes the Trail Creek and Phi Kappa Formations and unnamed rocks' 171|'Carey Dolomite, Jefferson Formation, Picabo Formation in the Dry Canyon inlier; Hanson Creek Formation, unnamed Middle Silurian limestone unit, Roberts Mountain Formation, and unnamed carbonate, conglomerate, and breccia units of Devonian age in the Wildhorse Canyon inlier.' 173|'Exposed only below the Pioneer thrust fault and Wildhorse detachment fault in the core of the Pioneer Mountains' 174|'Calc-silicate, gneissose quartzite, pelitic schist, and marble' 175|'Layered quartzofeldspathic, quartzitic, and mafic gneiss, as well as calc-silicate marble' 300|'Bridger Range area: Alluvium, colluvium, talus, landslide deposits, rock glaciers, and glacial and glaciofluvial deposits. Only selected major areas, generally exposed for at least 1 mi in a single direction, are shown. Crazy Mountains area: Alluvium, talus, rock glaciers, and glacial deposits. Only selected major areas, generally exposed for at least 1 mi in a single direction, are shown.' 301|'Sandy and conglomeratic tuffaceous siltstone with interbedded volcanic ash, freshwater gastropod limestone, and fossiliferous lenses of conglomerate and gravel. Coarse fractions mostly derived from Proterozoic Belt Supergroup and Paleozoic limestone. Shown only in extreme northeast corner of Bridger Range. Exposures more than 100 ft thick; actual thickness not measured (McGrew, 1977).' 302|'Bridger Range area: Dacite, diorite, diabase, and basalt dikes and sills. Diorite dike in northern part of Bridger Range is olive-gray, coarsely crystalline, pyroxene phenocrysts (Skipp and Peterson, 1965); composite dike in southern part of range may be similar in composition (McMannis, 1955). Sills differentiated into upper syenite layer and lower layer of biotite-augite diorite. Thickness 80-200 ft (McMannis, 1955). Crazy Mountains area: Diorite, diabase, and fine-grained rocks ranging from basalt to rhyolite; includes dikes and sills. Commonly porphyritic; phenocryst composition ranges from 5 to 50 percent. Transitional between alkaline and calc-alkaline compositions. Probably genetically related to quartz monzodiorite phase of Big Timber stock (Tqm) (du Bray and others, 1993). Probably includes alkalic bodies (Ta) on west flank of Crazy Mountains and north of Shields River.' 303|'Bridger Range area: Cliff-forming, massive, nonmarine conglomerate, sandstone, and siltstone; conglomerate clasts derived from pre-Cretaceous rocks. Contains fossil spores, plants, wood, freshwater mollusks, and vertebrates. About 6,600 ft thick (Roberts, 1964b, c; 1972). Crazy Mountains area: Siltstone, mudstone, sandstone, and pebble conglomerate. Unit is mainly hornfels in a several-mile-wide aureole of contact metamorphism adjacent to Big Timber stock (du Bray and others, 1993). Near Livingston, 6,615 ft thick (Roberts, 1972).' 304|'Bridger Range area: In descending order, includes Hoppers, Billman Creek, and Sedan Formations. Sedan Formation correlates with Miner Creek and Cokedale Formations, which are exposed in southernmost part of Bridger Range and in a narrow band extending to the east past Cokeville and Livingston (Skipp and McGrew, 1977). Livingston Group is volcaniclastic conglomerate, sandstone, and mudstone; and volcanic flows, sills, tuff, and breccia (Roberts, 1972). Hoppers Formation is purple-gray to gray-green, epiclastic volcanic sandstone, siltstone, mudstone, and conglomerate; 2,400 ft thick (Skipp, 1977). Billman Creek Formation is grayish-red, grayish-green, and gray volcanic mudstone and siltstone interbedded with minor volcanic sandstone, conglomerate, and vitric tuff. Includes freshwater gastropods and dinosaur bones (Skipp, 1977); 2,500 - 3,000 ft thick (Roberts, 1972; Skipp, 1977). Sedan Formation is primarily greenish-gray, epiclastic volcanic sandstone, mudstone, and conglomerate interbedded with mudflow conglomerate, welded tuff, devitrified silicified vitric tuff, bentonite, and lignitic coal (Skipp and McGrew, 1977). Freshwater mollusks, wood, and dinosaur bones present in upper part of Sedan Formation. Lower part contains ironstone nodules, magnetite-rich beds, volcanic granule-and-pebble conglomerate, and carbonaceous plant material. Lignite coal locally present at base. Sedan Formation about 3,000-4,500 ft (Skipp and McGrew, 1977). Crazy Mountains area: Volcaniclastic conglomerate, sandstone, and mudstone; tuff and volcanic breccia. Contains flows and sills (Roberts, 1972). Thickness 6,455 ft (Roberts, 1972).' 305|'Upper part is gray, thin- to thick-bedded, very fine grained to conglomeratic, largely calcareous, cross-bedded sandstone; calcareous concretions and intercalated coal and shale. Lower part is massive, indurated, ledge-forming, cross-bedded sandstone and greenish-gray and grayish-orange volcanic sandstone and conglomerate; intercalated coal, siltstone, and shale. Thickness 150-300 ft (Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965).' 306|'In descending order, includes Telegraph Creek Formation, Cody Shale, and Frontier Formation. Telegraph Creek Formation is predominantly gray, biotitic, silty shale that contains thin interbeds of fine-grained sandstone, limestone, and altered crystal vitric tuff. Upper part contains large calcareous cannonball concretions; lower part contains pelecypods and ammonites. About 285-400 ft thick (Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Cody Shale is dark-gray shale and mudstone that contain thin interbeds of gray-green, glauconitic, calcareous to argillaceous, cross-laminated and rippled, fossiliferous (ammonites) sandstone and siltstone; ironstone nodules and cone-in-cone structures occur locally. About 600-1,100 ft thick (McGrew, 1977; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Frontier Formation is mostly gray, green, or brown, thin- to medium-bedded, fine- to coarse-grained, calcareous to argillaceous, locally cross-laminated, ridge-forming sandstone; light-gray quartz, feldspar, and rock fragments contrast with dark-gray chert grains and give the sandstone a characteristic "salt-and-pepper" appearance. Contains interbeds of mudstone, siltstone, shale, quartzite, chert-pebble conglomerate, siliceous limestone, and fossil oyster banks; mudstone and siltstone intervals form valleys. About 500-700 ft thick (McGrew, 1977; Skipp, 1977; Skipp and Peterson, 1965).' 307|'In descending order, includes Mowry Shale (Upper Cretaceous) and Thermopolis Shale and Kootenai Formation (Lower Cretaceous). Mowry and Thermopolis Shales are dark-gray shaly mudstone and porcellanite interbedded with greenish-gray, medium-grained to very coarse grained, feldspathic, calcareous and argillaceous, thin-bedded sandstone; basal grayish-orange, ledge-forming quartzitic sandstone. Shales total about 600 ft thick (McGrew, 1977; Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968). Kootenai Formation is red, gray, and purple hematitic mudstone interbedded with yellowish-gray and grayish-purple, quartzose, medium- to coarse-grained sandstone. Gray, nodular, gastropod-bearing, freshwater limestone common in upper part (about 100 ft below top) and just above basal ledge- and talus-forming, salt-and-pepper sandstone. Lenses of granule- to pebble-size chert conglomerate in basal sandstone. About 200-400 ft thick (McGrew, 1977; Skipp, 1977).' 308|'Includes Morrison Formation (Upper Jurassic) and underlying Ellis Group (Upper and Middle Jurassic). Ellis Group comprises Swift, Rierdon, and Sawtooth Formations; near Livingston the Sawtooth’s lithic equivalent is the Piper Formation. Morrison Formation is red, gray, and purple mudstone and hackly shale intercalated with light-gray, yellowish-brown, and yellowish-orange siltstone and sandstone, some conglomeratic; calcareous, quartzose, commonly cross-bedded. Black shale near top; gray freshwater limestone in beds and nodules; thick red mudstone at base. About 200-400 ft thick (McGrew, 1977; Roberts, 1964a; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Ellis Group is light-gray to light-brown, quartzose, calcareous, commonly iron stained sandstone and siltstone, interbedded with gray, fossiliferous, ledge-forming oolitic or sandy limestone and chert-pebble conglomerate. Basal bed grades from chert breccia or chert-pebble conglomerate on the west to thin gray mudstone to fossiliferous limestone on the east. Thickness ranges from 2 to 213 ft (McGrew, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965).' 309|'In descending order, includes Phosphoria (Permian), Quadrant (Pennsylvanian), and Amsden (Lower Pennsylvanian and Upper Mississippian) Formations. Phosphoria Formation (McKelvey and others, 1959) (probably better correlated with Shedhorn Sandstone in this area) consists of a thin (no thicknesses given- presumably not more than several feet thick) section of phosphorite, phosphatic mudstone, carbonaceous mudstone, and chert (McKelvey and others, 1959); mapped with Quadrant Formation by Roberts (1964a, 1972). Quadrant Formation is well-sorted calcareous quartzite to quartzose sandstone; light-gray dolomite in lower part; lower part forms ledges. Thickness 0 to 150 ft (McGrew, 1977; Roberts, 1964a; Skipp and Hepp, 1968). Amsden Formation is light-gray, weathering to white, thin- to thick-bedded dolomite. Interbeds of pale-grayish-orange sandstone and siltstone in upper part. Massive red sandstone, gray sandstone, siltstone and mudstone, and red claystone interbedded with gray argillaceous limestone in lower part; dolomite-limestone breccia at base. About 250-600 ft thick (McGrew, 1977; Skipp and Hepp, 1968).' 310|'Includes Mission Canyon Limestone and underlying Lodgepole Limestone. Mission Canyon Limestone is gray, aphanitic to medium-crystalline, cherty, massive, cliff-forming limestone and dolomite. Red-siltstone-filled cavernous zones, limestone breccia beds, and gray dolomite locally in upper part. About 700-1,200 ft thick (McGrew, 1977; Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965). Lodgepole Limestone is gray, thin- to medium-bedded, fossiliferous, locally cherty limestone and dolomite; weathers into platy blocks. Red and yellow silty limestone interbeds; thin dark-gray shale at base. Forms step-like ledges. About 500-750 ft thick (McGrew, 1977; Roberts, 1964a; Skipp, 1977; Skipp and Hepp, 1968; Skipp and Peterson, 1965).' 311|'Three Forks Formation is yellow calcareous siltstone, impure gray fossiliferous limestone, and dark-gray and green mudstone and shale. Thin beds of dolomite in upper part; olive-gray and reddish-brown, calcareous, fossiliferous shale in lower part. Thickness 150-200 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Underlying Jefferson Formation is brownish- to olive-gray, thick-bedded to massive, sparsely fossiliferous, fetid dolomite and interbedded gray limestone; local stromatolites. About 400-600 ft thick (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965).' 312|'In descending order, includes Snowy Range Formation (and laterally equivalent Red Lion Formation) and Pilgrim Limestone (Upper Cambrian); and Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone (Middle Cambrian). Locally may include Devonian Maywood Formation (McGrew, 1977), a red and yellow calcareous siltstone and gray dolomitic limestone (Skipp and Peterson, 1965). Snowy Range Formation is mainly limestone and shale distinguished by rounded limestone pebbles in the Grove Creek Limestone Member; underlain by thick red, gray, and green shale, gray limestone, and siltstone-pebble conglomerate of undivided lower members; bed of columnar limestone locally in lower part of formation (Roberts, 1964a; Skipp and Peterson, 1965). Combined thickness of Maywood and Snowy Range Formations 100-200 ft (Skipp and Peterson, 1965). Pilgrim Limestone is gray, thin-bedded to massive limestone and dolomite; forms cliffs. Oolites, grayish-orange and yellow mottling, and flat-pebble conglomerates are common. Interbeds of greenish-gray shale in lower part; glauconitic and fossiliferous at base. Thickness 350-500 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Park Shale is grayish-green to multicolored clay shale; thin beds of argillaceous limestone, siltstone, and sandstone; sparse fossiliferous limestone and limestone-pebble conglomerate in upper part. Nonresistant; forms valleys. Thickness 100-200 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Meagher (pronounced "mar") Limestone is thin- to medium-bedded, mottled grayish-orange and medium-gray, fine-grained limestone and yellowish-gray dolomite; locally oolitic; crumpled bedding. Thickness 300-600 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965). Wolsey Shale is olive- to brownish-gray micaceous clay shale; contains gray impure limestone in upper part and fine-grained, thin-bedded, glauconitic sandstone in lower part. Abundant trace fossils ("worm cast" markings). Forms valleys. Thickness 100-500 ft (Roberts, 1964a; Skipp and Peterson, 1965). Flathead Sandstone is yellowish-gray to reddish-gray, fine- to coarse-grained, thin- to thick-bedded, locally cross-bedded quartzite and quartz sandstone. Locally conglomeratic in lower part. Very resistant; forms ridges. Thickness 0-300 ft (McGrew, 1977; Roberts, 1964a; Skipp and Peterson, 1965).' 313|'Includes the Spokane and underlying LaHood (formerly North Boulder Group) Formations of the Belt Supergroup. Spokane Formation is light-grayish-green argillite, gray to yellowish-gray quartzite and sandstone, and minor thin-bedded grayish-red limestone; rippled beds are common. Thickness 2,200-3,200 ft (McGrew, 1977). LaHood Formation is dark-greenish-gray, medium-grained to very coarse grained to conglomeratic, micaceous, feldspathic sandstone that contains interbedded dark-gray limestone and impure aphanitic laminated calcareous argillite; sandstone forms about 30 percent of sequence and is most common in upper part; limestone, with cone-in-cone structures, and argillite dominate lower part. More than 2,500 ft thick (Skipp and Peterson, 1965).' 314|'Primarily gneiss. Includes schist, granite, quartzite, pegmatite, amphibolite, and mafic intrusive rocks (Roberts, 1964a).' 316|'Mafic alkalic rocks (including malignite, nepheline syenite, analcite syenite, and theralite), trachyte porphyry, and related intrusive rocks. Typically these rocks are sodium rich, silica undersaturated, and strongly alkaline (Harlan, 1986). No plagioclase feldspar and calcic amphibole (Dudas, 1990). Includes dikes, sills, laccoliths, and small stocks.' 318|'Quartz monzonite to granodiorite. Locally contains inclusions of diorite and gabbro (Tdg) along contact. Forms an irregular-shaped mass at center of Big Timber stock (du Bray and others, 1993). 40Ar/39Ar ages on biotite: 49.23±0.1 and 49.33±0.12 Ma (S.S. Harlan, USGS, unpub. data, 1996).' 319|'Granular diorite and gabbro form main part of Big Timber stock. Locally contains stoped blocks of pyroxene-rich gabbro. Coarser grained and darker in color than quartz monzodiorite phase (Tqm) (du Bray and others, 1993). 40Ar/39Ar ages on biotite: 49.2±0.07 and 49.33±0.11 Ma (S.S. Harlan, USGS, unpub. data, 1996).' 320|'Quartz monzodiorite porphyry, compositionally similar to quartz monzodiorite phase of Big Timber stock (Tqm). Separated from main body of Big Timber stock by sedimentary rocks of Fort Union Formation (TKf). At southern end of area of outcrop, unit has a quenched appearance with plagioclase phenocrysts (2- to 5-mm long) in an aphanitic to glassy groundmass (du Bray and others, 1993).' 400|'Sedimentary and volcanic deposits, undivided.' 401|'All Miocene and Pliocene volcanic and sedimentary rocks including rocks of the Magic Mountain eruptive center, the Banbury Basalt and coeval rocks, the Idavada Volcanics, the tuff of Cannonball Mountain, and the Payette Formation.' 402|'Eocene extrusive volcanic rocks, volcaniclastic sedimentary rocks, and smaller hypabyssal bodies of the Challis Volcanic Group.' 403|'Mostly hypabyssal bodies, many of which are too small to show.' 404|'Pink to light-gray granite in the Sawtooth, Trinity, and Soldier batholiths and many smaller plutons.' 405|'Diorite, granodiorite, dacite porphyry, latite, quartz monzonite, and hornblende gabbro in the border areas of major batholiths and many smaller plutons.' 406|'Granite and leucogranite.' 407|'Major phases of the Idaho batholith.' 408|'Potassium-rich granodiorite and quartz diorite, mainly in the southwestern border areas of the Idaho batholith.' 409|'Includes the Eagle Creek and Hailey Members of the Wood River Formation and equivalent parts of the Dollarhide and Grand Prize Formations.' 410|'Includes the Wilson Creek Member of the Wood River Formation and equivalent parts of the Dollarhide and Grand Prize Formations, the Milligen Formation, unnamed Devonian and Silurian rocks, and the Trail Creek and Phi Kappa Formations.' 411|'Mainly carbonate and sandstone turbidite. Includes the McGowan Creek and Copper Basin Formations.' 414|'Includes the White Knob Limestone, the Surrett Canyon, South Creek, Scott Peak, Middle Canyon, and Jefferson Formations, and the Laketown and Fish Haven Dolostones.' 417| 425|'Unconsolidated silt, sand, and gravel along modern-day streams.' 426|'Unconsolidated deposits consisting mainly of slope wash and talus.' 427|'Unconsolidated mantle of alluvium, colluvium, and, near Lake Helena, wind-blown silt.' 428|'Unconsolidated earthflow deposits that contain a mixture of rock materials ranging from silt to boulders in size.' 429|'Glacial till, moraine, glaciofluvial deposits, and minor alluvium and colluvium.' 430|'Unconsolidated deposits of unsorted and unstratified clay, silt, sand, gravel, and boulders; in general lacks morainal form.' 431|'Unconsolidated clay, silt, sand, and gravel deposited from meltwater streams that flowed from glaciers; generally stratified.' 432|'Sand, silt, and clay that is sorted, stratified, and firmly compacted. Present in vicinity of Missouri River.' 433|'Unconsolidated gravel and fan deposits, and conglomerate in northern part of Big Belt Mountains.' 434|'Sandstone, siltstone, claystone, shale, marl, siliceous sinter, and minor poorly sorted conglomerate; locally carbonaceous and tuffaceous; rare coal and limestone. Volcaniclastic components common.' 435|'In southeastern most part of Big Belt Mountains, interbedded siltstone and tuff. Along central and southwestern most flank of Big Belt Mountains, composed of tuffaceous conglomerate and sandstone, subordinate stream-laid siltstone, freshwater limestone, and volcanic ash.' 436|'Sandstone and conglomerate that includes both volcaniclastic and non-volcaniclastic units; and lava flows and flow breccias. Present only in westernmost part of Forest.' 437|'Slim Sam Formation lies at top of sequence and is composed of interbedded volcaniclastic sandstone, tuff, and lapilli tuff; present only in Elkhorn Mountains. Eagle Sandstone consists of ledge-forming chert-rich sandstone; in Elkhorn Mountains locally contains fossil marine placers of magnetite. Telegraph Creek Formation consists of marine mudstone, siltstone, and sandstone.' 438|'In descending order, includes Vaughn (Upper part is dark siliceous sandstone and interbedded siliceous shale and siltstone; light gray porcellanite and lapilli tuff. Middle part is lenticular conglomerate of quartzite and chert clasts in matrix of sandstone. Lower part is sandstone interbedded with shale, bentonitic mudstone, and siltstone; some interbeds of limestone and limestone concretions.), Taft Hill (sandstone and dark shale, mudstone, and siltstone), and Flood Members (sandstone in upper part and dark gray fissile shale and minor interbedded siltstone in lower part).' 439|'Upper part is dark siliceous sandstone and interbedded siliceous shale and siltstone; light gray porcellanite and lapilli tuff. Middle part is lenticular conglomerate of quartzite and chert clasts in matrix of sandstone. Lower part is sandstone interbedded with shale, bentonitic mudstone, and siltstone; some interbeds of limestone and limestone concretions. Present in western part of Forest.' 440|'Taft Hill consists of sandstone and dark shale, mudstone, and siltstone. Flood consists of sandstone in upper part and dark gray fissile shale and minor interbedded siltstone in lower part. Present in western part of Forest.' 441|'In descending order, includes Cody Shale (Upper Cretaceous), Frontier Formation (Upper Cretaceous), Mowry Shale (Upper Cretaceous), and Thermopolis Formation (Lower Cretaceous). Cody consists of dark gray mudstone, and minor interbedded siltstone and sandstone. Frontier consists of light-gray sandstone, siltstone, and lesser interbedded mudstone. Mowry consists of sandstone, siltstone, tuffaceous siltstone, mudstone, and tuffaceous mudstone. Thermopolis consists of dark-gray fissile shale in upper part and rusty-brown quartz-rich sandstone and interbedded siltstone in lower part. Present in Elkhorn Mountains and southern part of Big Belt Mountains.' 442|'In Elkhorn Mountains and southern part of Big Belt Mountains. upper part is light-gray freshwater limestone, medial part is reddish orange mudstone and interbedded siltstone, and lower part is gray chert-rich sandstone and conglomeratic sandstone. In remainder of Forest, uppermost part is interbedded red to green shale, mudstone, siltstone, and light-gray sandstone; light-gray freshwater limestone and dolomite, and minor interbedded shale, siltstone, and sandstone; medial part is reddish orange mudstone, siltstone, shale, and light-gray sandstone; dark-gray shale and interbedded sandstone and siltstone; basal part is light-gray vitreous quartzite.' 443|'Includes the following map units: Blackleaf Formation; In descending order, includes Vaughn (Upper part is dark siliceous sandstone and interbedded siliceous shale and siltstone; light gray porcellanite and lapilli tuff. Middle part is lenticular conglomerate of quartzite and chert clasts in matrix of sandstone. Lower part is sandstone interbedded with shale, bentonitic mudstone, and siltstone; some interbeds of limestone and limestone concretions.), Taft Hill (sandstone and dark shale, mudstone, and siltstone), and Flood Members (sandstone in upper part and dark gray fissile shale and minor interbedded siltstone in lower part). Kootenai Formation; In Elkhorn Mountains and southern part of Big Belt Mountains. upper part is light-gray freshwater limestone, medial part is reddish orange mudstone and interbedded siltstone, and lower part is gray chert-rich sandstone and conglomeratic sandstone. In remainder of Forest, uppermost part in interbedded red to green shale, mudstone, siltstone, and light-gray sandstone; light-gray freshwater limestone and dolomite, and minor interbedded shale, siltstone, and sandstone; medial part is reddish orange mudstone, siltstone, shale, and light-gray sandstone; dark-gray shale and interbedded sandstone and siltstone; basal part is light-gray vitreous quartzite. Jurassic rocks, undivided (Upper and Middle Jurassic) In descending order, includes Morrison Formation (Upper Jurassic), Swift Formation (Upper and Middle Jurassic), Riordan Formation (Middle Jurassic), and Sawtooth Formation (Middle Jurassic). Morrison consists of gray to reddish purple and orange mudstone, shale, siltstone, and sandstone. Swift consists of gray calcareous sandstone and conglomeratic sandstone, and minor interbeds of siltstone and shale. Riordan consists of gray calcareous shale and siltstone and interbedded limestone. Sawtooth consists of gray calcareous sandstone and siltstone, and lesser amounts of shale and limestone. Present only in northern part of Big Belt Mountains and directly west of Helena.' 444|'In descending order, includes Morrison Formation (Upper Jurassic), Swift Formation (Upper and Middle Jurassic), Rierdon Formation (Middle Jurassic), and Sawtooth Formation (Middle Jurassic). Morrison consists of gray to reddish purple and orange mudstone, shale, siltstone, and sandstone. Swift consists of gray calcareous sandstone and conglomeratic sandstone, and minor interbeds of siltstone and shale. Rierdon consists of gray calcareous shale and siltstone and interbedded limestone. Sawtooth consists of gray calcareous sandstone and siltstone, and lesser amounts of shale and limestone.' 445|'Includes the following map units, described below: Phosphoria Formation and Quadrant Formation, undivided; Amsden Formation and Big Snowy Group, undivided. Present only in northern part of Big Belt Mountains. Phosphoria consists of gray phosphorite, phosphatic and calcareous sandstone, siltstone, shale, black phosphatic chert, and limestone. In Big Belt and Elkhorn Mountains and near Helena, in southern part of Big Belt Mountains, consists of chert, phosphatic sandstone, limestone, and quartzose sandstone; in northern part of Big Belt Mountains, consists of chert and cherty sandstone; in Elkhorn Mountains and near Helena, consists mainly of chert and quartzitic sandstone, but locally contains one to two thin beds of phosphatic rock. Quadrant consists of light-gray tightly cemented quartz-rich sandstone, locally with interbeds of siltstone and dolomite at base Amsden consists of reddish orange mudstone, shale, sandstone, and gray limestone and dolomite; present only in Big Belt Mountains and Elkhorn Mountains. Big Snowy Group, in descending order, comprised of Heath, Otter, and Kibbey Formations. Heath consists of gray limestone, petroliferous limestone and shale, and calcareous sandstone. Otter consists of varicolored shale, limestone, and siltstone. Kibbey consists of gray shale, siltstone, limestone, and calcareous sandstone. Big Snowy Group present only in Big Belt Mountains.' 446|'Shedhorn consists of gray siliceous sandstone. Phosphoria consists of gray phosphorite, phosphatic and calcareous sandstone, siltstone, shale, black phosphatic chert, and limestone. Park City consists of gray limestone, cherty limestone, calcareous siltstone, and sandstone. Snowcrest Range Group, in descending order, consists of lateral equivalents of Conover Ranch Formation (Lower Pennsylvanian and Upper Mississippian), Lombard Limestone (Upper Mississippian), and Kibbey Sandstone (Upper Mississippian) Conover Ranch and its lateral equivalents consists of reddish brown to reddish purple calcareous shale and siltstone. Lombard consists of gray limestone and dolomite. Kibbey consists of gray to red calcareous shale, siltstone, and sandstone. In Big Belt and Elkhorn Mountains and near Helena: Phosphoria: in southern part of Big Belt Mountains, consists of chert, phosphatic sandstone, limestone, and quartzose sandstone; in northern part of Big Belt Mountains, consists of chert and cherty sandstone; in Elkhorn Mountains and near Helena, consists mainly of chert and quartzitic sandstone, but locally contains one to two thin beds of phosphatic rock. Throughout Forest: Quadrant consists of light-gray tightly cemented quartz-rich sandstone, locally with interbeds of siltstone and dolomite at base.' 447|'Amsden consists of reddish orange mudstone, shale, sandstone, and gray limestone and dolomite; present only in Big Belt Mountains and Elkhorn Mountains. Tyler Formation consists of pale brown sandstone; present only in Dry Range area, northeast of Big Belt Mountains. Big Snowy Group, in descending order, comprised of Heath, Otter, and Kibbey Formations. Heath consists of gray limestone, petroliferous limestone and shale, and calcareous sandstone. Otter consists of varicolored shale, limestone, and siltstone. Kibbey consists of gray shale, siltstone, limestone, and calcareous sandstone. Big Snowy Group present only in Big Belt Mountains.' 448|'Includes Mission Canyon Formation (Upper and Lower Mississippian) and Lodgepole Formation (Lower Mississippian); and in Choteau quadrangle, Castle Reef Dolomite (Upper and Lower Mississippian) and Allan Mountain Limestone (Lower Mississippian). All units are composed of limestone and (or) dolomite; local chert.' 449|'Includes the following map units. Madison Group; Includes Mission Canyon Formation (Upper and Lower Mississippian) and Lodgepole Formation (Lower Mississippian); and in Choteau quadrangle, Castle Reef Dolomite (Upper and Lower Mississippian) and Allan Mountain Limestone (Lower Mississippian). All units are composed of limestone and (or) dolomite; local chert Three Forks Formation (varicolored shale. siltstone, limestone, and locally minor thin beds of sandstone) and Jefferson Formation (gray to brown dolomite, limestone, limestone breccia, and dark-gray fetid dolomite; minor siltstone and shale), undivided; Cambrian formations, undivided. Includes Upper snowy Range Formation (Upper Cambrian), Pilgrim Limestone (Upper Cambrian), Park Shale (Middle Cambrian), Meagher Limestone (Middle Cambrian), Wolsey Shale (middle Cambrian), and Flathead Sandstone (Middle Cambrian). Also includes Maywood Formation (Lower Devonian), described above, in southern part of Big Belt Mountains. Snowy Range consists of limestone and dolomite, including pebble conglomerate, and intervals of micaceous shale and siltstone. Pilgrim consists of yellowish gray to brown limestone and dolomite; green clay shale in lower part. Park consists of green fissile clay shale and minor interbeds of siltstone and limestone; local limestone pebble conglomerate. Meagher consists of light gray limestone and dolomite. Wolsey consists of greenish gray micaceous shale and glauconitic siltstone, and minor limestone and quartzite. Flathead consists of gray to maroon quartz sandstone, locally conglomeratic. Present only in northern part of Big Belt Mountains.' 450|'Three Forks consists of varicolored shale. siltstone, limestone, and locally minor thin beds of sandstone. Jefferson consists of gray to brown dolomite, limestone, limestone breccia, and dark-gray fetid dolomite; minor siltstone and shale. Maywood consists of reddish and yellowish gray siltstone and interbedded limestone and dolomite; sandstone present locally. Maywood in southern part of Big Belt Mountains is included with Cambrian formations, undivided, below.' 451|'Includes Upper Snowy Range Formation (Upper Cambrian), Pilgrim Limestone (Upper Cambrian), Park Shale (Middle Cambrian), Meagher Limestone (Middle Cambrian), Wolsey Shale (middle Cambrian), and Flathead Sandstone (Middle Cambrian). Also includes Maywood Formation (Lower Devonian), described above, in southern part of Big Belt Mountains. Snowy Range consists of limestone and dolomite, including pebble conglomerate, and intervals of micaceous shale and siltstone. Pilgrim consists of yellowish gray to brown limestone and dolomite; green clay shale in lower part. Park consists of green fissile clay shale and minor interbeds of siltstone and limestone; local limestone pebble conglomerate. Meagher consists of light gray limestone and dolomite. Wolsey consists of greenish gray micaceous shale and glauconitic siltstone, and minor limestone and quartzite. Flathead consists of gray to maroon quartz sandstone, locally conglomeratic. Map unit present only east of 112Ί longitude.' 452|'Red Lion consists of gray limestone and discontinuous bands of siliceous dolomite, and interbedded gray limestone and varicolored shale. Hasmark consists of gray dolomite and limestone, minor local interbeds of green shale and siltstone. Map unit present only west of 112Ί longitude.' 453|'Park consists of green fissile clay shale and minor interbeds of siltstone and limestone; local limestone pebble conglomerate. Meagher consists of light gray limestone and dolomite. Wolsey consists of greenish gray micaceous shale and glauconitic siltstone, and minor limestone and quartzite. Flathead consists of gray to maroon quartz sandstone, locally conglomeratic. Map unit present only west of 112Ί longitude.' 454|'Silver Hill consists of green clay shale, medium-gray limestone, and, in lower part, minor interbedded siltstone, sandstone, and shale. Flathead consists of gray to maroon quartz sandstone, locally conglomeratic. Present only in vicinity of Elliston.' 455|'Varicolored argillite, siltite, and metasandstone that ranges from feldspathic to quartz-rich; locally contains glauconite and chert.' 456|'Pale pink to light brown fine- to coarse-grained feldspathic metasandstone; sparse dark-red argillite and siltite; commonly cross-laminated, local ripples.' 457|'North of St. Mary’s fault, western part of Forest: Brown, red, and gray-green interbedded feldspathic metasandstone, siltite, and argillite. Thin interbeds of pale-green to light-gray quartzite. Contains ripples, desiccation cracks, raindrop impressions, salt casts, and cross-laminated beds. Formation divided into three members, in descending order: Yms3-Member 3-Gray siltite, argillite, and quartz-rich metasandstone and feldspar-quartz metasandstone. Yms2-Member 2-Varicolored quartzite and feldspar-quartz metasandstone. Yms1-Member 1-Varicolored argillite, siltite, feldspathic metasandstone.' 458|'Gray siltite, argillite, and quartz-rich metasandstone and feldspar-quartz metasandstone.' 459|'Varicolored quartzite and feldspar-quartz metasandstone.' 460|'Varicolored argillite, siltite, feldspathic metasandstone.' 461|'Green, white, and yellowish-gray calcareous and dolomitic argillite and siltite; argillaceous limestone and dolomite, locally stromatolitic; local beds of quartzite that contain rounded quartz grains. Ripples, fluid-escape structures, desiccation cracks, mud-chip rip-up clasts, and cross-lamination common.' 462|'North of St. Mary’s fault, in western part of Forest: On Hoadley thrust plate, interbedded red-brown siltite, gray-green siltite, and dark-red argillite. Red-purple feldspathic metasandstone at base. South of St. Marys fault, in western part of forest: Grayish red quartzite and feldspathic metasandstone, and interbedded zones of siltite and silty argillite. Zones of green beds of siltite, argillite, and fine-grained metasandstone, locally calcareous or dolomitic, distributed unevenly through formation. White to light-gray lenticular quartzite distributed throughout formation. On Hoadley thrust plate, interbedded red-brown siltite, gray-green siltite, and dark-red argillite. Red-purple feldspathic metasandstone at base. South of St. Marys fault, in western part of forest: Grayish red quartzite and feldspathic metasandstone, and interbedded zones of siltite and silty argillite. Zones of green beds of siltite, argillite, and fine-grained metasandstone, locally calcareous or dolomitic, distributed unevenly through formation. White to light-gray lenticular quartzite distributed throughout formation. On Eldorado thrust plate in Big Belt Mountains: Interbedded gray-green sandy siltite, dark-green argillite, and in basal part coarse-grained metasandstone. Ripple and rip-up clasts common; local stromatolitic beds.' 463|'Dark-gray dolomitic limestone, calcareous siltite, and argillite, commonly cyclically interbedded; local light-gray fine-grained metasandstone. Stromatolite beds common in upper and lower parts of formation. Common sedimentary structures include ripples, syneresis cracks, scour-and-fill structures, and fluid-escape structures.' 464|'Interlaminated gray-green dolomitic siltite and pale-green argillite. Light-gray fine-grained quartzite beds present in lower part of formation. Sedimentary structures include syneresis cracks, water-expulsion structures, and ripple cross-lamination.' 465|'Interbedded grayish red purple argillite and siltite, and lesser gray-green siltite; middle part, and to lesser extent lower part contains light-gray feldspathic metasandstone. Local dolomitic limestone. Common sedimentary structures include load casts, rip-up clasts, fluid-escape structures, ball-and-pillow structures, syneresis cracks, and ripple cross-lamination.' 466|'Interlaminated gray-green dolomitic siltite and pale-green argillite. Light-gray fine-grained quartzite beds present in lower part of formation. Sedimentary structures include syneresis cracks, water-expulsion structures, and ripple cross-lamination.' 467|'Includes the Spokane, Greyson, and Newland Formations, described above and below. Present only in northern part of Big Belt Mountains. Interbedded grayish red purple argillite and siltite, and lesser gray-green siltite; middle part, and to lesser extent lower part contains light-gray feldspathic metasandstone. Local dolomitic limestone. Common sedimentary structures include load casts, rip-up clasts, fluid-escape structures, ball-and-pillow structures, syneresis cracks, and ripple cross-lamination. Spokane Fm: Interbedded grayish red purple argillite and siltite, and lesser gray-green siltite; middle part, and to lesser extent lower part contains light-gray feldspathic metasandstone. Local dolomitic limestone. Common sedimentary structures include load casts, rip-up clasts, fluid-escape structures, ball-and-pillow structures, syneresis cracks, and ripple cross-lamination. Greyson Fm: In northwestern part of Forest: Interbedded dark greenish gray siltite and argillite, a few beds of lenticular quartzose metasandstone, and dolomitic siltite that contains algal laminations. In Big Belt Mountains: Upper member consists chiefly of argillite in lower part that grades upward into siltite; local gray fine-grained feldspathic metasandstone. Minor limestone and calcareous siltite and metasandstone present in upper part. Lower member consists of interbedded gray fine- to coarse-grained feldspathic metasandstone and, locally, quartzite; some beds ripple cross-laminated; conglomeratic beds, locally graded from small boulders in lower part to pebbles in upper part; medium-gray feldspathic siltite; local yellowish brown finely laminated (porcellanitic?) siltite; and gray finely laminated argillite. Newland Fm: Upper member consists of interbedded units of medium-gray limestone, some beds mottled dark gray with algal-like structures; calcareous and noncalcareous finely laminated siltite, and locally ripple cross-laminated siltite; locally light-gray, yellowish-orange-weathering porcellanitic argillite; minor feldspathic metasandstone and, locally, quartzite. Lower member consists of medium-gray, light-brown-weathering argillite and dolomitic argillite that is finely laminated. Minor interbeds of gray fine-to medium-grained feldspathic metasandstone beds a few inches thick. Formation exposed only in Big Belt Mountains.' 468|'In northwestern part of Forest: Interbedded dark greenish gray siltite and argillite, a few beds of lenticular quartzose metasandstone, and dolomitic siltite that contains algal laminations. In Big Belt Mountains: Upper member consists chiefly of argillite in lower part that grades upward into siltite; local gray fine-grained feldspathic metasandstone. Minor limestone and calcareous siltite and metasandstone present in upper part. Lower member consists of interbedded gray fine- to coarse-grained feldspathic metasandstone and, locally, quartzite; some beds ripple cross-laminated; conglomeratic beds, locally graded from small boulders in lower part to pebbles in upper part; medium-gray feldspathic siltite; local yellowish brown finely laminated (porcellanitic?) siltite; and gray finely laminated argillite.' 469|'Upper member consists of interbedded units of medium-gray limestone, some beds mottled dark gray with algal-like structures; calcareous and noncalcareous finely laminated siltite, and locally ripple cross-laminated siltite; locally light-gray, yellowish-orange-weathering porcellanitic argillite; minor feldspathic metasandstone and, locally, quartzite. Lower member consists of medium-gray, light-brown-weathering argillite and dolomitic argillite that is finely laminated. Minor interbeds of gray fine-to medium-grained feldspathic metasandstone beds a few inches thick. Formation exposed only in Big Belt Mountains.' 470|'Basalt flows, flow breccia, and plugs.' 471|'Andesite flows, flow breccia, and plugs.' 472|'Flows, porphyry plugs, breccia plugs, dikes, tuff, and welded tuff; includes minor andesite and latite flows and plugs.' 473|'Volcanic flows and dikes of basaltic, andesitic, and latitic composition; present only west of 112° longitude.' 474|'Volcanic rocks ranging from rhyolite to quartz latite to dacite in composition; includes plugs, dikes, stock, diatreme, flows, welded tuff, and air-fall tuff, locally volcaniclastic rocks. Present in Elkhorn Mountains and to west, south of U.S. Highway 12.' 475|'Lava flows, breccia, plugs, tuff, and volcaniclastic sedimentary rocks. Present only in southwesternmost part of Forest, south of Elliston.' 476|'Dark gray dike of highly altered igneous rock composed chiefly of mafic minerals; present along west flank of middle part of Big Belt Mountains.' 477|'Light gray intrusions of porphyritic latite to quartz latite. Present along west flank of central part of Big Belt Mountains.' 478|'Medium to dark gray lava flows, flow-breccia, air-fall tuff, and ash-flow tuff; composition ranges from rhyolitic to andesitic, and locally basaltic. Unit also includes related intrusive rocks (described below) where they are areally too small to show separately at this scale. Volcaniclastic sedimentary rocks, derived from debris of volcanic rocks, include conglomerate, sandstone, and mudstone. Present only in Elkhorn Mountains and area to west.' 479|'Dacite to andesite sills, dikes, and irregularly shaped intrusions. Shown only in Elkhorn Mountains.' 480|'Welded tuff and lava flows ranging from rhyolite to dacite in composition; dikes of andesitic composition; and volcaniclastic conglomerate, sandstone, siltstone, and mudstone. Unit present only in central and southern part of Big Belt Mountains.' 481|'Porphyritic dacite plug of Knopf (1963). Present northwest of Helena.' 482|'Dark-gray gabbro pluton. Present only east of Elliston.' 483|'Gray granitic intrusion of monzodiorite composition. Present only southeast of Lincoln.' 484|'Light-gray monzonite in stocks, dikes, and sills; present only in northwesternmost part Forest.' 485|'Light-gray aegirine-augite-bearing granitic intrusions of monzodiorite composition. Present only in Boulder Baldy-Boulder Mountains area of Big Belt Mountains.' 486|'Light-gray quartz monzodiorite intrusions. Present only in Big Belt Mountains.' 487|'Light- to medium-gray quartz monzonite intrusion. Present only in eastern part of Elkhorn Mountains.' 488|'Light-gray monzogranitic intrusion. Present only south of Lincoln..' 489|'Dark-gray dioritic rocks that form small stocks, dikes, and sills. Present in area between Helena and Elliston in western part of Forest.' 490|'Medium-gray granodiorite stocks and plugs.' 491|'Medium-gray granodiorite stock near Marysville, northwest of Helena.' 492|'Medium-gray granodiorite stock near Ogden Mountain, southwest of Lincoln.' 493|'Medium-gray intrusion granodiorite stock near Blackfoot City, northwest of Elliston.' 494|'Medium-gray granodiorite stock near Dalton Mountain, south of Lincoln.' 495|'Dark-gray granodiorite pluton along the northern margin of the Boulder batholith. Present south of Helena and in northwestern part of Elkhorn Mountains.' 496|'Light- to medium-gray granitic intrusions with many separate phases; composition ranges from monzogranite to granodiorite. Includes small areas of aplite (described below) too small to show separately. Forms the major rock type of the Boulder batholith in Elkhorn Mountains and in area south of U.S. Highway 12.' 497|'Light-gray, small leucomonzonite plutonic masses; present only west of Helena, north of U.S. Highway 12.' 498|'Dark-gray monzogabbro of plagioclase and clinopyroxene phenocrysts in groundmass of micropegmatite; present only south of Helena along northern margin of the Boulder batholith.' 499|'Light-gray to white rock of feldspar and quartz. Forms sheets, pods, dikes, and irregularly shaped masses within Butte quartz monzonite. Only larger bodies are shown. Present only in Elkhorn Mountains and in area south of U.S. Highway 12.' 500|'Dark-gray to gray-green sills and dikes.' 501|'Dark gray-green mafic sills; present only in northwestern part of Forest.' 575|'Gravel, sand, silt, and clay in flood plains and low terraces along present drainages.' 576|'Angular blocks of bedrock in finer grained matrix; mixed at places with glacial debris.' 577|'Includes till (ground and end moraines), outwash, and other fluvioglacial deposits. Dune sands are common in a few places. Locally includes lake sediments where not mapped separately as unit Ql.' 578|'Buff, laminated clay and silt containing a few lenses of gravel and scattered dropstones. Tends to form vertical cliffs along present stream courses; topographic surfaces are gently rolling. Sediments represent bottom deposits of various glacial lakes.' 579|'Conglomerate, sandstone, siltstone, marlstone, oil shale, and coal representing fill in Kishenehn basin; presently restricted to the valley along the North Fork of the Flathead River in northeastern part of map area. As much as 11,000 ft thick (Constenius, 1988).' 580|'Quartz latite porphyry, diorite, and lamprophyre intruded along faults. Only dikes thicker than 20 ft shown; one crosses Callahan Creek and one is at southeastern edge of map area.' 581|'Biotite or hornblende-biotite granite, quartz monzonite, and granodiorite. Occurs as stocks and also as small unmapped dikes in and near the Cabinet Mountains. The Dry Creek stock in the central Cabinet Mountains has a minimum Rb-Sr age of about 73 Ma (Wells and others, 1981).' 582|'Gray to buff, coarse-grained stocks and dikes. Composed largely of orthoclase, andesine, and small amounts of pyroxene and hornblende (Gibson, 1948). Largest exposures are in pyroxenite-syenite complex at Vermiculite Mountain about 6 mi east from Libby, Mont. Syenite interpreted to be essentially the same age as the pyroxenite (Kpy), which the syenite intrudes (Boettcher, 1967).' 583|'Found only in the Rainy Creek-Vermiculite Mountain area. A core of biotitite is surrounded by coarse-grained biotite pyroxenite, which in turn is ringed by magnetite pyroxenite (Boettcher, 1967). Biotitite yielded a Rb-Sr age of 104±3 Ma (C.E. Hedge, U.S. Geological Survey, unpub. data, 1982). Biotite pyroxenite has been altered at places to rocks containing vermiculite; Rainy Creek stock is main source of vermiculite in United States.' 584|'An incomplete section of deformed Fernie is exposed on a thrust plate in northeast Whitefish Range adjacent to north boundary of map. Upper part is gray dolomitic sandstone and interbedded dark-gray siltstone and shale. Lower in the section are layers of black fissile shale and gray limestone that contain belemnite fossils. Base not exposed. Minimum thickness about 500 ft.' 586|'Found only on thrust plates in northeast Whitefish Range. Light-gray, well-sorted, laminated and cross-laminated quartzite. Commonly has carbonate cement. Ripple marks present locally. Upper part includes beds of sandy dolomite that contain chert nodules. Contact with underlying Mississippian strata may be disconformable. Minimum thickness about 600 ft.' 587|'Includes Etherington, Mount Head, Livingstone, and Banff Formations as identified and described by Barnes (1963), and as published by Johns (1970). These rocks, found only on faulted thrust plates in northeast Whitefish Range, are a series of limestone and dolomite units that include some shale and calcarenite. Some units are bituminous, and several are cherty. Fossils that help distinguish the formations include brachiopods, crinoids, bryozoans, and corals. Estimated minimum thickness about 3,000 ft.' 588|'Found only in fault blocks and thrust plates across north end of Whitefish Range. Primarily dolomite and minor dolomitic sandstone and siltstone (Barnes, 1963). Estimated thickness about 2,000 ft.' 589|'In descending order, carbonate-bearing rocks, shale, and sandstone found in a series of fault blocks in northern Whitefish Range, in a southerly trending zone of faults beginning about 14 mi south of Libby, Mont., and in an isolated exposure of carbonate-­bearing strata about 6 mi south of Troy, Mont. In more complete exposures along Fishtrap Creek, about 12 mi south of southernmost exposures in Kalispell quadrangle, the carbonate-bearing rocks have been called informally the Fishtrap dolomite (Keim and Rector, 1964; Bush and Fischer, 1981) or the dolomite of Fishtrap Creek (Harrison and Cressman, in press). The carbonate-bearing rocks consist principally of gray crystalline to pelletoidal or oolitic dolomite and interbedded shale, limey mudstone, and minor laminated siltstone. Conformably beneath the carbonate-bearing strata is an olive, fissile shale unit, a few hundred feet thick, that is equivalent to the Wolsey (or Gordon) Shale. Conformably beneath the shale is a buff-colored sandstone unit, a few tens of feet thick, that is equivalent to the Flathead Quartzite. The Flathead rests unconformably on the underlying Libby Formation. Maximum thickness of the Middle Cambrian sequence is about 430 ft in northern Whitefish Range and about 2,400 ft in southern exposures at Fishtrap Creek.' 590|'Used locally in northern Whitefish Range where units are too thin to show individually at map scale.' 592|'Dioritic to gabbroic rocks that commonly show alteration of mafic minerals. Sills range in thickness from less than 3 to 500 ft. Thicker sills tend to have contact-metamorphosed zones around them. Sills commonly persist for many miles; some maintain approximately the same stratigraphic position, whereas others cut across the section at a low angle. Intrusion probably occurred three times (at about 1,430, 1,100, and 800 m.y. ago; Harrison, 1972). Unit ZYd used for younger two generations of sills above the Prichard Formation; unit Yd used for oldest generation of sills and confined to Prichard.' 593|'Upper half of unit is dark, olive-gray, silty-argillite that is very thinly laminated or apparently structureless; about 415 ft thick near Libby. Lower half of unit is dark greenish-gray, blocky-weathering hummocky cross-laminated, coarse siltite to very fine grained quartzite; siltite or quartzite beds commonly 3-15 ft thick and separated by partings of even-parallel-­laminated silty argillite 1-3 in. thick; about 510 ft thick near Libby. Grades by interlayering into the lower part of the Libby. Minimum thickness of upper part is about 925 ft near Libby but increases to at least 3,200 ft in northern Whitefish Range where upper part overlies the McNamara Formation.' 594|'In addition to separately mapped upper part (Ylu), includes Kidder’s lower five informal members, which on Flagstaff Mountain are, in descending order: (1) about 1,850 ft of alternating beds of dark-gray argillite and wavy-laminated green siltite, (2) about 1,380 ft of wavy-laminated green argillite and siltite that contain scattered carbonate concretions, (3) about 950 ft of stromatolitic and oolitic parallel- to wavy-laminated green siltite, (4) about 280 ft of light- and dark-green parallel-laminated siltite containing abundant small-scale sedimentary structures and thin chert laminae and chips, and (5) about 180 ft of dark-gray argillite thinly interlaminated with dark-green siltite. Rests sharply on the Bonner Quartzite or its siltite facies. Lower five members are probably a distal black and green facies of the green and red McNamara Formation into which it grades laterally by interlayering and intertonguing.' 595|'Predominantly grayish-green, interbedded and interlaminated argillite and siltite that contain thin chert laminae and chips. Oolites, stromatolites, quartzarenite, and stratabound copper minerals present at places. Three relatively thin red-bed sequences of laminated argillite and siltite occur at top, middle, and bottom of section in Whitefish Range. Strata commonly exhibit ripple marks, shrinkage cracks, scours, and small-scale cross-bedding. Red-bed sequences drop out to west where green strata pass to dark-gray and green strata of lower five members of the Libby Formation. Sharply overlies the Bonner Quartzite. Maximum thickness of about 4,200 ft exposed in northern Whitefish Range.' 596|'Red to pink, micaceous, arkosic, cross-bedded, fine- to medium-grained quartzite containing red argillite intraclasts; tabular and trough cross-beds and climbing ripple marks common. Interbeds of red, laminated argillite and pink, planar-laminated siltite increase in abundance to north, and siltite facies (Ybos) mapped where quartzite beds decrease to minor component. Sharply overlies the Mount Shields Formation. Maximum thickness about 1,000 ft.' 597|'Interbedded fine- to medium-grained feldspathic siltite and red argillite seen as beds and tongues in all exposures of Bonner; siltite facies is a minor component in the southern Libby thrust belt and southernmost Purcell anticlinorium. Cross-bedding and ripple cross-lamination common. Contains a few 10- to 20-ft-thick beds of typical Bonner Quartzite. Also contains some beds of green, parallel-laminated argillite and siltite, particularly near base. Sharply overlies the Mount Shields Formation. Thins from about 900 ft in south to 600 ft in north.' 598|'West of the lead thrust in the Libby thrust belt, the upper two members (shown as members 6 and 5 on fig. 2) are mapped together as the upper part of the Mount Shields where exposure permits. Member 6 (uppermost member) is black and green or white, thinly laminated argillite and siltite commonly displaying small-scale slump folds and shrinkage cracks. Maximum thickness about 300 ft. Member 5 (conformably underlying member 6) is gray, silty, stromatolitic limestone and dolomite that has conspicuous boxwork structure formed by siliceous laminae intersected by thin quartz veinlets along vertical joints. Green, dolomitic, silty argillite beds as thick as 30 ft are interlayered with the boxwork limestone. Maximum thickness about 500 ft.' 599|'In western part of map area, includes members 6 and 5 (where not mapped as Ymsu) plus members 4 through 1 (from upper to lower, as shown on fig. 2). Member 4 is blocky, green, dolomitic, silty argillite that shows parallel-laminated graded couplets; 1-ft-thick carbonate beds are scattered through member, and stromatolites are common near base; rare salt casts; ranges in thickness from about 350 ft in south-central part of map area to 550 ft in north- and east-central parts. Member 3 is predominantly alternating red and green beds of interlaminated argillite and siltite; mud chips, mud cracks, ripple marks, and fluid-escape structures are common; salt casts are abundant, particularly in red beds; stratabound films of chalcopyrite or chalcocite occur in some green beds; thickness ranges from about 700 ft in south-central part of map area to about 1,200 ft in north-central part. Member 2 is pale-red or green, flat-laminated, coarse-grained siltite to fine-grained quartzite that is blocky, feldspathic, and dolomitic; dolomite in cement, streaks, and pods parallel to bedding; minor layers of red or green argillite; red or buff stromatolite and oolite zone at top; ranges in thickness from about 700 ft in south-central part of map area to a maximum of 1,700 ft in west-central part. Member 1 is red to maroon, feldspathic, medium- to coarse-grained quartzite interbedded with red siltite; red argillite partings common between beds; cross stratification, mud chips, and heavy mineral streaks common; stromatolites at a few places; thickness about 1,000 ft in southern Libby thrust belt but thins in all directions from there (fig. 2). Sharply overlies Shepard Formation. In eastern part of map area in Whitefish Range, member 1 is missing, member 2 is thin in the south and missing in the north, member 3 is lowest member exposed in the north and is about 1,000 ft thick, and member 4 thickens to about 700 ft and includes at the top about 180 ft of thinly laminated, grayish-green argillite and siltite that contain a few beds of olive, fine-grained arenite (Whipple, 1984). Sharply overlies Shepard Formation.' 600|'Dark-gray, amygdaloidal basalt. Occurs only in lower part of Mount Shields Formation in Glacier National Park. About 30 ft thick.' 601|'Consists of a variety of rock types, most of which are carbonate bearing or carbonate rich. Exposures in northern and northeastern parts of map area are mostly gray, fine-grained quartzite and siltite, some of which is pyritic and calcareous; less abundant interbeds include limestone (some of which is stromatolitic and oolitic), dolomite, and green calcareous argillite. Exposures in western and southwestern parts of map area contain a few quartzite beds but are predominantly (1) green, platy, dolomitic argillite, (2) green, slightly dolomitic, interlaminated argillite and siltite, and (3) silty dolomite that at places is stromatolitic and oolitic and may also display molar-tooth structure or horizontal calcite pods. Pyrite is common in the more carbonate-rich rocks. Zones of red interlaminated argillite and siltite are limited to a few places in the green argillitic rocks of south-central part of map area and locally in Whitefish Range. Black laminated argillite first appears in the green argillitic beds in southwestern part of map area and thickens to the west. The Shepard is commonly sheared or folded into chevrons where found in tight to overturned major folds. Grades by interlayering into the Snowslip Formation or its green facies, but the contact is marked at places by shallow channels of quartzite or, in the northeastern part of map area, by an angular unconformity that separates the Shepard from the Purcell Lava. The Shepard is lithologically similar to and correlates with members 4 and 5 of the Wallace Formation as mapped by Harrison and Jobin (1963) in the Clark Fork 15-minute quadrangle, which is adjacent to southwest corner of map area. Thickness ranges from about 280 ft in northern Whitefish Range (Whipple, 1984) to about 2,800 ft in southwest corner of map area.' 602|'Black to blackish-green basalt of continental alkaline basalt affinity (McGimsey, 1985). Multiple flows, commonly 5-10 ft thick, in the upper part of the Snowslip Formation or at the Snowslip-Shepard contact. Generally can be divided into two or three groups of flows in most areas. In northeastern part of map area, upper, middle, and lower flows are present. Upper flows are fine grained in their lower parts and amygdaloidal and vesicular in their upper parts (Whipple, 1984); ropy flow structures cap many flows. Middle flows have indistinct contacts and fine-grained porphyritic zones. Lower flows are porphyritic and contain long (1-2 in.) tabular plagioclase crystals in a fine-grained and highly altered groundmass. Flows are locally pillowed. Thickness ranges from 0 to about 700 ft. In western and southwestern parts of map area, only upper and lower flows were observed. Upper flows are similar to those of northeastern part of map area except rope flow tops are absent. Lower porphyritic flows have vague pillow structures and vesicles or amygdaloids at their tops. At a few places, individual flows are separated by a few inches to a few feet of beds similar to those of the Snowslip. Southern end of exposed flows is found at two places in southern Whitefish Range and near the mouth of the Fisher River in western part of map area. Thickness ranges from 0 to about 160 ft.' 603|'Generally contains units from a hundred to several hundred feet thick of thinly laminated red to purple argillite and siltite interbedded with thinly laminated green argillite and siltite that alternate with similarly thick units of couplets of greenish-gray siltite and olive argillite. Some units contain beds of arenite, carbonate, and stromatolites. Whipple and Johnson (1988) recognized six informal members in northeast corner of map area; similar members were separated on the basis of amount of arenite or presence of carbonates and stromatolitic limestone. Mud cracks, ripple marks, and fluid-escape structures are common in argillite beds. Arenite beds display small-scale cross laminae at many places. In west-central part of map area, red beds become fewer and thinner, green laminated argillite and siltite predominates, and a marker bed of thinly laminated black argillite and green siltite is found in green strata about one-third of the distance below the top of this facies. Green beds commonly have chlorite on bedding surfaces and some contain small amounts of stratabound copper minerals. This facies of the Snowslip either grades by interlayering into the upper member of the Wallace Formation or rests sharply on the middle member. Thickens in all directions from about 600 ft in northern Whitefish Range to 3,000 ft or more in northern and central parts of map area (fig. 2).' 604|'Predominantly evenly laminated couplets of green argillite and siltite containing sparse or no red beds. Chlorite common on bedding surfaces. Laminated black argillite and green siltite increases in abundance to the west (fig. 2). The black argillite commonly displays small soft sediment folds, shrinkage cracks, small-scale cut-and-fill structures, and fluid-escape structures. Grades laterally by intertonguing into red and green facies of the Snowslip and grades vertically by interlayering into the underlying Wallace Formation. This informal member is lithologically similar to and correlates with member 3 of the Wallace Formation as mapped by Harrison and Jobin (1963) in the Clark Fork 15-minute quadrangle, which is adjacent to southwest corner of map area. Thickness ranges from about 2,000 to 3,000 ft.' 605|'Thinly to very thinly laminated dark-green or black argillite and light-green siltite. Beds locally show small-scale cut-and-fill structures and shrinkage cracks. Dark- to light-green beds are always very thinly laminated and have parallel bedding. Differs from overlying green beds of the Snowslip in displaying thinner laminae, being less silty, and having no chlorite on bedding surfaces. Thickness ranges from 0 to 1,200 ft.' 606|'Black argillite alternating with white slightly dolomitic siltite or very fine grained quartzite layers and lenses that range from 1 to 20 in. in thickness; characteristically uneven and wavy bedded. Thicker siltite layers and lenses commonly display load casts and flame structures. Zones a few hundred feet thick in upper and lower parts are at places carbonate rich and show irregular, vertical calcite ribbons (molar-tooth structure) or small (2 in.), vertical calcite pod structures that transect tens of feet of beds. Also includes some beds of black laminated argillite, green interlaminated argillite and siltite, and dolomitic argillite. Slump folds in zones a few feet thick are present in southern Libby thrust belt (fig. 2). Cycles of quartzite or black silty argillite overlain by dolomitic siltite or dolomite a few feet thick are sparsely present in southwest corner of map area; these cycles are similar to those that characterize the main body of the Helena Formation, Pyrite and chalcopyrite common in any of the carbonate-bearing lithologies. Thickness ranges from about 500 ft in Whitefish Range to about 6,000 ft at west edge of map area.' 607|'Predominantly blocky, green, parallel-laminated and interbedded dolomitic argillite and argillitic siltite, some of which contains molar-tooth structure and pods or irregular blobs of calcite. Includes a few beds a foot or two thick of white to buff, medium-grained quartzite. Resembles the Empire Formation but differs in containing molar-tooth structures and not exhibiting fluid-escape structures that characterize the Empire. Interfingers laterally with the lower member of the Helena in northwestern part of map area where the interfingered zone has been mapped as either lower Wallace or lower Helena depending on the predominant lithology. Where underlain by the St. Regis Formation, the contact is fairly sharp; where underlain by the Empire Formation, the contact grades by interlayering. Thickness ranges from 0 to about 900 ft.' 608|'Wallace Formation' 609|'Used only in southern part of Cabinet Mountains for unit mapped by Wells and others (1981).' 610|'Hundreds of distinct lithologic cycles in units a few feet to a few tens of feet thick form the main body of the Helena. Each complete cycle consists of three parts: an upper bed of dense conchoidal-fracturing dolomite that weathers orange brown, a middle bed of gray dolomite that commonly displays horizontal calcite pods or blobs that grade successively upward to vertical pods and blobs and then into molar-tooth structures, and a lower clastic bed rarely more than a foot thick of white quartzite or quartzite and black argillite resting on a cut surface. Complete cycles are seen in many exposures, but most commonly the lower bed is missing and less commonly one of the other beds is missing, Pyrite and some chalcopyrite common. Stromatolite beds present at some places, particularly east of the Rocky Mountain trench. A zone a few tens to a few hundred feet thick at top of map unit is black wavy-bedded argillite and tan-weathering white dolomitic siltite typical of the middle member of the Wallace Formation. At places includes the lower member of the Helena Formation where not mapped separately. Main body of Helena interfingers with middle member of Wallace in northern and central parts of map area (fig. 2). Thickness of main body of Helena ranges from about 9,000 ft in southeastern part of map area to 0 where it fingers out westward into the middle member of the Wallace.' 611|'Alternating beds a few feet to a few tens of feet thick of dense, orange-weathering dolomite and thinly laminated, apple-green to tan argillite. Half-inch-thick beds of brown-weathering quartzite scattered through unit. Dolomite commonly pyritic and displays irregular-shaped pods of calcite and molar-tooth structure. Tongues and layers of main body of Helena a few hundred feet thick are common in unit. Contact with main body of Helena placed on top of uppermost green and tan argillite bed. Grades by interlayering into the underlying Empire Formation, and grades by intertonguing laterally into lower member of the Wallace Formation. Maximum thickness about 3,000 ft.' 612|'Mapped in north-central part of map area where beds of the main body of the Helena and middle member of the Wallace are interlayered and intertongued on a scale of a few tens of feet to a few hundreds of feet. Minor interlayering at other places is mapped as either Helena or middle Wallace depending on the predominant lithology. Includes lower member of the Helena where not mapped separately.' 613|'Thinly laminated, dark-green and light-green dolomitic argillite and silty argillite or siltite. Laminae mostly wavy and discontinuous although some are even parallel. Fluid-escape structures are characteristic, and horizontal pods of white or pink calcite are particularly abundant in upper part. Ripple marks, syneresis cracks, and mud chips common in places. Lower part contains white dolomitic quartzite beds as thick as 2 ft. White, rounded-grain quartzite beds and lenses a few inches thick occur in northeast quarter of map area; cross-bedding and mud chips common in these beds. Pyrite cubes common in more carbonate-rich strata, and a few exposures display stratabound copper minerals. A few purple interlaminated argillite and siltite beds commonly occur near base and at places near middle of unit. Intertongues with underlying St. Regis Formation and probably with overlying lower member of Wallace. Thickness ranges from 0 to about 2,000 ft.' 614|'Purple interlaminated argillite and siltite beds that alternate with green interlaminated argillite and siltite beds. Upper part has carbonate specks and some carbonate cement; lower part has scattered beds of pink, fine-grained quartzite or siltite. Sedimentary structures include abundant mud cracks, mud chips, ripple marks, and fluid-escape structures. Formation present only in western part of map area. Grades by interlayering over a few tens of feet into underlying Revett Formation. Maximum thickness is about 1,000 ft, but thins to about 600 ft in north and to a few feet in southwest (fig. 2) before thickening again to about 1,100 ft near Clark Fork, Idaho, a few miles west of map area (Harrison and Jobin, 1963).' 615|'Characterized by blocky cosets of white, green, or pale-purple- gray, cross-bedded quartzite beds. In most areas, the Revett can be divided into three informal members: upper and lower quartzite-rich members and a middle member that contains significantly more argillite and siltite. Some quartzite is purple striped and commonly shows Liesegang rings of purple hematite coloration both along and across bedding. Many beds display climbing ripple marks, ripple cross laminae, load casts, and 1- to 5-ft-deep channels. Heavy minerals are common on cross laminae. Argillite beds are purple or green, generally display even- to wavy-parallel laminae, and contain mud cracks, mud chips, ripple marks, and fluid-escape structures. Siltite beds are commonly pale purple and parallel laminated; siltite alternates with beds of argillite. In southwestern part of map area, parts of the upper and lower quartzite members are green or white, show conspicuous iron-carbonate cement, and contain major ore deposits of stratabound copper-silver. In northern and western parts of map area, the characteristic quartzite beds are purple gray, thinner and less abundant, and enclosed in argillite and some siltite. In some of the easternmost exposures, the characteristic quartzite beds are only a few tens of feet thick. Small flakes of secondary biotite and tiny euhedral magnetite crystals are displayed in the formation throughout map area. Grades by interlayering into underlying Burke Formation. Thickness ranges from about 2,000 to about 2,800 ft in exposures west of the Rocky Mountain trench; formation is not recognized east of the trench.' 616|'Three lithologic members are found in most areas. Upper member has beds a few feet to a few tens of feet thick of green laminated argillite and siltite that alternate with beds of purple laminated argillite and siltite. Dolomitic cement is common as are ripple marks, mud chips, desiccation cracks, fluid-escape structures, ball-and-pillow structures, and flute casts. Beds a few inches to a few feet thick of white, rounded-grain, medium-grained, cross-bedded quartzite that displays mud chips and balls on cross strata are abundant in eastern part of map area but decrease rapidly in both number and thickness west of the Rocky Mountain trench. A section of this member on Blacktail Mountain about 13 mi south of Kalispell was drilled extensively to evaluate stratabound copper mineralization in green beds. Detailed descriptions of the surface geology of that area are given by Harrison and Reynolds (1979) and of selected cores by Reynolds (1979). Middle member is predominantly pink to purple-gray, very fine-grained feldspathic quartzite or coarse siltite that has planar lamination or long tabular cross lamination. Interbeds of purple argillite are common. Lower member is similar to the upper member but has more purple argillitic beds, is more dolomitic, and has scattered iron-carbonate specks and cement. The Spokane intertongues westward with the St. Regis and Revett Formations, but exact nature of intertonguing is uncertain owing to lack of critical exposures in and near the Rocky Mountain trench. Where the Spokane loses the distinctive white, rounded-grain quartzite beds westward and the underlying Revett loses the definitive cosets of cross-bedded quartzite eastward, the formations are difficult to distinguish, and some identifications of the mapped units were arbitrary. Thickness ranges from about 5,000 ft in eastern part of map area to 0 ft in western part.' 617|'Formation nomenclature as used by Whipple (in press). Lithologically the same as upper and lower members of the Spokane Formation, but the upper part of the Grinnell contains as much as 20 percent of white, rounded-grain quartzite beds (Whipple and others, 1984). Mapped only in Glacier National Park where it is about 2,600 ft thick.' 618|'Divisible into three informal members in most areas (fig. 2). Upper member consists predominantly of blocky beds of purple-gray interlaminated argillite and siltite interbedded with greenish-gray interlaminated argillite and siltite. Mud cracks and mud chips common in upper part of member. White to purple-gray parallel-laminated siltite and quartzite beds a few feet thick are interbedded in the upper part of the member and increase in abundance upward. Contact with the overlying Revett is placed where cosets of cross-bedded quartzite first appear. Middle member is predominantly gray to purple-gray parallel- and thinly-laminated siltite in blocky planar beds that have minor argillite partings or beds. Some siltite has zebra-striped markings of hematite coloration. Outcrops of middle member along northern part of Lake Koocanusa show wavy, wispy, discontinuous laminae. Lower member is predominantly green parallel-laminated interbedded argillite and siltite that generally are in beds about 2-20 in. thick and weather to give a blocky-flaggy outcrop. Upper part of lower member has some purple-gray interbeds of argillite and siltite. Siltite and some argillite lithologies in all members are commonly speckled by tiny euhedral magnetite crystals and display small flakes of secondary biotite. Grades by interlayering into the transition member of the underlying Prichard Formation. In the southern Cabinet Mountains, the Burke as mapped by Wells and others (1981) includes the transition member of the underlying Prichard Formation. Thickness ranges from about 2,500 ft to 3,600 ft west of the Rocky Mountain trench.' 619|'Predominantly thin to thick beds of olive siltite and very fine grained quartzite speckled by brown spots of limonite after iron carbonate and perhaps magnetite. Lamination faint but generally even parallel; minor cross lamination. Contact placed above highest black argillite bed in underlying transition member of the Prichard. Unit unnamed because authors differ in interpretation of the rocks as a facies of either the Burke or the Appekunny Formation. Mapped only along west side of Whitefish Range and west of Blaine Mountain. Thickness about 600 ft.' 620|'Formation nomenclature as used by Whipple (in press). The Appekunny Formation of Glacier National Park has been divided by Earhart and others (1983) into an upper part and a lower informal "Wolfgun member." Regional mapping of the Kalispell quadrangle and regional synthesis of the Prichard Formation by Cressman (1989) indicates that the Wolfgun is actually a stratigraphic and lithologic equivalent of the transition member of the Prichard Formation, which has been mapped over thousands of square miles to the west and southwest of Glacier National Park. As a consequence, we do not use "Wolfgun member" and assign those beds to the transition member of the Prichard (Ypt). The Appekunny as shown in the park consists mostly of interlaminated green argillite and siltite and some beds an inch or two thick of light-gray quartzite. Thickness is about 140 ft where exposed but thickens eastward.' 621|'Identical in composition and habit to unit ZYd but confined to the Prichard Formation and inferred to be the oldest of three generations of similar sills. Maximum thickness of a single sill is about 1,800 ft in the No.1 Gibbs borehole (near Lincoln County-Flathead County line in T. 28 N., R. 26 W.). One sill in lower part of Prichard along the Kootenai river about 22 mi northwest of Troy, Mont., yielded an age of 1,433±30 m.y. for igneous zircon in a granitic differentiate in the sill (Zartman and others, 1982). Sills are prominent seismic reflectors that decrease in abundance eastward from the crest of the Purcell anticlinorium to the Rocky Mountain trench (Harrison and others, 1985).' 622|'Generally consists of three units: (1) an upper unit of interlaminated light- and dark-gray siltite and argillite similar to the middle unit, but containing interbeds of light-olive-gray siltite and quartzite similar to those in the overlying Burke Formation, (2) a middle unit of interlaminated light- and dark-gray siltite and argillite, and (3) a basal unit of medium-gray blocky-weathering siltite and minor quartzite. All three units are present in nearly every outcrop area of member, but their relative proportions vary from place to place. Laminae are wavy to lenticular; scour-and-fill structures, fluid-escape structures, and syneresis cracks common. Iron sulfides weather to give rusty aspect to many outcrops. Randomly oriented biotite porphyroblasts common, particularly near top of upper unit, in all but easternmost exposures. Calcareous siltite and argillite beds make up a small part of the member (some in the middle unit but most in the upper unit) and gradually become more abundant eastward; in Glacier National Park contains some stromatolitic beds. Basal contact is sharp. Member is 2,000-2,500 ft thick throughout much of quadrangle but thins to about 1,050 ft in Whitefish Range and to about 860 ft in Glacier National Park. Some variation in the mapped thickness results from inconsistencies in placement of the gradational upper contact.' 623|'Medium-gray argillite that contains planar laminae and thin planar beds of light-gray and dark-gray silty argillite. Alternating light and dark layers, 1-3 mm thick, give lined or banded appearance. Dark-gray silty argillite contains discontinuous laminae of carbonaceous matter and iron sulfide (mostly pyrrhotite, but locally pyrite). Iron sulfide weathers to give rusty appearance to outcrops. Randomly oriented biotite porphyroblasts common except in Glacier National Park. In the park, upper part of member contains thin lenticular beds and pods of calcareous and dolomitic silty argillite. Near Lake Koocanusa, upper part of member contains some thin beds of white quartzite that at places contains metamorphic garnet, hornblende, and calcite; white quartzite beds become less abundant westward and are absent on west limb of Sylvanite anticline. Slump folds locally in lower part of member. Basal contact ranges from sharp to gradational through several hundred feet. Member is 1,600-2,000 ft thick in most of map area west of Rocky Mountain trench, and about 3,500 ft thick in Whitefish Range; about 3,500 ft is exposed east of map area near head of McDonald Lake in Glacier National Park.' 624|'Interbedded quartzite and argillite that enclose a distinctive tongue of argillite mapped separately as unit Ypa. Mostly (65 percent) medium-dark- to medium-light-gray, fine-grained and very fine grained, slightly feldspathic, argillitic quartzite in even beds 0.7-2 ft thick and in sets mostly 3-5 ft thick. Quartzite beds commonly grade to siltite and argillite in top few inches, and some beds exhibit sole marks. Quartzite sets alternate with argillite sets mostly about 10 ft thick. Argillite is silty, dark gray, and weathers brownish gray. Xenoblastic biotite present throughout member in amounts of as much as 20 percent; xenoblastic chlorite present locally. Member is a minimum of 12,000 ft thick in the Sylvanite anticline and west of the Moyie thrust system. Quartzite member thins and intertongues eastward with argillite in lower part of the Prichard (Ypl); is less than 4,000 ft thick and included in upper part of the lower Prichard (Ypl) immediately west of the Pinkham thrust; and probably pinches out near the Rocky Mountain trench.' 625|'Slabby-weathering, medium-gray, silty argillite in beds 0.5-1.5 ft thick. Parts of member display dark-gray laminae and thin beds of silty argillite that contain abundant pyrrhotite and carbonaceous matter; pyrrhotite weathers to impart rusty color to outcrops. Slump folds common near base. Xenoblastic biotite and chlorite common. Upper and lower contacts with enclosing quartzite member are sharp. About 800 ft thick in the Sylvanite anticline. Argillite member exposed in the Sylvanite anticline and west of the Moyie thrust system is a tongue of the argillite that makes up most of the Prichard Formation (Ypl) in central and eastern parts of quadrangle (see cross section B-B’).' 626|'Where exposed on the Purcell anticlinorium, consists of quartzite interbedded with argillite and is similar in character to the quartzite member. About 1,000 ft exposed. In No. 1 Gibbs borehole, lower part consists of six lithologic units that correlate with the informal members A-G described by Cressman (1985, 1989). These are from top to bottom: Quartzite interbedded with argillite and similar to quartzite member (member G). Total thickness of unit including core and surface exposures is about 4,000 ft. Dolomitic siltite, 3,000 ft thick (facies of member F). Interlaminated siltite and argillite interbedded with well-sorted quartzite, 2,800 ft thick (member E). Argillite, 500 ft thick (member D). Argillitic quartzite, 160 ft thick (member C). Argillite and siltite, 2,500 ft thick (members A and B, undifferentiated). Lowest beds in borehole are schistose and garnetiferous. Inferred thickness of entire Prichard Formation as shown in cross sections is about 25,000 ft and is based on thickness at borehole plus interpretation of gravity, magnetotelluric, and seismic data of depth to basement (Harrison and Cressman, in press).' 650|'Silt, sand, and gravel in channels and flood plains of existing streams and rivers and in alluvial fans.' 651|'Angular fragments of bedrock mixed with soil or heterogeneous boulders and finer-grained material; characterized by irregular, hummocky topography.' 652|'Unsorted mixture of boulders, cobbles, pebbles, and sand deposited by glaciers and by glacial meltwater. Unit includes undivided till and outwash of at least two major glacial episodes.' 654|'Light gray to yellowish-brown partly tuffaceous sandstone, siltstone, and shale, and subordinate interbeds of limestone and marl and lenses of pebble and cobble conglomerate composed of locally derived rock fragments. Locally, unit includes thin beds of lignite and lignitic shale in the Medicine Lodge Creek basin. Commonly veneered with gravel and a thin layer of eolian silt.' 655|'Includes limonitic, calcareous tufa near Shenon Creek and in the upper part of Medicine Lodge Creek, and calcareous, siliceous tufa and massive chalcedonic silica near the North Fork and South Fork of Everson Creek. Locally, unit contains repeatedly fractured and rehealed siliceous breccias. The Shenon Creek deposit probably is the source of placer gold mined in the Chinatown diggings in Jeff Davis Creek.' 656|'Basaltic andesite, andesite, latite, rhyodacite, and rhyolite as flows, ash-flow tuffs, and tuff and flow breccias. Locally, unit contains thin interbeds of light olive-gray, biotitic, tuffaceous, fine- to coarse-grained sandstone and conglomerate composed of volcanic detritus. Age ranges from about 48 Ma to 40 Ma (Staatz 1979; Ruppel and Lopez 1988; M''Gonigle and Dalrymple 1996).' 660|'Unit includes rocks originally mapped as Quadrant Formation (Pennsylvanian) (Lucchitta 1966; Smith 1961; Ruppel 1968; and Ruppel and Lopez 1988) that are now included in the Bloom and Gallagher Peak Sandstone Members of the Snaky Canyon Formation (latest Mississippian to Late Pennsylvanian) and in the Bluebird Mountain Formation (latest Mississippian) (Skipp et al. 1979). Consists mainly of quartzitic sandstone, dolomite, and limestone, and includes three units: (1) an upper pale yellowish-brown, fine-grained, thickly bedded, prominently cross-bedded, calcareous quartzitic sandstone with thin interbeds of similarly colored cherty, sandy dolomite; probably equivalent to the Gallagher Peak Sandstone Member of the Snaky Canyon Formation; (2) a middle unit of medium gray to medium light gray, thin- to medium-bedded, partly cherty limestone and dolomite; probably equivalent to the Bloom Member of the Snaky Canyon Formation; and (3) a lower unit of interbedded medium-light gray to medium-dark gray quartzitic sandstone, dolomite, and limestone probably equivalent to the Bluebird Mountain Formation; the sandstone is fine-grained, thickly bedded to massive; dolomite and limestone are finely to coarsely crystalline, thinly bedded or thickly bedded to massive. The three units are roughly equal in thickness; total thickness of interval is 1,300 ft to 1,700 ft.' 661|'Unit includes the Upper Mississippian and Lower Pennsylvanian Snowcrest Range Group (Wardlaw and Pecora 1985) and the Lower to Upper Mississippian Tendoy Group (Sando et al. 1985), the Upper Mississippian Railroad Canyon Formation, and carbonate rocks originally mapped as the Mississippian Madison Group, Paine Limestone, and Milligen Formation that are now included in the Surrett Canyon, South Creek, Scott Peak, Middle Canyon, and McGowan Creek Formations (Huh 1967; Sandberg 1975). The Railroad Canyon Formation is the temporal and partly the lithologic equivalent of the Lombard Limestone in the Snowcrest Range Group, and includes rocks originally mapped as Big Snowy Formation in the Beaverhead Mountains. The upper two-thirds of the formation is medium-dark gray to pale red, thin-­ to thick-bedded, fine- to medium-grained limestone and interbedded brownish-gray to pale red siltstone and mudstone; the lower third of the formation is grayish-red to dark gray mudstone and papery shales. The formation is 850 ft thick (Ruppel and Lopez 1988). The Railroad Canyon Formation is underlain by thickly bedded to massive, medium-light gray to medium-dark gray limestones of the Middle Canyon and Scott Peak Formations. The formations have a combined thickness of about 3,000 ft to 3,500 ft, but are greatly thinned by faulting in much of this region. The McGowan Creek Formation at the base of the Mississippian consists of medium gray to dark gray, fissile to chippy, carbonaceous siltstone, mudstone, and shale, and thin interbeds of dark gray fine-grained limestone. Thickness ranges from 100 ft to 200 ft.' 662|'Dolomite, medium-dark gray to dark gray, finely to medium crystalline, fetid, locally sandy near base of formation, and locally conglomeratic. The Three Forks Formation is known to be present only in the Hawley Creek area where it consists of dark gray siliceous mudstone and is less than 100 ft thick (Lucchitta 1966). The Jefferson Formation is about 800 ft thick.' 663|'Unit includes rocks of the Jefferson, Three Forks, and Summerhouse Formations.' 666|'Unit includes rocks of the Summerhouse Formation and Kinnikinic Quartzite and may locally include rocks of Devonian age. Units typically are too thin to be mapped separately or are so poorly exposed as to be inseparable. Regionally, the Summerhouse Formation (Lower Ordovician) is composed of sandstone that is quartzitic or feldspathic to arkosic, light gray to white, medium- to coarse-grained, locally feldspathic and micaceous, and locally conglomeratic; Scolithus burrows locally abundant; thickness ranges from 0 ft to about 230 ft (Ruppel et al. 1975).' 667|'Quartzite, white or light gray, vitreous, fine- to medium-grained; partly mottled, with irregular lenses and blebs a few cm long of reddish-brown sandstone cemented by ferrodolomite. Present only in thin, thrust-faulted slices and thins depositionally to 0 ft in the Beaverhead Mountains.' 668|'Quartzite, pale purple to grayish-pink, medium- to coarse-grained and grains well sorted and well rounded, beds 3 ft to 6 ft thick, partly prominently cross-laminated; contains abundant hematite grains. Thickness ranges from 0 ft to about 10,000 ft as a result of later Proterozoic and early Paleozoic erosion.' 669|'Quartzitic sandstone, pale red, medium-­ to coarse-grained, feldspathic to arkosic, locally conglomeratic. Present only near the north border of the map area east of Horse Prairie.' 670|'In this area the group includes (Ruppel 1975; Ruppel and Lopez 1988): Gunsight Formation, Apple Creek Formation, and Big Creek Formation.' 671|'Light brownish-gray to grayish-reddish-purple, thin- to medium-bedded, fine- to medium-grained, feldspathic quartzite; many beds laminated or cross-laminated. Minimum thickness about 6,000 ft.' 672|'Grayish-green, thinly bedded siltite and very fine-grained quartzite; contains streaks and lenses of light-gray ferrodolomite-cemented sandstone. Thickness about 1,000 ft, but present only in thin, fault-bounded slices in the map area.' 673|'Pale greenish-gray to light gray, thickly bedded or massive, fine-grained feldspathic quartzite. Thickness about 10,000 ft.' 674|'Medium gray to medium-dark gray, mostly thin- to medium-bedded, very fine-grained to fine-grained, feldspathic, finely biotitic quartzite and subordinate, interbedded, similarly colored siltite and argillite. Probable thickness more than 20,000 ft.' 675|'Quartzofeldspathic gneiss and schist and related granitic crystalline metamorphic rocks of Late Archean age (M’Gonigle 1993); originally included in the Dillon granite gneiss of Heinrich (1960).' 676|'Basaltic andesite, andesite, latite, rhyodacite, and rhyolite as flows, ash-flow tuffs, and tuff and flow breccias. Locally, unit contains thin interbeds of light olive-gray, biotitic, tuffaceous, fine- to coarse-grained sandstone and conglomerate composed of volcanic detritus. Age ranges from about 48 Ma to 40 Ma (Staatz 1979; Ruppel and Lopez 1988; M''Gonigle and Dalrymple 1996).' 700|'Well-rounded gravel and sand with lesser amounts of silt and clay.' 701|'Includes alluvial terrace deposits, braided stream deposits, alluvial fan deposits, and glacial outwash deposits.' 702|'Well-rounded cobbles, gravel, and sand in deposits with flat-topped surfaces that are 10-30 ft (3-9 m) above the present flood plain.' 703|'Unconsolidated material, typically clay-rich, that has moved down slope. Common northwest of Missoula in Tgc unit, particularly where it is overlain by more permeable Taf unit (Harris, 1997).' 704|'Varved, light-brown clay and silt of Glacial Lake Missoula.' 705|'Poorly sorted deposits of boulders and finer material in high mountain valleys.' 706|'Unconsolidated material consisting of weathered cobbles and finer-grained sediment. Cobbles in the upper Petty Creek area are Belt Supergroup, primarily quartzite, suggesting either a local source or a northerly source. Exposure immediately north of Missoula is described as conglomerate by Harris (1997).' 707|'Locally derived, poorly sorted, angular to rounded boulders, cobbles, gravel, sand, and silt. Probably equivalent to the Sixmile Formation of southwest Montana (Jim Sears, pers. comm., 1997).' 708|'Channel and flood plain deposits of the ancestral Bitterroot and Clark Fork rivers. Includes well-sorted and well-rounded cobbles, gravel, sand, silt, clay, and volcanic ash deposits. Clasts are not locally derived. Marked angular unconformity at top of unit near Missoula is overlain by Taf unit. To the south in the Bitterroot Valley the underlying Tgc beds are not as steeply dipping and this contact is a disconformity. Coarser intervals are permeable, but clay-rich zones are not. Probably equivalent to the Renova Formation of southwest Montana (Jim Sears, pers. comm., 1997).' 709|'Buff to light gray with variable amounts of smoky quartz and potassium feldspar phenocrysts. Entirely glassy (gray vitrophyre) in exposures north of Montana Creek. Large dike west of the Lolo Hot Springs batholith is locally syenitic in composition and, like many of the dikes, follows preexisting faults (Simpson, 1985; Lewis 1992a).' 710|'Pink to buff crystal-rich tuff containing approximately 30% phenocrysts of smoky quartz and sanidine in an aphanitic matrix.' 711|'Gray, porphyritic rocks with plagioclase, biotite, and hornblende phenocrysts in an aphanitic matrix.' 712|'Gray, porphyritic, volcanic tuffs or lava flows northwest of the Lolo Hot Springs batholith (Simpson 1985). Unit includes poorly exposed rhyolite flows(?) or dikes(?) southwest of Florence.' 713|'Light-gray to pale-pink, nonfoliated biotite monzogranite. Largest mass is the Lolo Hot Springs batholith, which is mostly medium grained but which contains some fine-grained zones, particularly near the margins (Leischner 1959; Simpson 1985). Alkali feldspar is strongly perthitic, and miarolitic cavities contain smoky quartz and alkali feldspar crystals. Smaller body in extreme southwest part of map is poorly mapped but is distinguished from the Idaho batholith by K2O>Na2O, <400 ppm Sr, and well-developed perthite. It has been mapped only in the Ranger Peak area on the Idaho side of the state line (Lewis et al. 1992b) and is projected into Montana on the basis of geochemical analyses (samples IG32A, IG33A, and IG37A) of Shuster and Bickford (1985).' 714|'Well-foliated and well-lineated mylonite along the front of the Bitterroot Range. Rocks have a granitic protolith south of Bass Creek and a metasedimentary protolith to the north. Movement is down-to-the-east based on shear-sense indicators (Hyndman et al. 1975; Hyndman and Myers 1988).' 715|'Primarily medium-grained, nonfoliated biotite granodiorite but also includes fine-grained biotite granodiorite, hornblende-biotite granodiorite, tonalite, and quartz diorite, some of which is foliated. These rocks, previously mapped as the Skookum Butte stock (Nold 1968, 1974), almost certainly represent more than one period of intrusion. Granodiorite in the west in the Sally Basin area is characterized by euhedral biotite crystals, interstitial potassium feldspar and quartz, and a relatively narrow zone of contact metamorphism. These features suggest shallow emplacement levels and a probable Eocene age. More nearly equigranular rocks at Skookum Butte are similar in appearance to Cretaceous granodiorite of the Idaho batholith. Rocks in the central part of the unit weather to large boulders that, although they resemble glacial deposits, are probably in situ.' 716|'Fine-grained diorite and gabbro consisting of plagioclase, hornblende, and pyroxene. Typically occurs as thin dikes and sills, but also present as larger masses.' 717|'Foliated and nonfoliated fine- to medium-grained (muscovite-) biotite granodiorite. Includes rocks mapped as granitic sheets by Wehrenberg (1971). Largest mass in the southern part of the area is part of the Idaho batholith, and there it contains zones with megacrystic potassium feldspar (Chase 1973; Shuster and Bickford 1985). The batholith in general is characterized by Na2O>K2O, except where megacrystic, in which case K2O exceeds Na2O. The age of the batholith in this area is uncertain, in part due to inheritance of Precambrian zircon and the possible disturbance of the U-Pb system during Eocene mylonitic deformation (see discussions in Bickford et al. 1981; Shuster and Bickford 1985; and Foster and Fanning 1997). Three U-Pb zircon ages of 69 to 75 Ma (Shuster and Bickford 1985; Toth and Stacey 1992) from along the Lochsa River west of the map area are probably the best age estimate for the Kgd unit. Samples near and in the mylonite zone give Eocene U-Pb zircon ages (Chase et al. 1983; Foster and Fanning 1997).' 718|'Foliated, medium-grained, biotite- and hornblende-biotite tonalite and granodiorite. Sphene fission track ages of 81 and 85 Ma from similar rocks just west of the map area (Ferguson 1975) provide the best age estimates for this unit.' 719|'Described by Hall (1969) as interbedded silty dolomite, dolomitic siltstone, and green shale (Dry Creek Shale Member) overlain by interlaminated limestone and siltstone (Sage Member). The Sage Member is nodular and composed of distinctive gray micritic mottles that weather recessively within a groundmass of darker, more resistant silt and dolomite (Winston 1998). Thickness 300-373 ft (92-114 m).' 720|'Hall (1969) describes unit as cliff-forming, medium- to coarsely-crystalline, gray dolomite. Contains lenses, stringers, and irregular patches of chert. Thickness 1,860-2,000 ft (570-610 m).' 721|'Hall (1969) reports a limestone member, consisting of gray, orange-mottled limestone, local brown calcareous siltstone, and green shale, and a shale and glauconitic sandstone member consisting of interbedded green shale and greenish-gray glauconitic sandstone. Thickness 465 ft (142 m).' 722|'Vitreous quartzite. Not mapped by Hall (1969), who showed all quartzite below the Silver Hill as Pilcher Quartzite. Projected into area based on mapping to the north by Wells (1974), who estimated the thickness to be 40-100 ft (12-31 m). According to Winston (1998) the Flathead in the Missoula area is only 10-20 ft (3-6 m) thick and composed of green, glauconitic quartzose sandstone with tracks and trails.' 723|'Massive to foliated, light-gray, medium-grained anorthosite. Berg (1965) speculated that the anorthosite bodies were residual from a melting event rather than intrusive in origin, in which case a Cretaceous age would be more likely than Proterozoic.' 724|'Coarse- to medium-grained, reddish or buff quartzite containing thin beds of sandy argillite as well as detrital muscovite. Characterized by trough cross-beds with distinctive alternating purple and light gray cross laminae (Winston 1998). Thickness uncertain, but 350-400 ft (110-120 m). Probably remains below the unconformity with the overlying Flathead Quartzite.' 725|'Tan-weathering, grayish-green micaceous quartzite and impure argillite. Thickness was probably on the order of 3,000 ft (920 m) in the area based on estimates from adjoining areas (Hall 1969), but a complete section is no longer present.' 726|'Red and green interbedded argillite and siltite, and buff quartzite. Contains distinctive green and red chert beds and clasts. Mudcracked, rippled and even silt and fine sand to clay couplets are common; less common are horizontally laminated sand beds (Winston 1998). Thickness in the map area is uncertain but probably on the order of 2,000-3,000 ft (610-915 m).' 727|'Pink or buff, medium- and coarse-grained feldspathic quartzite. Matrix-supported granules and pebbles present locally. Abundant trough cross-beds. Contains 15 to 25% potassium feldspar, but only trace amounts of plagioclase. Thickness in the map area is uncertain, but probably on the order of 1,000-2,000 ft (300-610 m).' 728|'Quartzite and subordinate argillite and siltite in lower part of section. Upper part is predominantly argillite and siltite. Typical reddish color is bleached out to light gray in the Fish Creek drainage, probably as a result of metamorphism and (or) hydrothermal alteration. Exposures do not permit subdividing the unit in this area.' 729|'Reddish argillite and siltite with lesser amounts of green siltite and argillite and thin-bedded quartzite. Thin dolomitic zone in uppermost part of section. Contains salt casts that are otherwise rare in the Belt Supergroup. Thickness is uncertain but probably on the order of 2,000-2,500 ft (610-760 m).' 730|'Reddish very fine- to medium-grained quartzite and lesser amounts of argillite and siltite. Thin layers of dolomitic rock present locally. Abundant planar laminations and ripple cross laminations. Upper 200 ft (61 m) of section are the coarsest, and can be difficult to distinguish from the Bonner Quartzite. Sampling in the northern part of the area indicates subequal amounts of plagioclase and potassium feldspar (25-35% total) in contrast to the plagioclase-poor Bonner Quartzite. Hall (1969) measured a section of member 2 on Petty Mountain, which at the time was thought to be Bonner Quartzite. A total thickness of 900 ft (275 m) was determined. Mount Shields member 1 was not recognized in the Missoula West quadrangle, but is present to the east in the northern part of the Butte 1 x 2 degree quadrangle (Slover and Winston 1986).' 731|'Green microlaminated argillite, and thin lenticular beds of green dolomitic siltite and fine-grained quartzite. Dolomitic beds have characteristic orange-brown weathering rinds. Uppermost part is a distinctive pinkish-red, thinly bedded dolomitic quartzite and siltite. Abundant load casts and ripple marks. Poorly exposed and easily missed when mapping. Unit has formed calc-silicate rocks next to the Lolo Hot Springs batholith and contains scapolite in the Kid Lake area. Included in the uppermost part of the Miller Peak Formation by previous mappers in the Missoula area (Nelson and Dobell 1961). Section of the Miller Peak Formation measured by Hall (1969) on Petty Mountain included approximately 850 ft (260 m) of the Shepard Formation, but this part of section was poorly exposed. Unit appears thicker west of Petty Creek.' 732|'Green and red argillite and siltite in the lower and middle parts of the section. Uppermost 200 ft (61 m) of section is reddish quartzite that is difficult to distinguish from member 2 of the Mount Shields Formation. Abundant straight-sided mud cracks in lower part of section. Hall (1969) measured a section on Petty Mountain that was assigned at that time to the lower part of the Miller Peak Formation. The Snowslip in that section is 2,975 ft (910 m) thick.' 733|'Primarily carbonate-bearing rocks of the middle member of the Wallace Formation but may include green siltite and argillite of the lower member along the lower part of Lolo Creek.' 734|'Green, diopside- and hornblende-rich rocks that are interlayered with thin quartzitic beds forming a calc-silicate gneiss (Nold 1968). Unit results from metamorphism of the middle member of the Wallace Formation (Ywm). More resistant to weathering than Ywm.' 735|'Dolomitic and calcareous siltite and quartzite, and black argillite. Minor amounts of green siltite and argillite. Quartzite beds show evidence of scour, along with hummocky cross stratification. Some quartzite is carbonate free. Tan-weathering dolomitic siltite with black argillite caps ("black and tan" rock type) is common. Unit is scapolite-bearing in the extreme western part of the area. Presence of black argillite and abundant calcite distinguishes these rocks from the Shepard Formation. Vertical fractures in quartzite are common, as are "birdsfoot" cracks on argillite surfaces. Orange-weathering dolomite-rich sedimentary breccia is present within the middle member. Clasts are predominantly white quartzite and range in size from less than a centimeter to several meters. Thickness of middle member highly uncertain, but probably on the order of 2,500-4,500 ft (760-1,370 m). No upper member of the Wallace Formation was mapped in this area, unlike in the Coeur d’Alene Mining District to the west. The upper member there is probably a lateral equivalent to the Snowslip and Shepard formations mapped in this study.' 736|'Green siltite and light green argillite predominate, typically with carbonate pods, but includes dolomitic siltite and quartzite. Bedding typically cyclical, with thin carbonate beds at the top of each cycle. Lower and middle parts are thinly bedded and the least quartzitic, and are probably lateral equivalents of the Empire Formation mapped just north of the area by Wells (1974). Thickness highly uncertain, but probably on the order of 3,000-5,000 ft (900-1500 m).' 737|'Highly recrystallized quartzite and lesser amounts of phyllite and schist. These rocks are along strike with rocks of the Mount Shields Formation and were considered metamorphosed equivalents of the Mount Shields by C. Wallace and Lidke (1991). Previous workers assigned them to the Ravalli Group (Wehrenberg 1971; Winston 1991). Because these rocks are at structurally low levels, they are probably Ravalli Group equivalent.' 738|'Gray-weathering, well-layered, quartz-rich feldspathic gneiss. Unit contains areas of unmapped granitic sills and dikes. Quartz content averages 51% and biotite 13% in the southwest part of the area (Chase 1973), where the rock clearly had a sedimentary protolith. Williams (1976) reports a lower quartz content (average 32%) for the Bass Lake area, and there the unit may include unmapped granitic rock. Includes the gray-weathering gneiss units of Anderson (1959) and Wehrenberg (1971). Chase (1977) correlates these rocks with the Ravalli Group of the Belt Supergroup, but previous assignments to the Prichard Formation (Anderson 1959; Wehrenberg 1971; Chase 1973) are more likely given the overall lack of quartzite in the unit.' 739|'Brown-weathering biotite- and muscovite-biotite schist, biotite-sillimanite gneiss, muscovite-biotite quartzite, and minor calc-silicate rocks. Quartz content averages 38%, biotite 24%, and sillimanite 12% in the southern part of the area (Anderson 1959; Chase 1973). Typically has feldspar augen, and contains an abundance of unmapped granitic sills and dikes, as well as minor amounts of garnet amphibolite (metamorphosed mafic sills). Increasing amounts of gneiss and igneous material southward. Calc-silicate rocks are present along Kootenai Creek one mile west of the range front. Includes rocks mapped as the brown-weathering gneiss of Wehrenberg (1971), pelitic schist of Williams (1976), and sillimanite gneiss of Anderson (1959) and Berg (1965). Previous workers have correlated these rocks to the Prichard Formation of the Belt Supergroup, at present considered the most likely protolith.' 925|'Gravel, sand, and silt deposited in modern channel or on present flood plain (tf).' 926|'Unsorted landslide debris (tf)' 927|'Gravel and sand, typically at or near present alluvial channel level (tf).' 929|'Quaternary-Tertiary sediments, undifferentiated.' 930|'Gravel and sand deposits, primarily above present alluvial channel levels (tf).' 931|'Upper greenstone facies; lower unit includes calcareous shale, siliceous limestone, argillite, and phyllite.' 932|'Light to medium gray and black massive and thin units of limestone, marble, phyllite, and sandstone.' 933|'Light gray to black, coarse marble and limestone, some quartzitic sandstone, argillite, phyllite, and chloritic schist, with basal conglomerate.' 934| 935|'Occasional phenocrysts of plagioclase and rare olivine.' 936|'Aphyric, dense, fine-grained basalt.' 937|'Phyric, coarse-grained basalt with large plagioclase and olivine phenocrysts.' 938|'Dark, fine-grained vesicular basalt.' 939|'Dikes of granitic composition (tf).' 940|'Light pink to gray rhyolite and dark purple to black volcanic breccia; contains xenoliths of older metamorphic and igneous rocks.' 941|'"Bovill" syenite (may be related to Potato Hill Volcanics or Gold Hill syenite).' 942|'Gray to reddish vesicular andesites.' 943|'Light gray quartz latite; contains widely spaced glassy bands.' 944|'Gray, medium- to coarse-grained rock with biotite and hornblende.' 945|'Gray to reddish, fine- to coarse-grained rock.' 946|'Coarse-grained rock with hornblende and biotite.' 947|'Medium-grained foliated rock containing biotite.' 948|'Coarse-grained plagioclase-hornblende rock.' 949| 950|'Moderately weathered hornblende syenite.' 951|'Biotite-hornblende-plagioclase quartz dioritic orthogneiss.' 952|'Coarse- to medium-grained hornblende-plagioclase-epidote rock.' 953|'Green to grayish green metamorphosed complex of lava flows, pyroclastics, and calcareous and noncalcareous sedimentary rocks.' 954|'Dark, well-foliated, medium-grained plagioclase-hornblende rocks.' 955|'Medium gray to olive-colored, thin-bedded siltite (which produces a soil similar to loess).' 956|'Vitreous quartzite which includes feldspathic quartzite with micaceous sheen.' 957|'Coarse-grained, garnet-mica schist.' 958|'Diopside gneiss, biotite gneiss, and biotite quartzite interbedded with schist.' 959|'Libby Formation through Prichard Formation, undifferentiated.' 960|'Medium- to coarse-grained garnet-mica schist.' 961|'Thick-bedded, coarse- to medium-grained pure quartzite with thin micaceous laminae.' 962|'Gray, coarse- to medium-grained garnet-mica schist.' 963|'Micaceous, medium-grained granular quartzite interbedded with schist.' 964|'Coarse-grained, garnet-mica schist; diopside gneiss; biotite gneiss; and biotite quartzite interbedded with schist..' 971| 975|'Stream deposits primarily of silt, sand, and gravel in floodplains, terraces, and valley bottoms. Locally the unit includes lacustrine, paludal, and eolian deposits in depressions.' 976|'Brown to yellow-brown fibrous peat, sedimentary peat, and muck present in flatlands near lakes. Peat is locally mixed with sand and silt and mixed or interbedded with one or two medium to thin beds of volcanic ash. The peat was mapped by Rigg (1958) at the southern end of Liberty Lake, the northwestern edge of Newman Lake, in Saltese Flat, the south end of Bailey Lake, and around Little Trout Lake. Only larger deposits are shown on Plate 1.' 977|'Active and stabilized dunes of predominantly fine to medium sand; mostly composed of quartz and basalt grains reworked from older sedimentary rocks (Kiver and others, 1979).' 978|'Unstratified and poorly sorted clay, silt, sand, and gravel deposited by rotational and translational slides and flows (Kiver and others, 1979). Slides are generally restricted to the sides of the "prairies and bluffs" and are as much as several kilometers in length, and some are more than 1.5 km across. Some slides contain individual blocks of basalt that are nearly 8 m in diameter. Well logs show that the Latah Formation generally underlies the slides. Most slides are covered with or partially buried by Pleistocene flood deposits (Qfg) and appear to be late Pleistocene in age. A few slides, such as along Deep Creek, have been active in historic time (Griggs, 1976). ' 979|'Light- to medium-brown, unstratified, eolian silt, clay, sand, and ash. The unit contains 5 percent sand, 60 percent silt, and 35 percent clay. Clay is mostly montmorillonite and illite in a ratio of 3:1; it also contains minor kaolinite. The sand and silt fraction is composed of angular quartz grains and lesser amounts of feldspar and muscovite. Heavy minerals include limonite, monazite, tourmaline, hornblende, garnet, epidote, and apatite. The loess mantles the Columbia River basalt and is most common on the tops of low hills and plateaus where erosion by water has been minimal. The loess is as thick as 23 m in Spokane County; it averages 6 m thick south of the Spokane River and thins to the north where there is more topographic relief (Hosterman, 1969). Evidence for several episodes of loess deposition comes from exposures in the Washtucna area in the Connell 1:100,000-scale quadrangle. Near Washtucna, buried paleosols with negative magnetic polarity are interstratified with loess that is interbedded with flood gravel. The polarity indicates the loess there was deposited prior to 790 ka (McDonald and Busacca, 1988). Near Washtucna, the presence of younger loess is indicated by an interbed of set C tephra from Mount St. Helens dated at 36 ka (McDonald and Busacca, 1988) and loess blanketed by 7 ka Mount Mazama tephra (Foley, 1982).' 980|'Light-gray, friable, poorly bedded, fine sand, silt, and clay deposited in pre-late Wisconsin Lake Spokane (Kiver and others, 1989; Weissenborn and Weis, 1976; Weis, 1968). This unit, as mapped, includes some Holocene deposits along Liberty Creek and Saltese Flat (Weis, 1968).' 981|'Till, outwash, and ice-contact stratified deposits in moraines, till plains, and meltwater channels and terraces deposited during the last glacial advance. The unit is generally restricted to the Chamokane Creek valley (Kiver and others, 1979).' 982|'Poorly sorted, stratified mixture of boulders, cobbles, gravel, and sand resulting from multiple episodes of catastrophic outbursts from glacier-dammed lakes, such as glacial Lake Missoula. The Spokane Valley was one of the main channelways for outburst flood waters from glacial Lake Missoula which inundated much of the present Clark Fork River drainage in Montana and Idaho. Floods flowed from the Spokane Valley through the Cheney Palouse Scabland tract to the Columbia Basin. Flood waters also traveled north of Mount Spokane through the Hillyard trough to the Little Spokane and Spokane Rivers (Molenaar, 1988). The upper elevation of effective erosion by flood water in the Spokane Valley was more than 800 m near Dishman (Kiver and Stradling, 1989). Radiocarbon dating and paleomagnetic measurements indicate that flood deposits are late Wisconsin and pre-late Wisconsin in age. Examination of graded couplets in flood deposits in the Spokane quadrangle and elsewhere led Waitt (1985) to the interpretation that in excess of 40 flood episodes took place during the late Wisconsin. Waitt (1985) has estimated that glacial Lake Missoula existed for 2,000 to 2,500 years, between 15,300 and 12,700 yr B.P. A giant ripple field near Airway Heights contains Mount St. Helens ash in the troughs of dunes overlain by flood gravel, indicating that one or more floods occurred at about 13,000 ka (Kiver and others, 1989). Tephra correlation and accelerated radiocarbon dating near Washtucna show that some of the pre-late Wisconsin floods occurred at 40 ka (McDonald and Busacca, 1988, 1989). On the basis of paleomagnetic measurements on interbedded loess, McDonald and Busacca (1989) suggest that more than five episodes of flooding occurred prior to 790 ka in the Cheney Palouse Scabland tract. In the Acme gravel pit in the Spokane Valley, flood deposits consist of cobble- to boulder-size (to 3 m long) clasts; large-scale, west-dipping foreset beds are 5 to 6 m thick and are overlain by a lag deposit of horizontally bedded sand and gravel (Kiver and Stradling, 1989). In a pit near Barker Road in the Spokane Valley, rhythmic bedding from lower energy flood waters in a relatively protected site are exposed. Each set of rhythmites consists of coarse, poorly bedded, pebbly sand disconformably overlain by well-bedded, fine sand. Pebbles and rip-up clasts of Pleistocene lake deposits and Miocene Latah Formation are present near the base of each coarse sand unit (Kiver and Stradling, 1989).' 983|'Glaciolacustrine deposits containing silt and clay couplets interbedded with coarser clastic deposits of catastrophic floods. The deposits are generally restricted to Latah Creek and the lower Spokane River. Along Latah Creek, 1.5- to 5-m-thick units of cyclic, upward-fining and -thinning sequences containing coarse clastic sediments are interbedded with 2-cm- to 20-cm-thick beds of varve-like, fine-laminated clay and silt couplets. The sand- to granule-size fragments in the coarse-grained material are chiefly composed of metamorphic rocks and lesser amounts of basalt, plutonic rocks, and quartz grains; pebble- to boulder-size clasts (to 106 cm in diameter) are similar in composition to the sand- and granule-size fragments and include clasts of semiconsolidated material, some of which is recognizable as Latah Formation. Unconformably overlying and incised into the cyclic sediments is a 7- to 9-m-thick, cut-and-fill channel filled with coarse gravel. Generally above the gravel are cyclic, upward-fining and -thinning, rhythmically bedded, buff-colored, sand and silt-rich units which are similar to the Touchet beds in southern Washington. The sequence is overlain by Mazama ash that is radiocarbon dated at 6,700 ka (Rigby, 1982). Rigby (1982) concluded that the sediments below the unconformity are turbidites generated by catastrophic floods from glacial Lake Missoula and deposited into a lake that was present in the Latah Creek area from 37 ka to 18 ka. Sediments above the unconformity are thought by Rigby (1982) to be slackwater deposits derived from pulses of catastrophic floodwaters surging upstream from the Little Spokane River between approximately 17 ka and 13 ka.' 984|'Well-indurated, manganese- and iron-cemented conglomerate that is present 14 km north of Wellpinit. The conglomerate overlies horizontal beds of arkosic sand that also contain manganese and iron cements. The conglomerate contains as much as 0.023 percent U2O3 in the matrix (Ingersoll and others, 1980). Directly to the north in the Chewelah 1:100,000-scale quadrangle, the unit overlies the rocks of the Columbia River Basalt Group (F. K. Miller, USGS, oral commun., 1988 in Waggoner, 1990).' 985|'Fine- to coarse-grained, aphyric basalt flows of reversed magnetic polarity (Swanson and others, 1979). They generally contain fine phenocrysts of plagioclase and olivine. The basalt is of the Rosalia chemical type and has higher titanium and lower magnesium and chromium contents than other flows of the Wanapum Basalt (Wright and others, 1989). The unit overlies and is commonly invasive into the lakebed sediments of the Latah Formation or overlies the Grand Ronde Basalt N2 unit. Flows of the Priest Rapids Member are thought to have been extruded between 15.3 and 14.5 Ma (Reidel and Fecht, 1987). This unit includes the "Rim rock" flows of Pardee and Bryan (1926).' 986|'Upper flows of the Grande Ronde Basalt, of normal magnetic polarity. The basalt is aphyric to sparsely phyric and contains minor amounts of small plagioclase laths. The flows, which are commonly pillowed, are as much as 50 m thick, but are generally 15 to 25 m thick. Flows of the Grande Ronde Basalt are thought to have been extruded between 15.6 and 16.5 Ma (Reidel and Fecht, 1987), and they overlie pre-Tertiary rock in the Spokane quadrangle. The Grande Ronde Basalt makes up the "Valley flows" of Pardee and Bryan (1926). The base of the basalt reportedly was penetrated at about 130 m below the valley floor in the well at the Davenport Hotel in downtown Spokane (Griggs, 1976).' 987|'Gray to tan to yellow-orange siltstone, claystone, and minor sandstone named by Pardee and Bryan (1926) for lacustrine and fluvial deposits along Latah Creek. Kirkham and Johnson (1929) suggested extending the formation name west to Grand Coulee; however, Griggs (1976) has suggested that the name Latah be restricted to the drainage of the Spokane River, as originally intended by Pardee and Bryan (1926). Pardee and Bryan (1926) included in the Latah Formation both the sediments interbedded with the Miocene Columbia River Basalt Group and the 305 meters of sediments below the base of the basalt in the Latah-Texas well near the mouth of Latah Creek. They also include the 73 meters of sediment below the lowest basalt in the well at the Davenport Hotel (Pardee and Bryan, 1926). A large part of the exposure of the Latah Formation is located between flows of the Grande Ronde Basalt and the Priest Rapids Member. Floral assemblages in the Latah Formation indicate a Miocene age (Knowlton, 1926). More than 95 forms make up the floras in the Spokane and Coeur d''Alene areas. These include well-preserved species of Ginkgo, Sequoia, Taxodium, Populus, Castanea, Quercus, Ulmus, and Acer.' 988|'Medium- to thick-bedded, fine-grained, muscovite-bearing quartzite to silty quartzite that contains minor chlorite, and thin-bedded, light- to dark-gray, quartz-rich phyllite and muscovite schist that also includes minor biotite. These rocks are contiguous with the unit of metasedimentary rocks of uncertain age mapped by Becraft and Weis (1963) to the west on the Turtle Lake 15'' quadrangle (Coulee Dam 1:100,000-scale quadrangle). The rocks mapped by Becraft and Weis (1963) appear to be thicker than those generally exposed in the lower Paleozoic sequence in this part of northeastern Washington, suggesting a Precambrian age. However, in the Coulee Dam 1:100,000-scale quadrangle, the unit contains a thick section of calcite-bearing marbles; carbonate rock in the Precambrian section is generally dolomitic. Because of the uncertainty in the age of the rock and the lithologies present, this unit is probably Cambrian or Middle Proterozoic (Precambrian Y) in age.' 989|'Medium- to thick-bedded, white to light-gray, fine-grained, vitreous quartzite that contains authigenic feldspar. The quartzite is interbedded with medium- to thin-bedded buff and light-green siltite. This poorly exposed unit is most probably Precambrian Y in age; however, it could be part of the Cambrian-Precambrian Z Addy Quartzite.' 990|'White to light-gray, thick-bedded quartzite, pebbly quartzite, and medium- to dark-gray, thin-bedded siltite. Pebble-bearing quartzite is present on Fancher Butte and Booth Hill. The pebbles are 3- to 6-mm-long, white and blue, opaque, rounded to subrounded quartz clasts that define bedding in the quartzite. Larger pebbles, as much as 25 mm long, are composed of purplish and pinkish quartzite and constitute as much as 50 percent of the pebbly quartzite. On the western hill of Fancher Butte medium- to dark-gray, thin-bedded, thin- to very thin laminated siltite and minor quartzite are present as float. Quartzite on Fancher Butte and Booth Hill could be the Addy Quartzite, but it is most likely part of the Buffalo Hump Formation. The thickness of this unit in the Spokane quadrangle can not be accurately estimated because of poor exposure and probable structural complexities.' 991|'White to light-gray, medium- to thick-bedded, medium- to coarse-grained, grain-supported quartzite interbedded with medium- to thin-bedded muscovite-biotite schist. The unit is exposed as isolated outcrops on ridge tops and in small drainages in the northwest part of the map area. This quartzite locally contains minor disseminated pyrite, muscovite, and chlorite. The unit is most likely part of the Buffalo Hump or Togo Formations of the Deer Trail Group, but it could be part of the Cambrian-Precambrian Z Addy Quartzite. Outcrops of the quartzite are too scattered to permit accurate thickness estimates.' 992|'Medium- to dark-gray argillite interlaminated with tan, light-gray and white siltite to quartzite laminae that crops out on Magnison Butte and Hanning Butte. Bedding thickness ranges from laminations on the sub-millimeter scale to fining-upward couplets as thick as 30 mm. Even parallel laminations and soft-sediment deformation are common. The McHale Slate in the Spokane quadrangle appears to be similar to the lower 100 to 120 m of the formation as described by Miller and Whipple (1989) in the Chewelah 1:100,000-scale quadrangle. The unit exhibits weak cleavage.' 993|'Calc-silicate, hornfels, calcite- and epidote-cemented quartzite, gray fissile argillite, phyllite, mica schist, and quartz-plagioclase-biotite gneiss that is present near Medical Lake. Original lithologies were probably similar to those of rocks mapped as the lower Wallace Formation by Griggs (1973) in the Rosalia 1:100,000-scale quadrangle. There, the upper part of the unit consists of couplets of black argillite and light-gray to greenish-gray siltite, and medium- to thick-bedded carbonate-bearing quartzite.' 994|'Quartzite, silty quartzite, schist, and siltite present south and west of Spokane (Griggs, 1973). The unit is dominantly vitreous, medium- to coarse-grained, white to light-gray, medium- to thick-bedded quartzite and interbeds of quartz-biotite-muscovite schist. The quartzite contains as much as 10 percent authigenic feldspar. The unit is generally thermally upgraded south of Spokane and near Cheney; greenschist-facies rocks are present immediately west of State Route 195. The contact relation with older gneiss is undefined. On Krell Hill, the contact of the heterogeneous metamorphic rocks (PChm) with the Ravalli Group is obliterated by the intrusion of a body of alaskite (TKiaa). This contact, however, is likely a fault, as rocks of different metamorphic grade and probable age are in juxtaposition. Outcrops of the quartzite abruptly terminate at the drainage south of Browns Mountain, suggesting a possible structural contact with the higher grade rocks there. The maximum thickness of the unit south of Spokane is 1,200 m.' 995|'Light- to dark-gray hypabyssal, porphyritic dikes generally in the northern and western part of the map area. The fine-grained matrix is composed of feldspar, quartz, hornblende, and biotite. Phenocrysts include hornblende (as much as 0.5 cm long), subhedral biotite, plagioclase (as much as 1 cm long), and minor K-feldspar (Miller, 1974c). A dike that has a similar mineralogy on the Chewelah 1:100,000-scale quadrangle northeast of Elk yielded a K-Ar age of 47.3±1.6 Ma on hornblende and 46.8±1.4 Ma on biotite (Miller, 1974c).' 996|'Lineated (nearly east-west lineation) and foliated, medium- to coarse-grained, biotite-hornblende monzodiorite to granodiorite. The unit is present west of Tumtum and near Medical Lake. West of Tumtum the unit is characterized by: aligned, euhedral, black hornblende; pink and white poikilitic K-feldspar phenocrysts aligned subparallel to the hornblende; and abundant wispy mafic xenoliths. The biotite to hornblende ratio is 1:2; hornblende commonly has pyroxene cores. The rock contains 1 to 2 percent titanite. Near Medical Lake the unit includes leucocratic, medium-grained, equigranular to porphyritic, foliated and lineated diorite to granodiorite that has macroscopic mineralogy and fabric similar to the rock near Tumtum. The rock at Medical Lake contains subhedral quartz that is interstitial to feldspar, anhedral plagioclase, white euhedral to subhedral K-feldspar, subhedral black biotite which is associated with pyrite, and hornblende as euhedral phenocrysts and/or in clots with pyroxene cores. The rock contains accessory magnetite and abundant titanite. Schlieren is locally present. The unit near Tumtum is thought by F. K. Miller (USGS, oral commun., 1989) to be similar to the border phase of the Silver Point Quartz Monzonite in the Chewelah 1:100,000-scale quadrangle. North of the Spokane River, the unit is cut by brittlely deformed alaskite pegmatite dikes which cut across the fabric in the monzodiorite and cement a breccia that contains fragments of a deformed monzodiorite that is well exposed in NE1/4 sec. 16, T. 27 N., R. 40 E. The contact of the intrusive rock near Tumtum with the Silver Point Quartz Monzonite is generally gradational. South of the Spokane River, the unit is probably in fault contact with the biotite-bearing intrusive rock near Four Mound Prairie.' 997|'Leucocratic (color index 14-21), coarse-grained, porphyritic hornblende-biotite quartz monzonite that is present on Scoop Mountain. The formation was named by Miller and Clark (1975) for exposures near Silver Point on Loon Lake in the Chewelah 1:100,000-scale quadrangle. The groundmass has a characteristic bimodal texture and consists of K-feldspar, quartz, hornblende, biotite, and plagioclase; K-feldspar and quartz are coarser grained than the other constituents in the matrix. Biotite is more common in the fine-grained phase of the groundmass than in the coarse-grained part. Microperthitic orthoclase phenocrysts are 6 to 37 mm in length and make up 1 to 5 percent of the rock. Quartz is interstitial to other minerals. Hornblende is subhedral to euhedral, has a long dimension as much as 10 mm, and is associated with biotite. The hornblende to biotite ratio is consistently .75:1. Titanite is the most common accessory mineral, followed by magnetite, apatite, zircon, and rare allanite (Miller and Clark, 1975). The pluton is generally massive, but it is foliated in Rail Canyon parallel to the contact with the biotite-bearing intrusive rock in Corkscrew Canyon (TKiac) and foliated near the contact with the biotite-hornblende-bearing intrusive rock near Tumtum. The Silver Point unit is cut by north-northwest-trending, biotite-chlorite-silica-bearing zones that display cataclasis (Miller and Clark, 1975). Two samples from the localities in the Chewelah 1:100,000-scale quadrangle returned whole-rock Rb-Sr ages of 39±4 Ma and 46±2 Ma (Armstrong and others, 1987). Recalculated K-Ar ages on rocks from the Chewelah 1:100,000-scale quadrangle are 51 Ma on biotite and 62 Ma on hornblende (Miller and Clark, 1975). The initial 87Sr/86Sr ratio of 0.7063±0.0002 from the pluton was the lowest found by Armstrong and others (1987) after studying several plutons in the area. Armstrong and others (1987) suggest that the low initial isotope ratios confirm that the pluton has an Eocene age and not a reset Cretaceous age.' 998|'Massive to very weakly foliated, leucocratic, fine- to medium-grained, equigranular, muscovite-biotite quartz monzonite to granite that forms small plutons and dikes that sharply cut the foliation of the older mylonitic rocks. The unit was named by Rhodes and Hyndman (1984) for exposures on Mount Rathdrum in Idaho and is present in the Spokane quadrangle near Hauser and Newman Lakes. The granite contains quartz, K-feldspar, plagioclase containing myrmekitic intergrowths, biotite, and minor muscovite (Weissenborn and Weis, 1976). The bodies are intruded along the axis of the south-plunging Spokane dome of the Priest River complex. Larger bodies of this intrusive unit are generally massive, whereas dikes commonly have structural fabric. Dikes of Mount Rathdrum granite commonly dip 10o to 15o more steeply to the west than mylonitic foliation of the enclosing metamorphic rock. Partings in the dikes display moderate to strong slickenside-like lineation parallel to the N. 70o E. lineation in the mylonitic rocks of the Hauser Lake Gneiss (Rhodes and Hyndman, 1984). Preliminary discordant U-Pb geochronology from undeformed samples suggest a minimum early Eocene (52 Ma) age for the pluton (Bickford and others, 1985). Rhodes and Hyndman (1984) suggest that the granite is a late synkinematic rock intruded during mylonitic deformation associated with the Priest River complex, and thus it experienced only a small increment of the total strain. Dikes and sills which cooled more rapidly exhibit more deformation and likely cooled before the last movement along the mylonite zone (Rhodes and others, 1989).' 999|'Discontinuous sill- and dike-like bodies of alaskite, pegmatite, aplite, and fine- to coarse-grained quartz monzonite. This unit consists of quartz, K-feldspar, plagioclase, muscovite, and minor red subhedral garnet; it locally contains minor biotite. Leucocratic, white to tan alaskite intrudes and is gradational with the biotite-muscovite-bearing Mount Spokane granite. This alaskite consists of medium- to coarse-grained quartz, megacrystic K-feldspar, plagioclase, and muscovite; biotite is absent except near intrusive contacts. Accessory minerals include garnet, apatite, zircon, and monazite. Pegmatite and graphic intergrowths are common (Weissenborn and Weis, 1976; Weis, 1968). As mapped, the alaskite unit locally includes outcrops of the Mount Spokane granite. Autunite and meta-autunite are present as fracture coatings and solid masses of crystals on alaskite on the west side of Mount Kit Carson. A total of 90,000 lb of U3O8 were produced from nine properties; most of the ore came from the Daybreak mine (Weissenborn and Weis, 1976). The unit includes alaskite, pegmatite, and quartz monzonite that intrudes the heterogeneous metamorphic rocks (PChm) and metasedimentary rocks of the Ravalli Group (Ymsr) on Krell Ridge. Alaskite pegmatite with K-feldspar megacrysts (to 15 cm in length) generally has no fabric on Krell Hill; however, biotite, where present, is commonly clotted. The alaskite is intruded parallel to subparallel to the foliation in quartzite and gneiss of the heterogeneous metamorphic rocks, with local stoping. On Krell Ridge, the body forms small resistant knobs and is present along the contact, a probable fault, between the high-grade metamorphic rocks and the metasedimentary rocks of the Ravalli Group. Brittlely deformed alaskite cements breccia and stopes intrusive rocks on the east side of Chamokane Creek from the north border of the map area to near the Spokane River. In NE1/4 sec. 16, T. 27 N., R. 40 E., at the petroglyph site, the brittlely deformed alaskite is well exposed, cementing a breccia of and assimilating the foliated and lineated orthogneiss near Tumtum, which is of probable Eocene age. Thin (0.1-1 m), light-colored bands of mylonite that cut the Newman Lake Gneiss near the contact with the Mount Spokane granite on Beacon Hill probably had an alaskite parent. Rb-Sr ages of alaskite from the Mount Spokane area range from 40 to 159 Ma (Armstrong and others, 1987; Table 1).' 1000|'Scattered bodies of undeformed, leucocratic, coarse-grained, equigranular to porphyritic, biotite granite to granodiorite at several localities in the Spokane quadrangle. Some of these bodies are proximal to deformed plutons but are not themselves deformed, suggesting that they may be Eocene in age or not in the zone affected by mylonitization. Near Griffith Springs, south of the Little Spokane River, the unit includes equigranular biotite granite to granodiorite. This body contains 20 to 30 percent white to lavender, anhedral quartz; 10 to 15 percent subhedral, interstitial black biotite; white K-feldspar; and plagioclase. The intrusive rock is cut by garnet-bearing alaskite dikes, which are, in turn, cut by near-vertical lamprophyre dikes. Minor chlorite and epidote are present on fractures. Near Milan, leucocratic, equigranular, coarse-grained, biotite quartz monzonite to granite is present and is cut by alaskite pegmatite dikes. The unit also includes coarse-grained, highly weathered, biotite granite containing white K-feldspar phenocrysts that crops out in the valley bottoms near Hangman Valley.' 1001|'Coarse-grained, equigranular to porphyritic, biotite quartz monzonite to granite east of Chamokane Creek. Biotite is anhedral to clotted; lavender-gray quartz is anhedral; and K-feldspar phenocrysts are poikilitic. The weathered intrusive rock is cut by brittlely deformed alaskite, pegmatite, and aplite dikes, which also locally cement breccia made up of blocks of the unit. The biotite-bearing intrusive rock is foliated along the contact with the Silver Point Quartz Monzonite and the orthogneiss near Tumtum. These rocks are possibly part of the body of leucocratic intrusive rock exposed south of the Spokane River (TKiaf).' 1002|'Leucocratic, coarse-grained, equigranular, biotite quartz monzonite to granite with trace to 1 percent isolated flakes of muscovite. Quartz is present as light-gray, rounded grains in coarse-grained aggregates and pods. The intrusive rock contains coarse grains and phenocrysts of white, twinned K-feldspar, 3 to 8 percent subhedral black biotite, and 1 percent magnetite as isolated grains. The unit is megascopically similar to the alaskite, pegmatite, and aplite unit, except that the intrusive rock near Mud Creek has relatively increased biotite and decreased muscovite contents. It is possibly a late-stage phase of the muscovite-biotite-bearing intrusive rock near the Little Spokane River.' 1003|'Massive to weakly foliated, leucocratic, medium- to coarse-grained, equigranular, biotite granite to quartz monzonite present south of the Spokane River on Four Mound Prairie. As mapped, the unit probably includes more than one body, and it is possibly part of the biotite-bearing intrusive rock in Corkscrew Canyon (TKiac) on the north side of the Spokane River. Light-gray quartz is present in clots or aggregates of crystals and is intergrown with K-feldspar. White K-feldspar forms some phenocrysts and clots of phenocrysts. Anhedral to subhedral, black biotite comprises 2 to 7 percent of the rock and generally is interstitial to other minerals. The unit is cut by alaskite pegmatite dikes. Outcrops east of Davis Lake (secs. 5 and 6, T. 26 N., R. 40 E.) are foliated and lineated and contain clotted and stretched quartz grains.' 1004|'Leucocratic, medium- to coarse-grained, locally porphyritic quartz monzonite to granite that is present near Round Mountain. Similar rock was named by Becraft and Weis (1963) for exposures in the Coulee Dam 1:100,000-scale quadrangle to the west. The intrusive rock contains 26 to 32 percent K-feldspar, 20 to 42 percent quartz, 26 to 32 percent plagioclase, 1 to 4 percent muscovite, and 0 to 3 percent biotite that is partially replaced by chlorite and muscovite. K-feldspar, followed by quartz and plagioclase, are the dominant phenocrysts. Quartz is anhedral and has a smoky color; subhedral to euhedral plagioclase is twinned. Muscovite is present as inclusions within K-feldspar or as an alteration product of biotite and is associated with chlorite (Kinart, 1980). Accessory minerals include magnetite (to 1%), apatite, fluorite, garnet, zircon, and monazite. Alaskite and quartz porphyry comprise more than 10 percent of the pluton (Castor and others, 1982). The average U3O8 content of this intrusive body is 19 ppm; the range is from 1 to 46 ppm. Strong argillic alteration is locally present (Castor and others, 1982). A U-Pb age of 75 Ma from zircon is reported from the Coulee Dam 1:100,000-scale quadrangle by Ludwig and others (1981). Asmerom and others (1988) reported a Rb-Sr age of 74.7±3 Ma from samples taken in the Coulee Dam 1:100,000-scale quadrangle.' 1005|'Leucocratic, equigranular to porphyritic, medium- to coarse-grained granodiorite to monzogranite that crops out west of the Little Spokane River. The granodiorite was named by Miller (1974c) for exposures near Fan Lake in the Chewelah 1:100,000-scale quadrangle. The granodiorite contains zoned plagioclase of an intermediate composition, quartz, K-feldspar, biotite, and hornblende. Euhedral hornblende phenocrysts are locally as much as 1 cm in length. Accessory minerals include titanite, allanite, zircon, apatite, and opaque minerals. Quartz and K-feldspar are interstitial to plagioclase and mafic minerals. K-Ar ages are 95.1±4 Ma on hornblende and 93.4±2.6 Ma on biotite from the Fan Lake granodiorite in the Chewelah 1:100,000-scale quadrangle (Miller, 1974c).' 1006|'Leucocratic, massive, medium-grained, biotite-hornblende granodiorite that crops out south of Wellpinit. The granodiorite contains 25 to 30 percent quartz; 35 to 40 percent subhedral, zoned plagioclase (An40-55); 10 to 15 percent interstitial K-feldspar; 7 to 15 percent euhedral biotite in laths 2 to 5 mm long; and 7 to 12 percent subhedral to euhedral hornblende. Mafic minerals are partially altered to chlorite. Accessory minerals include titanite, apatite, magnetite, and zircon (Becraft and Weis, 1963). The body is cut by hypabyssal hornblende-biotite dacite dikes and alaskite, pegmatite, and aplite dikes. Three Pb-a ages ranging from 95±10 Ma to 105±10 Ma were obtained by Becraft and Weis (1963) from exposures on the Turtle Lake 15'' quadrangle in the northern part of the Coulee Dam 1:100,000-scale quadrangle.' 1007|'Leucocratic, coarse-grained, equigranular, foliated, muscovite-biotite quartz monzonite to granite that is present north of Milan. The unit is probably part of the deformed muscovite-biotite quartz monzonite mapped by Miller (1974c) near Eloika Lake in the Newport Number 3 quadrangle in the Chewelah 1:100,000-scale quadrangle (Waggoner, 1990). Biotite is chloritic; muscovite is present both as discrete crystals and intergrown with chlorite and biotite. The micas are commonly wrapped around quartz and feldspar. Sillimanite replacing biotite and cataclasis were observed by Miller (1974c) in thin sections of similar rocks from the Chewelah 1:100,000-scale quadrangle. The two-mica granite is highly weathered and cut by alaskite pegmatite dikes. This intrusive rock is most likely Cretaceous on the basis of its similarities to other muscovite-bearing plutons of probable Cretaceous age in the area.' 1008|'Leucocratic, foliated to massive, medium- to fine-grained biotite-muscovite granite to monzogranite that is present on Mount Spokane. The granite contains quartz, K-feldspar, plagioclase, muscovite, biotite, and accessory apatite, zircon, garnet, and rutile. Biotite and muscovite are shredded; muscovite replaces biotite and replaces feldspar along crystal boundaries. The granite is intruded by and gradational over tens of meters with alaskite, pegmatite, and aplite dikes and sill-like bodies (TKiaa). Muscovite-biotite-bearing granitic rocks on Mount Spokane were mapped as a biotite-muscovite quartz monzonite by Weissenborn and Weis (1976) and later informally called the Mount Spokane granite by Rhodes and others (1989). The Mount Spokane granite structurally overlies the Newman Lake Gneiss (Rhodes and Hyndman, 1984; Bickford and others, 1985). Various amounts of strain are exhibited at the contact of these two intrusive units. A 2- to 5-m-thick zone containing 0.25- to 1-m-thick bands of light-colored mylonite is present on Beacon Hill in lineated (N. 70o E.) Newman Lake Gneiss near the contact with the Mount Spokane granite. The granite on Beacon Hill is foliated and in places contains stretched quartz grains for approximately 15 to 40 m above the mylonite; it then grades upward into generally undeformed rock (DGER, unpublished mapping). Elsewhere to the north, the contact with the Newman Lake Gneiss is represented by a zone of foliated and lineated rock (TKqf and TKqh units of Weissenborn and Weis, 1976). The contact of the Mount Spokane granite with the orthogneiss near Mount Spokane (Kogs), which is thought by Rhodes and Hyndman (1984) to be deformed Mount Spokane granite in the mylonitic zone, is gradational over 500 to 1,000 m of structural thickness (Rhodes and Hyndman, 1984). East of Mount Spokane, south-trending, west-dipping foliation is generally present in the granite within 3 to 5 km of the contact with the orthogneiss (Kogs); foliation in the Mount Spokane granite fades to the west (Weissenborn and Weis, 1976). Two samples from the west side of Mount Kit Carson yielded concordant K-Ar muscovite and biotite ages of about 47 Ma and represent the age of emplacement or reset ages (Miller and Engels, 1975; Engels and others, 1976). Bickford and others (1985) report lower U-Pb intercepts indicating ages of 92±5 Ma on muscovite and 75±5 Ma on biotite. Additional dating of the pluton has been done by Armstrong and others (1987). Two zircons yielded discordant U-Pb ages of 94 to 143 Ma. Three granitic rocks and one pegmatite gave a maximum age of 84±4 Ma and an initial Rb/Sr ratio of 0.7108. Two other samples analyzed by Armstrong and others (1987) contain more radiogenic Sr; calculated Rb-Sr ages are 113±2 Ma and 159±3 Ma. Armstrong and others (1987) suggest that the U-Pb ages and the Rb-Sr isochrons indicate a middle to Late Cretaceous age for the pluton.' 1009|'Leucocratic, medium- to coarse-grained, foliated to massive, muscovite-biotite quartz monzonite to granite that is similar to the Mount Spokane granite. Quartz is light gray and present as clusters of broken and annealed quartz grains; quartz commonly forms graphic intergrowths with feldspar. White K-feldspar is present as phenocrysts as much as 2 cm in length. Plagioclase is commonly slightly altered. Subhedral biotite comprises as much as 10 percent of the rock and forms clots. Muscovite ranges from 0 to 3 percent of the rock and is present as single euhedral crystals, in clots, or with biotite. Foliation is strongest near the Little Spokane River and fades to the north, where the intrusive body is generally massive. The body is intruded by dikes and small bodies of massive hornblende-biotite granodiorite and is cut by alaskite dikes that in places make up as much as 50 percent of the total volume. This intrusive unit yielded discordant K-Ar ages of 48 Ma on biotite and 53 Ma on muscovite (Miller and Engels, 1975). These ages are probably reset; the muscovite-biotite intrusive near the Little Spokane River is most likely of the same age as the Mount Spokane granite.' 1010|'Medium- to dark-gray, medium- to coarse-grained, mylonitic, hornblende-biotite granodiorite gneiss named by Weis (1968) for exposures on the northwest side of Newman Lake. The unit is characterized by megacrysts of subhedral to euhedral crystalloblastic orthoclase as much as 2 cm in length and contains plagioclase, K-feldspar, quartz, biotite, hornblende, and accessory titanite, allanite, zircon, apatite, and opaque minerals (Miller, 1974c). Obvious effects of cataclasis can be seen in most thin sections. The rock was recrystallized during cataclasis, and development of a pervasive lineation is recorded by streaked-out clots of biotite (Miller, 1974c). The trend of the weak to strong lineation is consistent with the N. 70o E. to S. 70o W. trend of lineation observed elsewhere in the PRC. Crystalloblasts of K-feldspar are distinctively aligned parallel or subparallel to the foliation where the rock is strongly deformed. The Newman Lake Gneiss is structurally overlain by the Mount Spokane granite (Rhodes and Hyndman, 1984; Bickford and others, 1985). On Beacon Hill a 2- to 5-m-thick zone containing light-colored, 0.25- to 1-m-thick bands of mylonite is interlayered with deformed Newman Lake Gneiss at the contact with the Mount Spokane granite. Elsewhere, the muscovite-megacryst-bearing granite (Kogsm) is generally present at the contact of the Newman Lake Gneiss and the Mount Spokane granite (Weissenborn and Weis, 1976); minor thin (<15 mm) mylonite zones are also present. Rhodes and Hyndman (1984) suggest that the Newman Lake Gneiss structurally overlies the Hauser Lake Gneiss. Rb-Sr isotopic analyses of the Newman Lake Gneiss returned values similar to those for the Mount Spokane granite. Zircons yielded U-Pb ages of 45 and 139 Ma; however the data produced poor isochrons (Armstrong and others, 1987). The gneiss is thought by Armstrong and others (1987) to be Cretaceous in age. Bickford and others (1985) report a lower U-Pb intercept on mylonitic rock of 55 Ma.' 1011|'Coarse-grained, quartz-feldspar-biotite-muscovite orthogneiss that has distinctive coarse banding of light minerals and dark biotite-rich layers (Weissenborn and Weis, 1976). The structural fabric is dominated by a strong mylonitic foliation defined by mica and mylonitic lineation which lies in the plane of the foliation; lineation is defined by slickenside-like striae. The orientation of the lineation is consistently N. 70o E. to S. 70o W. (Rhodes and others, 1989). This unit is the banded gneiss of Weissenborn and Weis (1976), which they suggested was Precambrian in age. However, Weissenborn and Weis noted that it is difficult to distinguish the orthogneiss on Mount Spokane from the Mount Spokane granite and that contacts with the Mount Spokane granite are gradational. Rhodes and Hyndman (1984) suggest that the banded gneiss of Weissenborn and Weis (1976) is the Mount Spokane granite deformed in the mylonitic zone of the PRC. The contact between the orthogneiss near Mount Spokane and the Mount Spokane granite is transitional through 500 to 1,000 m of structural thickness; the mylonitic zone is overlain by undeformed granite (Rhodes and Hyndman, 1984). The unit structurally overlies the Newman Lake Gneiss (Weissenborn and Weis, 1976).' 1012|'Foliated and lineated biotite-muscovite quartz monzonite east of Mount Spokane. This unit generally separates the Mount Spokane granite from the Newman Lake Gneiss. The megacryst-bearing granite is similar in composition to the Mount Spokane granite, but it has books of muscovite as much as 2.5 cm in diameter; biotite is commonly interleaved with the muscovite. At the contact with the Newman Lake Gneiss, the rock is highly sheared, and the muscovite books are obliterated (Weissenborn and Weis, 1976). The unit contains thin to thick (<2 cm to 1 m) bands of mylonite which apparently developed from alaskite dikes. The unit forms gradational contacts with the Mount Spokane granite (Kiats), the orthogneiss on Mount Spokane (Kogs), and the Newman Lake Gneiss (Kogn) (Weissenborn and Weis, 1976).' 1013|'Local pods and small bodies of amphibolite containing an assemblage of plagioclase ± hornblende, titanite, garnet, diopside, ilmenite, and quartz (Rhodes, 1986). Individual bodies are locally as thick as 28 m (Miller, 1974c). Amphibolite is shown on Plate 1 only in the Hauser Lake Gneiss, but it is also present in gneiss south of the Spokane River (Weis, 1968) and in the heterogeneous metamorphic rocks (PChm) on Browns Mountain (P. L. Weis, retired USGS, oral commun., 1988).' 1014|'Rusty-weathering, medium-grained, well-banded, foliated and lineated mylonitic biotite-orthoclase-plagioclase-quartz gneiss, and schist that contains minor quartzite (Weis, 1968; Rhodes and Hyndman, 1984). Muscovite-biotite schist layers are less than 1 m thick and quartz-feldspar layers are more than 1 m thick (Miller, 1974c). The gneiss is locally intruded by abundant pods of garnet-bearing amphibolite (PCam) (Rhodes and Hyndman, 1984; Weis, 1968). The unit was named by Weis (1968) for exposures on the east side of Hauser Lake, Idaho. The unit extends in a continuous band to the north into the Chewelah 1:100,000-scale quadrangle; a structural thickness of more than 6,000 m was estimated by Weissenborn and Weis (1976). A Precambrian age for the protolith was suggested by Weis (1968), Weissenborn and Weis (1976), Miller (1974c), and Griggs (1973). The gneiss is thought by Miller (1974c), Griggs (1973), and Rhodes (1984) to have a protolith of the Prichard Formation (Belt Supergroup). Weis (1968), Armstrong and others (1987), and Rhodes and others (1989) have suggested a pre-Beltian age for the protolith. Armstrong and others (1987) suggest a pre-Belt protolith because samples are consistently more radiogenic (Rb-Sr) than Belt Supergroup rocks. Whole-rock 87Sr/86Sr analyses of 13 samples suggest an age of 2,053 Ma for the Hauser Lake Gneiss, whereas euhedral zircons indicate a discordant age of 1,668±32 Ma. Augen gneiss that intrudes the Hauser Lake Gneiss near Priest River, Idaho, yielded a single whole-rock Rb-Sr age of 1,440 Ma (Clark, 1973). Armstrong and others (1987) suggest that an old age is indicated for the metamorphic banding because of the large layer-to-layer variation in Sr isotopic composition (Armstrong and others, 1987). According to Armstrong and others (1987), a major metamorphic event to create that banding might be dated at 1,670 Ma, on the basis of the upper intercept obtained from zircons. Rhodes and others (1989) correlate the rusty-weathering sillimanite-bearing paragneiss and schist south of the Spokane Valley with the Hauser Lake Gneiss. The gneiss south of the river has a gently dipping lineation and non-penetrative mylonitic foliation and is thought by Rhodes (1986) to be below the mylonitic zone on the south limb of a synform that has its axis parallel to the Spokane River valley. The Hauser Lake Gneiss is structurally overlain by the Cretaceous Newman Lake Gneiss (Weissenborn and Weis, 1976; Rhodes and Hyndman, 1984; Rhodes and others, 1989). The gneiss unit is intruded by the foliated and lineated dikes of the Mount Rathdrum granite (Eiatr), which cut the foliation in the gneiss.' 1015|'Light- to medium-gray, medium- to coarse-grained, prominently layered gneiss, schist, and quartzite present south of the Spokane River. In the Dishman Hills and near Glenrose, the unit appears to be predominantly feldspathic orthogneiss that displays compositional banding and injected layers composed of biotite, muscovite, feldspar, and quartz. On Browns Mountain and south of hill 881 (3 km northwest of Chester), the unit is predominantly biotite-bearing, medium- to coarse-grained, medium- to thick-layered quartzite and interbedded medium- to coarse-grained biotite-muscovite and muscovite-biotite schists. Dark-colored graphitic schist and light- to dark-colored schist interlayered with quartzite are present on Silver Hill. Dark-gray to black, fine-grained graphitic schist contains quartz, pink crystalline andalusite, biotite, muscovite, and locally abundant tourmaline. Light- to dark-gray sillimanite-biotite schist to gneiss is interlayered with quartzite and also contains quartz, biotite, andalusite, tourmaline, and muscovite. Dark-gray to black, fine-grained, massive quartzite is composed of quartz and minor muscovite, biotite, feldspar, andalusite, sillimanite, and graphite (McLeod, 1923). McLeod (1923) suggests that the tourmaline content of the schist increases with proximity to the granitic rocks (TKia). The unit is cut by alaskite pegmatite dikes; alaskite on Krell Hill and near Big Rock generally intrudes the quartzite of the heterogeneous metamorphic unit parallel to subparallel to foliation. Schist and quartzite on Silver Hill are intruded by biotite-bearing intrusive rocks (TKia) (Page, 1942). On Browns Mountain biotite schist and paragneiss are cut by biotite-hornblende diorite and amphibolite of probable Precambrian age' 1016|'Medium-grained, thin- to thick-bedded quartzite. Five to 35 meters of massive, white quartzite are present at the base of the unit and grade upward into gray, thin- to medium-layered, arkosic quartzite and micaceous feldspathic quartzite containing 60 to 90 percent quartz. The unit is intruded by pegmatitic bodies. The contact with the underlying gneiss of Mica Peak (PCbgm) is sharp (Weis, 1968). The quartzite is thought to be part of the Prichard Formation by Griggs (1973) and to be older than the Belt Supergroup by Weis (1968).' 1017|'Light-gray, coarse-grained muscovite-quartz-feldspar schist and segregation gneiss that consists of mica-rich layers separating quartz-feldspar layers. The schist commonly contains more than 50 percent mica and is locally contorted. Sillimanite and biotite are locally present. Concordant and discordant granitic bodies make up as much as 50 percent of the unit. Small scattered amphibolite bodies intrude the gneiss in the eastern part of the unit. The contact with the underlying gneiss (PCscr) is gradational through 5 to 35 m (Weis, 1968). The unit is thought to be part of the Prichard Formation by Griggs (1973) and to be older than the Belt Supergroup by Weis (1968).' 1018|'Light pinkish gray, medium- to fine-grained, quartz-feldspar-muscovite-sillimanite schist and gneiss. The gneiss is poorly layered and contains abundant small folds. It is intruded by sparse pegmatite and granitic dikes and minor small amphibolite bodies. The contact with gneiss near Cable Peak (PCbgc) is poorly exposed (Weis, 1968). The gneiss near Round Mountain is thought to be part of the Prichard Formation by Griggs (1973) and to be older than the Belt Supergroup by Weis (1968).' 1019|'Light- to dark-gray, chiefly medium-gray, prominently layered gneiss and schist. Individual layers are generally less than 15 cm thick and include quartzite, feldspathic quartzite, and micaceous quartz-feldspar gneiss, granitic gneiss, amphibolite, and schist. The gneiss characteristically has a wide range of composition in adjacent layers (Weis, 1968). The gneiss is thought to be part of the Prichard Formation by Griggs (1973) and to be older than the Belt Supergroup by Weis (1968).' 1075|'Stream deposits in modern drainages.' 1076|'Poorly sorted and poorly stratified, unconsolidated deposits principally of glacial origin. Includes till in lateral and ground moraines as well as outwash and minor modern stream alluvium.' 1077|'Unconsolidated, poorly sorted, fluvial sediment 70 to 330 m above the present stream levels. Ranges in size from clay to boulders and is derived from local sources (Hosterman, 1956).' 1078|'Small stocks of hornblende monzonite, porphyritic hornblende monzonite, alkali-feldspar syenite, and pyroxene syenite (Schlack, 1989). Age determinations have yielded conflicting dates, but emplacement ages of about 100 Ma are likely (Marvin and others, 1984; Schlack, 1989).' 1079|'Interbedded quartzite and argillite with some arenaceous dolomitic beds; usually thinly bedded (Hobbs and others, 1965). Colors range from purplish-gray and very pale pink to gray and green. Mud cracks and ripple marks common.' 1080|'Predominantly thinly laminated medium- to dark-gray argillite; some interbedded light-gray quartzite and arenaceous dolomite (Hobbs and others, 1965).' 1081|'Light-gray dolomitic and calcareous quartzite interbedded with medium- to dark-gray argillite (Hobbs and others, 1965). Some impure dolomite beds near top. Ripple marks, small scale cross-bedding, and molar-tooth and ovoid structures in some layers. Mapped as YwI by Hobbs and others (1965).' 1082|'Thick-bedded impure to pure quartzite at base, grading upward to interbedded and interlaminated impure quartzite and argillite that comprise bulk of formation (Hobbs and others, 1965). Characteristically thin bedded and many layers laminated. Predominantly purplish red and grayish red; argillite is darker. Some carbonate-bearing beds, mostly in upper part. Ripple marks, mud cracks, and mud-chip breccia in some layers.' 1083|'Thick-bedded white to light-gray quartzite containing interbedded impure and nearly pure quartzite in upper and lower parts, and a few widely spaced argillite partings (Hobbs and others, 1965). Cross-bedded and laminated in part.' 1084|'Light- to greenish-gray fine-grained impure quartzite with lesser amounts of nearly white to light-gray nearly pure to pure quartzite (Hobbs and others, 1965). Beds predominantly 5 to 20 cm thick. Ripple marks and pseudoconglomerate are common.' 1085|'Light-gray to nearly white pure to impure quartzite interbedded with laminated argillite (Hobbs and others, 1965). Quartzite beds 5 to 45 cm thick. Ripple marks and graded bedding are common.' 1086|'Banded dark-gray argillite, laminated in part; weathers rusty red (Hosterman, 1956).' 1100|'Stream deposits in modern drainages.' 1101|'Unconsolidated deposits on terraces above the modern drainages. Some may be glacial outwash deposits.' 1102|'Poorly sorted and poorly stratified, unconsolidated deposits principally of glacial origin. Includes till in lateral and ground moraines as well as outwash and minor modern stream alluvium.' 1103|'Unconsolidated, poorly sorted, fluvial sediment preserved in erosional remnants 250 to 350 m (800 to 1150 ft) above the present river bottoms. Includes beds of cobble gravel, sand, and orange-weathering clay. Slumps are common.' 1104|'Small exposure of basalt in the extreme western part of the map area.' 1105|'Gray dikes that contain plagioclase, biotite, and hornblende phenocrysts in an aphanitic groundmass' 1106|'Light-gray, medium- to coarse-grained, equigranular to porphyritic granodiorite. The Roundtop pluton contains about 20 percent quartz, 50 percent zoned euhedral plagioclase, 15 to 25 percent orthoclase, and 5 to 15 percent hornblende and biotite combined (Hietanen, 1968). Fabric is weak to absent in most places, but gently-dipping igneous flow foliation is present in the central part of the pluton and strongly developed contact-parallel mylonitic foliation and lineation is present in the southern part (Steve Box, pers. comm., 1997). Age is 52±7 Ma by U-Pb methods on zircon (Marvin and others, 1984). The Herrick stock contains biotite, minor hornblende, and plagioclase that is in part strongly zoned (Holland, 1947). The Herrick stock is deeply weathered except along the St. Joe River canyon. Hornfels has formed adjacent to this intrusion and forms a resistant ridge along its northeast side.' 1107|'Medium- to fine-grained, equigranular hornblende-pyroxene gabbro and diorite. Hornblende is present as either rims on pyroxene or completely replacing pyroxene and olivine is present at a few localities (Hietanen, 1963; 1968). Plagioclase constitutes about 30 to 40 percent of the rock and has all anorthite content of 45 to 74 percent. Some exposures are highly altered, but others, particularly in the southwest part of the area , are unaltered. Typically intruded along or near fault zones. Includes the Wishard Sill of Pardee (1911) which is exposed along the state line from Wishard Peak northwest to beyond Dominion Peak. Age is highly uncertain, as no reliable radiometric dates are available. Hietanen (1968) assigned them a Tertiary age. Harrison and others (1986) assigned only the southern exposures this age, and thought the Wishard sill was Proterozoic. A whole-rock K-Ar age of 575±4 Ma on the Wishard sill (Marvin and others, 1984) may represent a partially reset Proterozoic age, or an incorrect old age resulting from excess argon.' 1108|'Light-gray, medium- to coarse-grained, massive to foliated, (muscovite-) biotite granodiorite. Pegmatite dikes and biotite schlieren are common in the intrusion along the Little North Fork of the Clearwater River (Hietanen, 1963). None of these rocks have been reliably dated, but the presence of a foliation, muscovite, and an abundance of pegmatites suggest a Cretaceous age.' 1109|'Gray, medium-grained, strongly foliated (hornblende-) biotite tonalite, granodiorite, and quartz diorite. Contains euhedral epidote interpreted to be magmatic in origin. Foliation is mylonitic at several localities. No reliable dating of these rocks exists, but a Cretaceous age is likely given the degree of fabric development.' 1110|'White to light-gray, foliated to massive, medium- to coarse-grained anorthosite. Contains bimodal plagioclase populations (andesine and bytownite) and minor amounts of hornblende, biotite, and chlorite (Hietanen, 1963). Weathers more readily than surrounding metasedimentary rocks. Hietanen (1963) speculated that the anorthosite formed by metamorphic rather than igneous processes.' 1111|'Dark-gray, foliated or lineated hornblende-plagioclase rocks, typically garnet-bearing. May in part be metamorphosed carbonate-bearing rocks, but most are thought to be metamorphosed igneous sills and dikes (Hietanen, 1963).' 1112|'Quartzite-dominated interval in the northwestern part of the area where subdivision into members has not been attempted.' 1113|'Red to light-gray, decimeter-­ to meter-scale beds of fine- to medium-grained parallel-laminated quartzite. Informally termed member three in this report. Exposed at the type section on Striped Peak where approximately 305 m (1,000 ft) is exposed. The upper part is truncated by faulting, so this represents a minimum thickness. Laterally equivalent to the Bonner Quartzite, described to the cast (Harrison and others, 1986). Contains abundant K-feldspar (18-23 percent) and lesser amounts of plagioclase (6-7 percent) based on two samples stained for feldspar.' 1114|'Red and subordinate green siltite and argillite interbedded with minor amounts of quartzite and carbonate. Beds are typically centimeter to millimeter scale. Informally termed member two in this report. Exposed at the type section on Striped Peak where it is approximately 105 m (350 ft) thick. Laterally equivalent to member 3 of the Mt. Shields Formation, described to the east (Harrison and others, 1986).' 1115|'Red or gray fine- to very fine-grained quartzite and subordinate amounts of siltite, argillite, and carbonate. At Cedar Mountain, north of Avery, unit consists of fine-grained quartzite in tabular, decimeter- to meter-thick beds with green siltite and argillite along partings. Lower part of unit there grades upwards from Ywu3 with increasing 5 to 10 cm white quartzite beds and dark green siltite, increasing ripple-drift cross lamination and rippled tops and more common mud cracked surfaces. Some beds are graded and there seem to be thinning and fining-upward cycles, Rocks highest in section are 20 to 40 cm thick flat-laminated quartzites with rippled tops and rarer mud-cracked thin argillite caps. Carbonate is present in some thin (~5 cm) zones at or near bedding surfaces. Entire member exposed at the type section on Striped Peak where it is approximately 245 m (800 ft) thick. Informal unit designated here is laterally equivalent to Mt. Shields Formation, members 1 and 2, described to the east (Harrison and others, 1986). A single sample of quartzite from Striped Peak stained for feldspar contained about 20 percent plagioclase and no K-feldspar.' 1116|'Carbonate-free microlaminated and wavy couplets of siltite and argillite. Most argillite is light green but some is black. Mapped and described by Hobbs and others (1965) in part of the Coeur d’Alene mining district, and by Vance (1981) south of the district. Vance (1981) estimated a thickness of about 400 m (1300 ft) on the northeast side of Foolhen Mountain. May be equivalent to Wallace 5 of Clark Fork section (Harrison and Jobin, 1963) or upper Shepard (Burmester, 1986; Lemoine and Winston, 1986).' 1117|'Rock is typically microlaminated dark-green siltite and light-green argillite, and dolomitic siltite beds 2 to 5 cm thick. Thin quartzite lenses and starved ripples produce a lenticular sediment type. More quartzitic in southeast part of area where unit contains an abundance of load casts on bed bottoms. Vance (1981) estimated a thickness of about 210 m (700 ft) on the northeast side of Foolhen Mountain. Likely correlates with Wallace 4 of Clark Fork section (Harrison and Jobin, 1963). May be equivalent to the middle or lower part of Shepard Formation (Lemoine and Winston, 1986).' 1118|'Dark-gray, thinly laminated argillite and siltite in the lower part grading upward to green thinly laminated argillite and siltite. Rare, thin (2-5 cm) coarse siltite or very fine-grained quartzite layers have ripple-drift cross lamination and ripple tops. Upwards, bedding becomes thinner and more planar with the highest gray rocks being microlaminated white-weathering siltite and black (biotitic?) argillite with characteristic planar partings. Above that, dark green siltite and light green argillite becomes more abundant and thickly and unevenly laminated, with more thin rippled quartzite beds and mud chips and mudcracked surfaces. Top is gradational into Ywu2 with siltite and argillite becoming more microlaminated. Equivalent to the Snowslip Formation in the Missoula area to the northeast (Winston, 1986). Unit lacks the red argillite and siltite of the Snowslip, and the gray thinly laminated lower part is missing from the Snowslip to the northeast. An estimated a thickness of about 460 in (1500 ft) on Foolhen Mountain, south of the Coeur d’Alene district (Vance, 1981) is probably a minimum because the lower contact is not exposed there.' 1119|'Unit in the Coeur d’Alene mining district (Ywl of Hobbs and others, 1965) where subdivision into separate members has not been attempted. Also includes structurally complex rocks in the southwest part of the map area.' 1120|'Characterized by pinch and swell couplets (Winston, 1986) of white quartzite that grade upward into black argillite caps. The quartzite is fine- to very fine-grained, commonly calcareous, with hummocky to low-angle cross-stratification and scoured or loaded basal contacts. Centimeter-scale beds of silty, molar-tooth limestone and dolostone are widespread. Zones of tan-weathering calcareous siltite to very fine-grained quartzite with black, non-calcareous argillite caps (black and tan rock type) are locally common. Microlaminated white siltite to black argillite is present, although not dominant. Minor zones with horizontal pods of non-resistant calcareous siltite are present low in the section. Amalgamated, decimeter beds of quartzite grading to cm-scale, tan, non-resistant, calcareous siltite and thin black argillite caps occur in some intervals, and commonly underlie or are interspersed with zones of sedimentary breccia. The breccia typically consists of rounded to angular, pebble- to boulder-sized calcareous quartzite clasts in orange-weathering calcitic siltite matrix with abundant soft-sediment deformation features. Outcrops of the breccia are commonly silicified and form prominent hoodoos. Several breccia zones mapped by Nord (1967) as fault breccias are interpreted here as sedimentary breccia.' 1121|'Characterized by wavy, even, thin laminations and less common couplets of green siltite with light green to white argillite caps. Lenticular couplets locally common. Intervals of "tri-color" white quartzite, dark green siltite, and pale green argillite alternate with the laminated green siltite-argillite lower in the unit. Carbonate is present in the lower part as punky-weathering, molar tooth silty limestone beds up to 1 m in thickness. Zones of horizontal pods of silty carbonate are common in the upper part of the unit; the carbonate is non-resistant and normally weathers out, leaving distinct voids. Pinch and swell couplets of white and quartzite and black argillite increase in abundance toward the top of the unit, as do thin intervals of the "black and tan" rock type, characteristic of Ywm.' 1122|'Gray to brown muscovite-biotite schist and phyllite that is coarsest in the southern part of the area and grades into argillite and siltite to the north and east. Micas grow to about 5 mm, but grain size in biotite quartzite (siltite part of protolith) remains small. Lowest rocks are scapolite-rich, graded dark-gray to black siltite and argillite in even to wavy couplets. Growth of scapolite may have destroyed microlaminae. Rocks commonly show metamorphic succession from tabular chloritoid, increasing size garnets, staurolite, then kyanite (Lang and Rice, 1985a). Compositional layering is typically parallel to foliation, but locally is folded isoclinally with centimeter- to outcrop-scale wavelengths. Mapped as schist within the Wallace Formation by Hietanen (1968). Unit is the metamorphic equivalent of Ywu1. Contact with Ywu1 drawn at the garnet isograd.' 1123|'Medium-grained, thin-bedded quartzite that contains minor amounts of calc-silicate minerals (primarily actinolite and diopside) and thin layers of phyllitic black argillite or schist that locally contain scapolite. The lower contact west of the Roundtop pluton appears to be gradational with downward decreasing quartzite content and bedding thickness and increasingly even bedding style. Equivalent to less-metamorphosed rocks of Ywm and Ywml units. Mapped by Hietanen (1968) as quartzite unit of Wallace Formation.' 1124|'Thick-bedded impure to pure quartzite at base, grading upward to interbedded and interlaminated impure quartzite and argillite that comprise bulk of formation (Hobbs and others, 1965). Characteristically thin bedded to laminated. Predominantly purplish red and grayish red; argillite is darker. Some carbonate-bearing beds, mostly in upper part. Ripple marks, mud cracks, and mud-chip breccia in some layers. Rocks at Ward Peak along the state line, tentatively assigned to Ysr, are green siltite and light green to white argillite with lesser amounts of wavy, decimeter-scale quartzite beds. Abundant white rounded argillite rip-up clasts are present, some with unusually equant dimensions. These rocks resemble Ywl, but are assigned to Ysr because of quartzite beds, scarcity of carbonate, and the presence of abundant rip-ups. Similar rocks arc described and mapped (where exposures permitted) as uppermost St. Regis in the Coeur d Alene mining district (Ysg unit of Hobbs and others, 1965). Along Gold Creek the St. Regis is characterized by rounded mud-chip rip-ups and evenly bedded, cm-scale white quartzite grading up into cm-scale black argillite caps.' 1125|'Thick-bedded white to light-gray quartzite containing interbedded impure and nearly pure quartzite in upper and lower parts, and a few widely spaced argillite partings (Hobbs and others, 1965). Cross-bedded and laminated in part.' 1126|'Light- to greenish-gray fine-grained impure quartzite with lesser amounts of nearly white to light-gray nearly pure to pure quartzite (Hobbs and others, 1965). Beds predominantly 5 to 20 cm thick. Ripple marks and pseudoconglomerate are common.' 1127|'Muscovite-rich schist, thin quartzite intervals, and minor calc-silicate rocks. Garnet present but neither abundant nor ubiquitous. Exposures west of Granite Peak show increasing amounts of quartzite down section, assuming the relict NE-dipping bedding is upright. Unit probably is equivalent to the St. Regis Formation of the Ravalli Group, but may include part of the Revett Formation. Tentatively assigned to the Ravalli Group on the basis of stratigraphic position.' 1128|'Mostly fine-grained, sugary and friable white feldspathic quartzite with muscovitic partings and rare, thin biotite quartzite tops. Minor fine- to medium-grained (1-2 mm) biotite-muscovite schist. Unit is probably equivalent to the Revett Formation of the Ravalli Group, but may include part of the Burke and St. Regis Formations. Tentatively assigned to the Ravalli Group on the basis of stratigraphic position. Thickness is about 800 m (2500 ft) west of Granite peak where this unit includes what Hietanen (1968) mapped as Wallace quartzite. It is not assigned here to the Wallace because features expected of metamorphosed Wallace were not found. Among these are relict pinch and swell bedding, evidence of carbonate such as scapolite or calc-silicate mineral assemblages, and schist and quartzite proportions and thicknesses similar to siltite-argillite and quartzite observed in the Wallace lithologies to the north.' 1129|'Light gray to nearly white impure to pure quartzite interbedded with laminated argillite (Hobbs and others, 1965). Quartzite beds 5 to 45 cm thick. Ripple marks, mud cracks, and graded bedding are common.' 1130|'Typically dark, rusty-weathering coarse-grained (5 mm micas) biotite-muscovite-feldspar-quartz schist. Commonly crenulated; locally garnetiferous or sillimanite-bearing [sic]. Also contains as discontinuous layers fine-grained (muscovite)-biotite-feldspar quartzite with moderately-developed foliation. This quartzite is similar to unit described below but generally represents less than 10 percent of the Ys unit. Includes rocks assigned by Hietanen (1968) to Prichard Formation. Some of the unit at Monumental Buttes originally assigned to the Prichard Formation (Hietanen, 1963) but later assigned to the Boehls Butte Formation and thought to be pre-Belt in age (Hietanen, 1984). Correlation with specific Belt Supergroup (or older) units is too speculative at the present time, but most are probably metamorphic equivalents of the Prichard Formation.' 1131|'Gray to white, coarse- to medium-grained quartzite. Includes rare garnetiferous and calc-silicate concentrations. Mapped by Hietanen (1968) as quartzite of the Prichard Formation and later subdivided to include quartzite of Boehls Butte Formation (Hietanen, 1984). Most is probably quartzite of the Prichard Formation, but present understanding of stratigraphy and structure in the area precludes assignment to a formation.' 1132|'Bluish gray-green, medium- to coarse-grained rocks rich in diopside, hornblende, or both (Hietanen, 1963). Quartz, plagioclase, tremolite, scapolite, and calcite are also present.' 1133| 1150|'Well rounded, well-sorted cobbles, gravel, and sand deposited in active stream channels and floodplains. Provenance appears to include much of west central Montana.' 1151|'Unsorted and unstratified mixtures of angular material that have been moved down slope by gravity. Landslides are uncommon in areas underlain by Belt rocks. The most dramatic example of a landslide in the Wallace quadrangle is the Little Joe Slide (T17N R29W sec 36) that formed catastrophically in 1968.' 1152|'Well sorted, well rounded boulders, cobbles, gravel, sand, and silt deposited by streams, later incised by modern streams and now stranded up to 15 m above the present watercourses. Includes both terrace deposits along rivers and outwash fan deposits emerging from the mountains.' 1153|'Light tan to light brown, varved silt and clay deposited in Glacial Lake Missoula. Supports vertical cuts along streams and in road cuts. On topographic maps, form flat surfaces that are usually dissected into a badlands topography.' 1154|'Unsorted, unstratified clay- through boulder-sized material deposited by glaciers; only occurring above 1250 m in elevation. Many small moraines in the mountain valleys are not shown on the map.' 1155|'Locally derived, poorly sorted, subangular to subrounded boulders, cobbles, gravel, sand, and silt deposited in fans along the mountain front just south of the Osburn Fault. Surfaces of these dissected fans generally stand 60 m above the present streams.' 1156|'Mostly fluvial channel deposits of well rounded, well sorted boulders, cobbles, pebbles, and sand. Clasts of Bonner Formation and some of granitic rocks and porphyritic volcanic rocks are from outside the area. Found up to 1500 m in elevation, and interpreted to have been deposited by the ancient Clark Fork River system. South of St. Regis, these rocks dip 30° into the Boyd Mountain fault indicating late Tertiary listric normal movement on the Boyd Mountain Fault.' 1157|'Medium- to fine-grained, equigranular hornblende-pyroxene gabbro and diorite. A Late Cretaceous-Early Tertiary age is inferred for dikes and sills from their common occurrence along faults, although Harrison et al. (1996) assigned a late Proterozoic age to some.' 1158|'Greenish-gray, micaceous, tabular and lensoidal, hummocky cross-stratified, fine-grained quartzite with argillite interbeds. Some chert chips and beds in the lower part. Entire thickness is not exposed, but at least 500 m.' 1159|'Dense, bright green and red interbedded siltite and argillite in microlaminae and couplets. Mudcracks and chips are common. Contains diagnostic thin chert beds and chips. The upper part contains some pink to gray cross-bedded quartzite in beds less than 0.5 m thick. Dominated by mudcracked even couplet and mudcracked lenticular couplet sediment types. Whole unit not exposed, but at least 600 m thick.' 1160|'Pink, medium-grained, feldspathic, cross-bedded quartzite. Lewis (1998) found 15-25% potassium feldspar but only a trace of plagioclase in the Bonner Formation. Mostly comprised of the cross-bedded sand sediment type. The sole exposure in the Wallace quadrangle is fault bounded, but it is probably 300 m thick.' 1161|'Red siltite to argillite couplets with abundant mudcracks, mud chips, and salt casts. Mostly mudcracked even couplet and mudcracked lenticular couplet sediment types. About 600 m thick.' 1162|'Pink to gray, flat-laminated, fine-grained quartzite, with tan weathering dolomitic blebs. Some cross bedding, which can make it difficult to distinguish from the Bonner Formation. However, in contrast to the Bonner, Lewis (1998) found subequal amounts of plagioclase and potassium feldspar and a total feldspar content of 25-35% in the Mount Shields. Characterized by the flat-laminated sand sediment type; about 900 m thick.' 1163|'Distinguished by dark green siltite and light green argillite in microlaminae and couplets that are dolomitic and have a characteristic orange-brown weathering rind. Poorly exposed but weathers into thin plates. Some 2-5 cm thick white quartzite beds are present in the upper part. Common sediment types are calcareous microlaminae and calcareous uncracked (non-mudcracked) even couplets. Estimated to be 250 m thick.' 1164|'Mostly silver-green siltite and argillite in planar parallel beds 2-5 cm thick that weather into small blocky pieces with a characteristic spotted, rusty-red-on-green appearance. Green siltite-argillite laminae are common near the base. Mudcracks are abundant. Some fine-grained feldspar-poor quartzite is present in beds less than 0.5 m thick, especially in the lower part. Dominant sediment types are mudcracked even couplet and mudcracked lenticular couplet. Faults must complicate thickness estimates, because apparent thicknesses range from 600 to 900 m in the quadrangle.' 1165|'Tan-weathering dolomitic siltite and quartzite is capped by black argillite in pinch and swell couplets and couples to form a distinctive lithology. The quartzite and siltite commonly have scoured bases or bases with load casts. Non-polygonal "birdsfoot" cracks common in black argillite. Carbonate-rich beds have molar-tooth structures. Sedimentary breccia, consisting mostly of white quartzite clasts in punky, orange weathering silty dolomite, is common, although we believe some sedimentary breccia zones shown by Campbell (1960) are actually tectonic in origin. The breccia weathers into spires and hoodoos. Euhedral scapolite crystals are present in breccia zones. Severe deformation within this unit makes thickness estimates problematic, but it is no more than 2000 m.' 1166|'Consists of cycles, from 1 to 10 m thick and usually incomplete, of a basal white quartzite or intraclast unit, overlain by uncracked (non-mudcracked) even and lenticular couplets of green siltite and argillite, and capped by dolomitic beds. However, these cycles are difficult to recognize in the typical small outcrop. The unit is more easily recognized by wavy but parallel, silver-green couplets of darker green siltite and lighter green argillite, by white quartzite, by beds of tan or brown weathering dolomite from 0.5 to 1 m thick, and by weathered-out pods of carbonate in the green siltite. Some couplets are mudcracked. About 150 m above the base of this unit is an interval of abundant thick black argillite beds and some pinch and swell couplets capped by black argillite like those of the middle member of the Wallace Formation. This is probably the severely deformed interval displayed in the footwall of the Camel''s Hump structure. Carbonate mud and uncracked (non-mudcracked) even couplets are the principal sediment types in this interval. The severity of deformation within the lower Wallace makes thickness unknown, but it is probably about 700 m (Don Winston, personal communication, 1999).' 1167|'Identified by gray-purple or, more commonly, bright green mud-cracked silt to clay couplets with abundant mud chips. Dolomitic silt beds in the upper part weather to a siderite-colored brown, darker than the carbonate in the Wallace Formation. Toward the base, gray-purple, flat-laminated quartzite beds 10 to 30 cm thick become common. Down section in the Revett Formation, the thickness of these quartzite beds increases to 0.5 to 1 m and mud cracks and chips become less common. Comprised mainly of mudcracked even couplet, mudcracked lenticular couplet, and even couple sediment types. Thickness is estimated to be 850 m.' 1168|'There are three informal members, here undivided. The uppermost member is white to purple or lavender, banded medium-grained quartzite in 1-2 m thick beds. Quartzite beds are capped by green argillite, 5-10 cm thick, that is not obvious in outcrop. Very thin mud skins and associated mud chips are common. Quartzite beds are flat-laminated to cross-bedded. Some cross-beds are overprinted and intersected by purple liesegang bands. The middle member consists mostly of gray-green siltite and argillite in indistinct wispy beds 5-20 cm thick. Load, pillow, and fluid escape structures are common. Some mud cracks and chips are present. This member is mostly of the discontinuous layer sediment type. The lowermost member is banded quartzite very similar to the upper unit. The contact with the Burke Formation is gradational and somewhat arbitrary and placed where the quartzite beds thin to less than 1 m. The Revett is 900 to 1250 m thick.' 1169|'Poorly exposed and relatively unknown. The uppermost part is purple-gray to white, flat-laminated and cross-bedded sericitic quartzite in beds 0.3-1 m thick interbedded with green siltite. Flat-laminated sand and even couples characterize the sediment types. The middle and lowermost parts contain purple, or alternating green and purple, mudcracked siltite-argillite couplets with abundant mud chips. In the siltite, abundant euhedral magnetite grains may be diagnostic. The middle and lowermost parts are mainly mudcracked even couplet and mudcracked lenticular couplet sediment types. Approximately 900 m thick.' 1170|'The upper part consists of pyritic, black and gray, evenly laminated argillite and siltite couplets and couples. Lower in the section, flat-laminated, gray, fine-grained, micaceous quartzite predominates. The whole formation weathers a rusty brown from the abundant pyrite and pyrrhotite. Principal sediment types are plane-laminated silt and clay, flat-laminated sand, and even couples. The entire formation, not exposed in the Wallace quadrangle, is probably more than 6000 m thick.' 1171|'Diorite and gabbro' 1200|'Gravel, sand, silt, and clay in flood plains and low terraces along present drainage. Includes alluvial fans.' 1201|'Angular blocks of bedrock in finer grained matrix; mixed, at places, with glacial debris.' 1202|'Buff to lavender varved clay and silt containing a few lenses of gravel and scattered dropstones. Tends to form vertical cliffs along present stream courses; old surfaces are gently rolling; eroded surfaces tend to show a badlands topography. Sediments represent bottom deposits of Glacial Lake Missoula, which was filled several times during the Pleistocene to a maximum altitude of 4,200 ft when ice dammed the mouth of the Clark Fork River (Pardee, 1910; Alden, 1953; Bretz and others, 1956).' 1203|'Includes till (ground and end moraines), outwash, and other glaciofluvial deposits and flood-related deposits. Particularly abundant and thick in drainages and basins connecting to or north of the Flathead-Clark Fork River system. Torrential gravel deposits related to catastrophic emptying of Glacial Lake Missoula are in perched valleys at places along the Clark Fork River, form large point bars as terraces, form giant ripples, and show coarse cross-bedding convex upstream at the mouths of many tributaries to the Clark Fork northwest of Thompson Falls, Mont.' 1205|'Semiconsolidated to consolidated conglomerate interlayered with shale, coal, and volcanic ash.' 1207|'In northeast: rhyodacite vent agglomerate, volcanic conglomerate and breccia containing thin interbeds of crystal tuff, andesitic to dacitic crystal tuff characterized by large hexagonal crystals of biotite, and minor basalt. In southeast: andesitic to dacitic welded crystal tuff and a few dikes; phenocrysts in tuff are commonly fragmented and consist of smoky quartz, sanidine, plagioclase, and minor biotite.' 1208|'Intrusive rocks ranging in composition from granodiorite to granite. Most are biotite or hornblende-biotite seriate porphyritic granodiorite; some plutonic phases contain muscovite; and a few have significant amounts of granite pegmatite. Also includes andesite porphyry in volcanic center in northeast. These rocks range in age from about 35 m.y. to about 50 m.y. (Marvin and others, 1984).' 1210|'Mostly porphyritic hornblende-biotite quartz monzonite and granodiorite. These rocks range in age from about 70 m.y. to 120 m.y. (Marvin and others, 1984). Some of these plutons have been offset by faulting.' 1211|'Mostly medium- to coarse-grained aegirine-hornblende-mica-bearing feldspathic rocks in a complex of plutons that include syenite, fenite, and quartz syenite. About 105 m.y. old (Marvin and others, 1984).' 1212|'Used in the southeast and in cross sections. Includes, in descending order, Red Lion Formation (shale, limestone, and dolomite), the Hasmark Dolomite, the Silver Hill Formation (shale and limestone), and the Flathead Quartzite. Rests unconformably on the Pilcher, Garnet Range, and McNamara Formations of Proterozoic age. Total thickness is about 2,700 ft.' 1213|'Unit mapped in north-central area. Unnamed gray to tan dolomite. Consists of a lower dolomudstone about 1,020 ft thick, a middle laminated shaly unit about 280 ft thick, and an upper dolomudstone about 1,200 ft thick that has an erosional top (Aadland, 1977). Total thickness exposed is about 2,500 ft.' 1214|'Unit mapped in north-central area. About 300 ft of olive fissile shale (Wolsey or Gordon Shale) that contains trilobites and brachiopods of early Middle Cambrian age (Keim and Rector, 1964). Underlain by red to maroon quartzite at base grading up into buff quartzite (Flathead Quartzite). Base of quartzite has sparse trace fossils. Quartzite thickness about 30 ft. Disconformably overlies Libby Formation of Proterozoic age; micaceous and hematitic weathered zone in upper few feet of Libby. Disconformity has thin secondary chert zone along it.' 1215|'Dioritic to gabbroic rocks that commonly show alteration of mafic minerals. Sills and dikes range in thickness from less than 3 ft to 1,500 ft. Thicker sills tend to have wide (600 ft) contact-metamorphosed zones around them. Sills commonly persist for many kilometers; some maintain approximately the same stratigraphic position, whereas others cut up through the section. Dikes are rare. Only the larger or more persistent sills and dikes are shown. Intrusion occurred at least twice (at about 1,400 m.y. and 800 m.y.; Harrison, 1972).' 1216|'Mostly red to light pink, cross-bedded, micaceous and feldspathic fine-grained quartzite containing thin, shaly, argillitic layers in the upper part. Pre-Flathead erosion has removed all the Pilcher in places, and a complete section is nowhere present. The maximum exposed thickness is about 1,770 ft.' 1217|'Predominantly gray-green micaceous siltite containing beds and zones of fine-grained quartzite and dark-green to black argillite; calcareous at places. Red argillite and siltite zones are rare and scattered through upper two-thirds of formation. Lower part is predominantly black or dark-green laminated argillite and siltite, calcareous at places, and contains a few stromatolite beds, oolite beds, and black or green chert in chips and layers. Most of the unit shows mud chips, mud cracks, ripple marks, small-scale cross-beds, and cut-and-fill structures. Red weathered zone a few feet thick is present beneath Flathead Quartzite where Flathead rests on Garnet Range. Anomalous amounts of chalcopyrite in places in lower black argillite; chalcocite and bornite in a few green strata. Thickness is about 8,200 ft.' 1218|'Consists mostly of interlaminated argillite and siltite and, in the upper part, quartzite. Contains three informal members. Upper member is mostly black interlaminated argillite and siltite and contains abundant mud-crack casts, ripple marks, and soft-sediment structures. Green fine- to medium-grained cross-bedded quartzite and siltite become common in upper half. A zone of red interlaminated argillite and siltite occurs in the north-central part of the quadrangle near the top of the section about 300 ft below the erosional unconformity that terminates the unit. Red weathered zone is present at top of Libby beneath the unconformity. Middle member is mostly evenly laminated dark-green argillite and light-green siltite and contains mud chips, chert chips and beds, cut-and-fill structures, and ripple marks. Anomalous amounts of chalcocite or bornite occur in a few beds. Lower member is black and white or green interlaminated argillite and siltite and contains scattered beds of dolomite and stromatolites. Maximum exposed thickness of Libby is about 5,500 ft.' 1219|'Interbedded, laminated and interlaminated red and green argillite and siltite; ripple marks, mud cracks, scours, and small-scale cross-bedding common; rare salt casts. Distinctive, very fine grained, dense, mostly pale-green but locally red siltites are common in beds generally less than 8 in. thick. A few chert beds are present, and rounded chert grains and chips are common. A few green strata contain anomalous amounts of chalcocite and bornite. Thickness is about 3,600 ft in the southeast. In the north-central area, the unit is mostly green laminated argillite containing only sparse red argillite, sparse chert, and rare stromatolites; thickness about 300 ft.' 1220|'Red to pink, micaceous, arkosic, cross-bedded, fine- to medium-grained quartzite containing red argillite interclasts: tabular and trough cross-beds and climbing ripples common. Interbeds of red laminated argillite increase in abundance to west and north. Northernmost exposures contain scattered beds of planar-laminated green fine-grained quartzite as much as 30 ft thick. Ranges in thickness from about 1,400 ft in the southeast to about 500 ft in the north-central area.' 1221|'Type locality for the Striped Peak Formation (Ransome and Calkins, 1908) is on Striped Peak, a few kilometers southwest of Wallace, Idaho. Here, and at more extensive exposures about 10 km southwest of the peak, the formation (Ysp) consists of four informal members. The upper member consists of interbedded red and green laminated argillite and siltite that contains mud chips, ripple marks, mud cracks, and salt casts. The erosional remnant of this member on Striped Peak is about 300 ft thick, but it is about 2,300 ft thick a few kilometers to the southwest. The member below is a buff-weathering stromatolitic dolomite about 200 ft thick. The third member down section is a red, medium-grained, planar-bedded, stromatolitic dolomite containing some dolomite cement and streaks and is about 650 ft thick. The basal member is interbedded argillite, siltite, and quartzite that are dominantly red in color but contain some green strata; mud cracks, ripple marks, and small-scale cross-beds are common; the member is about 1,000 ft thick. Anomalous amounts of copper sulfides occur at places in green beds of the lower part of the Striped Peak and in the carbonate members. The Striped Peak as originally defined is the red rock sequence above black argillites of the upper part of the Wallace Formation. Using this definition, Anderson (1930) called red strata above the Wallace and below the Libby Formation in the Clark Fork, Idaho, area the Striped Peak Formation (fig. 2). In the Clark Fork area, the sequence of members seen at Striped Peak is found in the lower 600 ft of section (fig. 2); above this, Harrison and Jobin (1963) mapped: a dolomite member about 400 ft thick characterized by boxwork structure, stromatolites, and oolites; a black interlaminated argillite and siltite member about 300 ft thick; and a red arkosic, planar-bedded, fine- to medium-grained quartzite member about 650 ft thick. All these units are included in the Striped Peak Formation (Ysp) mapped in the northwestern part of the area.' 1222|'In the north-central area, range from 0 ft to 400 ft in thickness and are mapped between the Mount Shields Formation, equivalent to the Striped Peak at Striped Peak, and the Bonner Quartzite, equivalent to the quartzite member of the Striped Peak at Clark Fork. These two members, along with some beds in the upper part of the Wallace Formation, commonly contain several hundred parts per million of boron (Harrison and Grimes, 1970)-a feature unique to the western facies in this part of the stratigraphic section.' 1223|'Consists of three informal members. The upper member is interbedded laminated argillite and siltite; dominantly red with minor green strata in the lower part, but dominantly green argillite and siltite containing dolomitic beds near the top. Red strata commonly contain salt casts, mud cracks, mud chips, and ripple marks. Some green strata contain anomalous amounts of chalcocite and bornite or, more rarely, chalcopyrite. Ranges in thickness from about 3,300 ft in the south, where upper green-bed zone is about 100 ft thick, to about 1,000 ft in the north where upper green-bed zone is about 330 ft thick: Middle member is red fine-grained arkosic planar-bedded quartzite containing at places tan dolomitic lenses, streaks, and cement; has a zone of red stromatolite beds near top. Ranges in thickness from about 3,300 ft in the southeast to about 650 ft in the north. Basal member is interbedded siltite, quartzite, and argillite; dominantly red in color but contains some green argillite and siltite; mud cracks, mud chips, ripple marks, and cut-and-fill structures abundant. Argillite is commonly in partings between planar beds of micaceous arkosic siltite and quartzite. Ranges in thickness from about 3,300 ft in the southeast to about 1,000 ft in the north.' 1224|'Predominantly thinly and evenly laminated, green and pale-green dolomitic argillite; weathers readily into tan platy rubble. Stromatolite heads or beds are scattered through the section but particularly in lower part. A few green strata have anomalous amounts of copper sulfides. Near middle of unit contains a zone about 300 ft thick that has two or more thin (10-ft-thick) beds of red interlaminated argillite and siltite. Ranges in thickness from about 1,200 ft in the southeast to about 2,200 ft in the north. Unit is missing in the west.' 1225|'Interbedded red and green interlaminated argillite and siltite contains mud cracks, mud chips, and ripple marks. Dolomitic beds scattered through section. Commonly has green-bed zone at base. Chlorite and muscovite on bedding planes. Green beds at places contain anomalous amounts of disseminated chalcocite and bornite. In northern area contains an interlaminated black and white argillite unit about 390 ft thick. Thickness of Snowslip ranges from about 1,600 to 1,800 ft. Unit is missing in southwest and west.' 1226|'The upper member of the Wallace, as exposed on the south side of Striped Peak and on Foolhen Mountain, was divided into three lithologic units by Shenon and McConnel (1939). The middle unit can be further subdivided, so that in those localities the upper member of the Wallace consists of, from youngest to oldest: (1) an upper zone of black, thinly laminated argillite similar to the lower zone but including beds showing intricate small folds from soft-sediment deformation; (2) a zone of dolomite, stromatolites, and oolites containing interbeds of green dolomitic argillite; (3) a zone of green planar-laminated argillite and siltite that is pyritic and dolomitic; and (4) a lower zone of thinly laminated black argillite containing some green laminated argillite, moderately abundant desiccation cracks, and cut-and-fill structures. The uppermost contact is fairly sharp and is marked by the appearance of green or red siltites and quartzites of the overlying Striped Peak Formation. The lowermost contact of the upper member grades into the alternating beds of black argillite and white siltite of the middle member of the Wallace. Lithologic units and their proportions vary in the upper member. To the north, the characteristic black laminated argillite is interbedded in various proportions with green laminated argillite and with minor amounts of red laminated argillite. In the type locality, the upper member is overlain by the basal unit of the Striped Peak (equals Mount Shields), but in other areas it is overlain by either the Snowslip or Shepard Formation. Ranges in thickness from 0 ft in the southeast to about 5,000 ft in the northwest.' 1227|'Middle member: Characteristic lithology is uneven and wavy bedded; black argillite alternates with slightly dolomitic white siltite or very fine grained quartzite layers that range from an inch to 20 in. in thickness; this is the unit exposed in the type locality of the Wallace in road cuts and valleys at the town of Wallace, Idaho. Also included are some beds of black laminated argillite, green interlaminated argillite and siltite, and dolomitic argillite. Zones of dolomitic argillite and siltite similar to rocks of the lower member, some of which show vertical ribbons of calcite ("molar tooth structure"), are commonly a few feet thick but may dominate in zones 1,300 ft thick. To the south and southeast, sedimentary breccias of high carbonate content are common and are associated with large slump folds in zones as thick as 60 ft. The breccias consist of angular fragments of quartzite and blocks of typical middle Wallace lithology suspended in a silty matrix and cemented by calcite, dolomite, iron-carbonate, and quartz; these breccias are commonly dotted by crystals or pseudomorphs of scapolite or shortite(?). Thicker breccias and slumped zones tend to be near the base, but occur throughout the middle member in the southwestern and south-central part of the area. The brecciated and slumped zones are well displayed in road cuts along Trout Creek south of Superior, Mont. Unit contains rare salt casts. Metamorphosed rocks in the south-central area have light-gray scapolite crystals speckled through black argillite. The contact with the lower member is gradational and is placed where the green laminated dolomitic argillites begin to dominate the section. Thickness ranges from about 7,000 ft in the south to about 4,400 ft in the north. Lower member: Dominantly green interlaminated dolomitic argillite and siltite, some of which contains irregular vertical ribbons ("molar tooth structure") and pods or irregular blobs of calcite. Most of the unit contains some carbonate, either as impure beds or cement. Beds containing stromatolites are scattered through the section. Unit also contains a few interbeds having lithologies similar to those characteristic of the middle and upper members. Lower contact is sharp where underlain by the quartzite member or by purple laminated argillites of the St. Regis, but is gradational where green laminated argillite occurs at the top of the St. Regis. Rocks typical of the unit are well exposed in cirques at the head of drainages on both sides of the Idaho-Montana line in the south-central area. Thickness ranges from about 8,200 ft in the south to about 500 ft in the north part of the map area.' 1228|'Characteristic lithology is uneven and wavy bedded; black argillite alternates with slightly dolomitic white siltite or very fine grained quartzite layers that range from an inch to 20 in. in thickness; this is the unit exposed in the type locality of the Wallace in road cuts and valleys at the town of Wallace, Idaho. Also included are some beds of black laminated argillite, green interlaminated argillite and siltite, and dolomitic argillite. Zones of dolomitic argillite and siltite similar to rocks of the lower member, some of which show vertical ribbons of calcite ("molar tooth structure"), are commonly a few feet thick but may dominate in zones 1,300 ft thick. To the south and southeast, sedimentary breccias of high carbonate content are common and are associated with large slump folds in zones as thick as 60 ft. The breccias consist of angular fragments of quartzite and blocks of typical middle Wallace lithology suspended in a silty matrix and cemented by calcite, dolomite, iron-carbonate, and quartz; these breccias are commonly dotted by crystals or pseudomorphs of scapolite or shortite(?). Thicker breccias and slumped zones tend to be near the base, but occur throughout the middle member in the southwestern and south-central part of the area. The brecciated and slumped zones are well displayed in road cuts along Trout Creek south of Superior, Mont. Unit contains rare salt casts. Metamorphosed rocks in the south-central area have light-gray scapolite crystals speckled through black argillite. The contact with the lower member is gradational and is placed where the green laminated dolomitic argillites begin to dominate the section. Thickness ranges from about 7,000 ft in the south to about 4,400 ft in the north.' 1229|'Dominantly green interlaminated dolomitic argillite and siltite, some of which contains irregular vertical ribbons ("molar tooth structure") and pods or irregular blobs of calcite. Most of the unit contains some carbonate, either as impure beds or cement. Beds containing stromatolites are scattered through the section. Unit also contains a few interbeds having lithologies similar to those characteristic of the middle and upper members. Lower contact is sharp where underlain by the quartzite member or by purple laminated argillites of the St. Regis, but is gradational where green laminated argillite occurs at the top of the St. Regis. Rocks typical of the unit are well exposed in cirques at the head of drainages on both sides of the Idaho-Montana line in the south-central area. Thickness ranges from about 8,200 ft in the south to about 500 ft in the north part of the map area.' 1231|'This formation is the eastern and more carbonate-rich facies of the "middle Belt carbonate." Predominantly dolomite, dolomitic siltite, and dolomitic argillite. Characterized by 3- to 10-foot-thick cycles that begin on a cut surface and consist of gray or green dolomitic argillite, fine-grained quartzite, and siltite that grade upward into impure dolomite, which is overlain by dense orange-weathering dolomite. Irregular calcite ovoids and ribbons commonly change from horizontal pods to vertical pods and, at some places, to vertical ribbons in the impure dolomitic beds. Stromatolites and oolites are present. Upper part has zones as much as 150 ft thick that are similar to the middle member of the Wallace Formation and consist of abundant black argillite irregularly interlayered with dolomitic siltite; these zones commonly display soft-sediment deformation structures. Pyrite and minor chalcopyrite scattered throughout unit. Thickness about 8,800 ft.' 1232|'Mostly thinly and evenly laminated light and dark-green dolomitic argillite or argillite and siltite. Flattened ovoids of pink or white calcite commonly weather out and leave distinctive zones of subhorizontal voids in the exposures. Mud chips and pebbles, mud cracks, and ripple marks are common. A few shallow channels are filled by subrounded plates of limestone (rip-up clasts). Contains red argillite zones near middle of unit. Pyrite common; anomalous amounts of chalcopyrite, chalcocite, and bornite in a few strata, particularly near base. Thickness from 0 ft in the west to about 1,200 ft in the east.' 1233|'Alternating beds of dark-purple and dark-green interlaminated argillite and siltite. Lower part has beds of light-purple siltite and very fine grained quartzite; upper part is dolomitic. Abundant deformed mud-crack casts, mud chips, and ripple marks. Some green strata contain anomalous amounts of copper sulfides. Type locality is at the headwaters of the Saint Regis River. Thickness ranges from about 1,000 to 3,000 ft.' 1234|'Three lithologic units can be found in most areas. The upper unit is about 400 ft thick and is similar to lower unit, but has more green beds and contains medium-grained white quartzite beds as much as 4 in. thick. In many areas, one or more of the green beds contains anomalous amounts of chalcocite and bornite. Middle unit is about 900 ft thick and consists of alternating beds of purple argillite and pink to purple-gray, very fine grained feldspathic quartzite or coarse siltite; quartzite beds show planar cross-bedding and are as much as 50 ft thick. The lower unit is about 1,700 ft thick and is dominantly purple to purple-gray, thinly laminated argillite or argillite and siltite; mud cracks, ripple marks, fluid-escape structures, mud-chip breccias, small ball-and-pillow structures, and flute casts are common. Some dolomite cement and scattered siderite specks. A few beds of pale-purple siltite and quartzite. Green dolomitic argillite layers a few feet thick occur near base and top; some contain anomalous amounts of copper sulfides.' 1235|'Characterized in its type locality (Coeur d’Alene district) by white medium-grained cross-bedded quartzite interbedded with white siltite and green laminated argillite. Quartzite shows horizontal lamination, planar cross-bedding, festoon cross-bedding, and climbing ripples; channels, load casts, and soft-sediment structures are common; heavy mineral laminations are present on cross-beds at many places. Argillitic rocks contain mud cracks, mud chips, and ripple marks. Northeast and east from type locality, unit contains progressively less quartzite and more siltite and argillite, much of which is purple or purple gray. Consists in most areas of three lithologic units: upper and lower units dominated by quartzite, and a middle unit containing quartzite interbedded with abundant argillite and siltite. Hematitic banding in quartzite and siltite commonly forms pseudocross-bedding that transects true bedding. Zone of abundant white quartzite trends north from type locality and contains, at places, anomalous amounts of silver and copper sulfide that reach ore grade in a few localities. Thickness of Revett ranges from about 500 ft to 2,500 ft.' 1236|'Several lithologic units can be identified at many places and commonly include the following four: (1) an upper zone similar to the one underlying it, except it contains white to purple-gray fine- to medium-grained quartzite beds 3-6 ft thick that increase in abundance upward; (2) a zone of green-gray to purple-gray interbedded blocky argillite and siltite that contain mud cracks and mud chips; (3) a zone dominated by parallel- and thinly laminated gray to purple-gray siltite in blocky planar beds that have minor argillite partings or beds; and (4) a lower zone of green parallel-laminated interbedded argillite and siltite that are generally in beds about 2 in. to 20 in. thick and give a blocky-flaggy outcrop (upper part of this zone has purple-gray interbeds of argillite and siltite). Siltites are commonly speckled by tiny euhedral magnetite crystals. Anomalous amounts of chalcocite occur near the top of the lower zone and in zone (2). Thickness ranges from about 2,500 ft in the type locality near Burke, Idaho, to about 3,450 ft in the north and northeast to the thickest known section of about 7,500 ft in the southeast.' 1237|'The upper part, as used here, includes a transition zone, as originally used by Hobbs and others (1965), consisting of pyritic black and gray laminated, wavy and lenticularly bedded argillite and siltite that contain thin blocky beds of pyritic green argillite and siltite, cut-and-fill structures, and ripple marks, plus a lower zone of pyritic, evenly laminated, black and gray argillite and siltite in planar couplets that range from 0.5 in. to 1 in. in thickness; the basal 650 ft contains some beds of planar-laminated quartzite. Base of this part is the top of the gray micaceous quartzite sequence that is present throughout the area and marks the top of the lower part of the Prichard Formation. Some mappers have limited the upper part to the transition zone only, whereas others have placed the transition zone in the Burke Formation; we have not changed their contacts in this compilation. Thickness of the unit ranges from about 1,970 ft to 5,250 ft, depending in large part on whether only the transition zone was mapped as the upper part of the Prichard.' 1238|'Consists of seven lithologic units. The informal members identified by Cressman (1981, 1982) in the area of Plains, Mont., include from top to bottom: (1)Planar-laminated medium-gray fine-grained micaceous quartzite interbedded with olive-gray siltite and medium-light-gray argillite; quartzite beds grade to argillite in top 0.5-1 in. Thickness 1,640 ft. (2)Interlaminated and very thinly interbedded dark-gray and light-gray argillite and siltite; pyrite or pyrrhotite laminae are present locally. Black tourmaline-bearing chert bed as much as 12 in. thick is present locally. Argillite pebble conglomerate that contains scattered clasts of galena-sphalerite is present locally in lower part. Thickness from 3,200 ft to 3,600 ft. (3)Interlaminated siltite and argillite interbedded with some quartzite, particularly in lower part; cross-bedding, scour-and-fill structures, mud cracks, and mud-chip breccias common. Thickness 2,710 ft. (4)Platy-weathering olive-gray silty argillite that contains abundant chlorite porphyroblasts and sparse garnet; some interbedded quartzite in lower part and interbedded siltite in upper part. Thickness about 950 ft. (5)Medium-gray fine-grained quartzite in beds 1-2 ft thick; planar laminated in part; cross-lamination in upper part of some beds; beds grade to argillite in top 0.5-1 in. Thickness 250 ft to 360 ft. (6)Graded siltite-argillite couplets several tenths of an inch to an inch thick; siltite is dark gray; argillite is light gray. Some black argillite laminations contain felted masses of tourmaline. About 15 percent of the member consists of slump folds in composite slump sheets that average 2.5 ft thick. Thickness about 3,280 ft. (7)A member similar in lithology to member above but without slump folds. About 1,970 ft exposed.' 1250|'Silt- to boulder-size, moderately sorted to well-sorted sediments in modern floodplains, including overbank deposits. Larger clasts are moderately rounded to well-rounded. Maximum thickness greater than 5 m.' 1251|'Black to dark-brown organic-rich clay, silt and sand in grassy or willow-covered flat outcrops. Mostly represents eutrophication of shallow lakes.' 1252|'Unconsolidated to slightly indurated, mostly massive, dark-brown to light-gray-brown deposits that mantle gently to moderately sloping surfaces; sediment types are intermixed by downslope movement. Colluvium contains cobbles and pebbles derived from weathering of bedrock; loess is very fine grained sand, silt, and minor clay. Commonly contains poorly to moderately developed soil profile in upper part. Includes alluvium in small channels and sheetwash on steeper hillsides. Some small areas (as in Tobacco Root Mountains) underlain by till in valleys above about 2,000 m. Unmapped in many areas, particularly where deposit is thin and forms discontinuous veneer. Maximum thickness probably less than 10 m.' 1253|'Angular and subangular cobbles and boulders at base of steep valley walls or cliffs. Boulders generally as large as 2 m, although in places as large as 10 m. Locally includes minor alluvial deposits and rock-glacier deposits. Maximum thickness greater than 20 m.' 1254|'Ranges from chaotically oriented debris to almost intact slump blocks of bedrock. About 10 m to greater than 50 m thick.' 1255|'Hummocky, lobate deposits of angular boulders having a frontal slope near the angle of repose; locally active. In places, grade into and include some talus (unit Qt). About 20 m thick.' 1256|'Moderately well sorted pebble- to boulder-size gravel in fan-shaped deposits at base of mountain fronts. Includes piedmont-slope deposits and debris-flow deposits.' 1257|'Moderately well-sorted cobble and boulder gravel that forms a prominent alluvial fan at mouth of Cedar Creek in Madison Valley. Clasts rounded to subrounded. Surface characterized by braided stream pattern. Locally mantled by loess less than 1 m thick. Fan formed mainly during Pinedale (about 20-14 ka) and Bull Lake (about 140-100 ka) glaciations; deposits of both ages exposed at surface (Ritter and others, 1990); minor proximal fan development during Holocene. Modern channel of Cedar Creek incised about 10 m into fan deposits near mouth of creek, indicating proximal fan deposits presently eroding. As much as 150 m thick. Unit shown separately from other fan deposits due to its prominent size and detailed studies (Ritter and others, 1990).' 1258|'Unsorted, unstratified, unconsolidated, subangular to subrounded boulders in an unsorted matrix as fine as silt. Most till deposited during Pinedale (about 20-14 ka) and Bull Lake (about 140-100 ka) glaciations. Pinedale-age till preserved in hummocky deposits that contain numerous closed depressions and have a thin or non-existent soil profile; deposits of Bull Lake-age till have more rounded topography, are more dissected, and generally exhibit a well-developed soil profile. As much as about 50 m thick.' 1259|'Unsorted, unstratified, nonconsolidated deposit composed of angular to subangular clasts of Archean gneiss as large as 2 m. Believed to be mostly debris-flow deposits, but may include some till and landslide deposits. Mapped only in Jack Creek area. Maximum thickness greater than 20 m.' 1260|'Moderately sorted, moderately rounded to well-rounded sand and gravel. Underlies about 12 recognized terrace surfaces in Madison Valley. Bearzi (1987) recognized 11 geomorphic surfaces along Jack Creek. In Madison Valley, east of Madison River, includes all but the highest (and oldest) terrace-gravel deposit, which is shown separately (unit Qgc). West of Madison River in Madison Valley, highest terrace-gravel deposits may correlate with terrace-gravel deposits of the Cameron bench (unit Qgc). Mantled by less than 2 m of loess at most places, although loess on many higher surfaces is thick enough to support cultivation. Mostly less than 10 m thick.' 1261|'Moderately sorted, moderately rounded to well-rounded sand and gravel underlying topographically highest (and oldest) terrace deposits in Madison Valley. Formed during Bull Lake glaciation (about 140-100 ka) (Schneider and Ritter, 1987). Mantled by less than 2 m of loess in most places.' 1262|'Interbedded, moderately sorted, poorly consolidated to unconsolidated, well-bedded to massive silt, sand, and well rounded cobbles that are exposed in slopes adjacent to, and below, terraces in Madison Valley. Exposed basin-fill deposits are interpreted to be no older than Pliocene, although they overlie deeply buried basin-fill deposits, as thick as 4,500 m, that may be as old as Eocene in the Madison Valley (Rasmussen and Fields, 1983).' 1263|'Light brown to gray, massive welded tuff that contains sparse phenocrysts of sanidine, quartz and plagioclase. Matrix is mostly devitrified glass shards, opaque minerals, and aphanitic minerals. Contains sparse pumice fragments. Lithophysae and vesicles not observed in quadrangle. Forms prominent cap to many ridges and buttes. Age of unit is 2.0 Ma and source is from Yellowstone caldera (Christiansen and Blank, 1972), the northern edge of which is about 30 km south of quadrangle’s southeastern corner. Thickness increases to south and maximum thickness in quadrangle about 50 m.' 1264|'White, light gray, and light-brownish-gray, moderately to well indurated, locally vuggy limestone, limy sandstone, ash-bearing sandstone, pebbly sandstone, and pebble conglomerate. Interlayered ash deposits common, especially is association with limestone. Limestone may be either lacustrine deposits or carbonate paleosols (Hanneman and others, 1991). Mapped along Madison River, east of Ennis Lake, and in Spanish Creek basin. One maximum 40Ar/39Ar age on sanidine from just east of Ennis Lake in Madison Valley (NE 1/4, Sec. 6, R. 1 E., T. 5 S.), is 16.2±0.19 Ma (K.K. Kellogg and S.S. Harlan, unpublished data, 1990). Deposits in Spanish Creek basin undated, but probably Miocene. Maximum thickness greater than 1,000 m.' 1265|'Light brownish-gray to yellowish-gray, locally vuggy, well-bedded, fine-grained limestone. Similar to thin limestones associated with ash-bearing sediments in unit Ts, although much thicker, so is considered to be Miocene in age; maximum thickness greater than 20 m. Mapped along west side of Madison Valley (Hadley, 1969a, b).' 1266|'Probably basaltic, contains fragments of clay and basalt (Wier, 1982). Intrudes both volcanic rocks and Archean gneiss. One oval outcrop mapped, about 200 m in diameter, just south of Virginia City.' 1267|'Mostly dense to scoriaceous basalt and andesite flows, volcanic agglomerates, sandy tuff, and interbedded basaltic cinder deposits. Flows are black and dark brown and contain small phenocrysts of calcic plagioclase, olivine, augite, opaque minerals, and, in places, potassium feldspar and biotite (Hadley, 1980). At least one silicic (latitic?) plagioclase-phyric flow occurs near base of unit, 4 km southeast of Virginia City. Three localities gave the following potassium-argon (K-Ar) dates: 1) 49.3±2.5 and 51.1±1.9 Ma (biotite) and 51.0±3.8 Ma (plagioclase); this flow is just north of Virginia City and overlies Archean basement, 2) 34.4±3.0 Ma (whole rock), and 3) 32.7±1.4 Ma (whole rock) (Marvin and others, 1974). Unit greater than 200 m thick.' 1268|'Dark-gray to black aphanitic rhyolite flows north and northwest of Ennis Lake (Kellogg, 1993b; Vitaliano and Cordua, 1979). Cut by many faults; local steep dips. Correlated with volcanic rocks of Virginia City volcanic field.' 1269|'Pale gray to white rhyolitic air-fall ash, tuffaceous sandstone, and sparse gravel lenses. Poorly exposed in most places and occurs locally at base of sequence of volcanic rocks of Virginia City volcanic field. Large areas mapped by Hadley (1969b) as felsic tuff are underlain mostly by chaotic, locally tuffaceous landslide deposits. Thickness as great as about 40 m.' 1270|'Coarse-grained, well bedded, cross-bedded volcaniclastic sandstone, pebbly sandstone, and pebble and cobble conglomerate that in places underlie the rocks of the Virginia City volcanic field (Hadley, 1969b, 1980).' 1272|'Mapped in Madison Valley in southern part of quadrangle (Hadley, 1969b); may be outlier of volcanic rocks of Virginia City volcanic field.' 1273|'Light- to medium-gray, and brownish-gray, dense, porphyritic dacite. Phenocrysts are euhedral plagioclase, hornblende, and biotite. Petrographically very similar to dacite porphyry of Fan and Lone Mountains, but at least some stocks intrudes Eocene volcanic rocks; other nearby rocks of similar composition intrude Cambrian strata and Archean gneiss. One potassium-argon biotite date of 49.5±1.5 Ma (Chadwick, 1969, p. 160).' 1274|'Dark gray, very fine-grained basalt or andesite intrusive breccia containing sparse, small phenocrysts of plagioclase. Forms two small necks in upper Porcupine Creek, east of Gallatin Valley. Intrudes Eocene volcanic rocks and Archean gneiss. Not included in Absaroka Volcanic Supergroup by Simons and others (1985), but unit probably formed feeder vents for younger rocks of Supergroup.' 1275|'Mapped where proportion of mostly andesitic and basaltic flows predominate over volcaniclastic rocks, as mapped by the U.S. Geological Survey (1972). Compositions similar to those in Sepulcher Formation, described below. Conformably overlies rocks of Sepulcher Formation.' 1276|'Mafic and intermediate-composition volcaniclastic rocks, flow breccias, and lava flows. Contains sparse beds of light-reddish-brown welded tuffs. Includes the Fortress Mountain and Daly Creek Members (Smedes and Prostka, 1972), and Hyalite Peak Volcanics (Chadwick, 1969; Hiza, 1994). Most of unit composed of dark grayish-brown laharic breccias, volcaniclastic conglomerate, and light- to medium-gray volcaniclastic sandstone. Both laharic deposits (vent facies) and conglomeratic deposits (alluvial facies) are greater than 100 m thick in places, crudely stratified, and well indurated. Laharic deposits composed of unsorted, angular to subrounded volcanic debris containing clasts as large as 3 m across. Both lahars and conglomerates contain much silicified wood, commonly as large trunks as thick as 3 m, in growth positions. Lavas are mostly medium-gray to dark-gray, fine-grained to sparsely porphyritic pyroxene or olivine-pyroxene andesite and basalt. Flows with compositions as silicic as latite or trachyandesite occur in upper part of section (as on Steamboat Mountain). Proportion of flows appears to increase upward in section. Basal Daly Creek Member of Sepulcher Formation commonly mapped on basis of lighter color and preponderance of alluvial-facies material. However, except for a gradual increase upward in percentage of flows, no distinct difference was noted between lower and upper parts of Sepulcher Formation.' 1277|'Cobbles and boulders predominantly of Archean rocks, with subordinate clasts of Paleozoic carbonate and Tertiary volcanic rocks, mapped near Garnet Mountain in the northeastern part of quadrangle (McMannis and Chadwick, 1964). Conglomerate is locally overlain by a thin, discontinuous siltstone of late early Eocene age. Not overlain by volcanic rocks in quadrangle, but similar rocks in adjacent areas occur at the base of volcanic sequences of the Absaroka Supergroup.' 1278|'Greenish-gray to pinkish-gray, sparsely porphyritic, massive, siliceous dacite or rhyolite. Most phenocrysts consist of as much as 15 percent white, strongly saussuritized plagioclase crystals as long as 3 mm. Rock commonly stained black by manganese oxide. Mainly forms thin sills, dikes, and irregular pods intruding exclusively Archean rocks. Described by Kellogg (1993b). Unit undated, but shares compositional and textural similarities with both Upper Cretaceous dacite porphyry of Fan and Lone Mountains and Eocene rhyolite and rhyodacite plugs near Norris (Kellogg, 1994, 1995), about 10-15 km north of quadrangle.' 1279|'Black, medium-grained plagioclase-clinopyroxene gabbro that typically weathers into spheroidal blocks. In places completely weathered into grussy orange-brown soil. Several larger sills contain irregular dikes, as wide as 2 m, of fine-grained pinkish-gray inequigranular clinopyroxene-biotite syenite, in which the clinopyroxene forms conspicuous rods as long as 1 cm. Syenite, in turn, intruded by thin aplitic dikes. Unit crops out near Meadow Village at Big Sky and intrudes Upper and Lower Cretaceous rocks.' 1280|'Gray to greenish-gray porphyritic dacite that weathers to a very light gray or tan. Euhedral to subhedral phenocrysts compose 30-50 percent of the rock; phenocrysts, 70-90 percent zoned plagioclase (An25-35) as long as 1 cm, trace to 15 percent green hornblende as long as 6 mm, 0-15 percent biotite as long as 3 mm, and 0-10 percent equant quartz as long as 5 mm. Mafic minerals commonly altered to chlorite and epidote; plagioclase sericitized. Matrix is very fine grained and dense, and contains about 5 percent opaque minerals. Fine-grained chill zone extends inward several centimeters from contacts. Commonly contains mafic autoliths as large as about 0.5 m. Forms sills, some greater than 80 m thick, in two intrusive centers underlying Fan Mountain and Lone Mountain. The sills are thought to form a "Christmas-tree laccolith" complex underlying both mountains (Swanson,1950), although Kellogg (1992) found no evidence for a central "trunk." Late Cretaceous beds dip away from the central peaks at both Fan and Lone Mountains. Intrudes rocks that range in age from Middle Cambrian to Late Cretaceous. Where less than about 20 m thick, unit is indicated on map by a single line. Potassium-argon (K-Ar) and 40Ar/39Ar ages from hornblende are about 68-69 Ma (Tysdal and others, 1986; K.S. Kellogg and S.S. Harlan, unpublished data).' 1281|'Gray, coarse-grained, inequigranular to porphyritic, massive hornblende-biotite granodiorite, monzogranite, and monzodiorite (classification of Streckeisen, 1976). Typically contains 50 percent normally zoned oligoclase (An25-An28), 15-25 percent green hornblende, 10-20 percent microcline (slight development of braid perthite) commonly as phenocrysts as long as 3 cm, 0-20 percent quartz, 5 percent biotite, 0-1 percent clinopyroxene cores in some hornblende, trace to 3 percent magnetite, and traces of apatite and conspicuous sphene and zircon; hypidiomorphic texture. Includes minor dark-gray hornblende diorite and granodiorite along border and in satellitic stocks east of main batholith. Weathers light gray, in rounded tors or flat grussy outcrops. K-Ar age of batholith is 71-74 Ma (Vitaliano and Cordua, 1979).' 1282|'Reddish-orange conglomerate with sandstone matrix cemented by calcite and hematite. Contains well-rounded cobbles and boulders derived from Paleozoic and Mesozoic strata shed from thrust plates to west; represents an unroofing sequence, with clasts generally increasing in age upwards (DeCelles and others, 1987). Lower contact is conformable with underlying Livingston Formation. Well exposed only on Sphinx Mountain and The Helmet. Minimum thickness about 610 m.' 1283|'Conglomerate and interbedded limestone containing well-rounded clasts of Archean gneiss and Belt Supergroup quartzite. Mapped by Hadley (1969b) as two separate units (Sphinx Conglomerate and Tertiary gravel) but both units reinterpreted on fossil and stratigraphic evidence as Beaverhead Group rocks of Cretaceous age (J.M. O’Neill, personal commun., 1997).' 1284|'Livingston Formation exposed on west side of Madison Range, near Sphinx Mountain, and east of Gallatin River, south of the Spanish Peaks fault. Does not crop out west of Madison Valley. Undivided unit shown only in southeastern part of quadrangle. Description after Tysdal (1990).' 1285|'Cobble and boulder conglomerate of well-rounded volcanic clasts in matrix of coarse-grained volcaniclastic sandstone; upper part predominantly sandstone. Lower contact is conformable. Thickness as much as 180 m.' 1286|'Brown, maroon, and gray dacite to basalt flows, autoclastic breccia, tuff breccia, and welded tuff; minor interlayered volcaniclastic sandstone. Lower contact is conformable. Thickness estimated 300-450 m.' 1287|'Complexly intertonguing units of dark-green to dark-brown, medium- to coarse-grained, locally pebbly volcaniclastic sandstone, olivine basalt, mudflow breccia, volcaniclastic conglomerate, and mudstone. Conformable with underlying Everts(?) Formation. Thickness ranges from about 60 to 210 m).' 1288|'May locally include Muddy Sandstone and Thermopolis Shale. Mapped in area of Cretaceous outcrops east of Gallatin River.' 1289|'Everts Formation - Thin- to medium-bedded light-gray to dark-gray, poorly to moderately sorted quartz-rich sandstone; intercalated with thin-bedded, greenish-gray to dark gray mudstone and siltstone. Contains a few thin beds of dense limestone, porcellanite, and coal. About equal quantities of sandstone and finer-grained sedimentary rocks. Does not crop out west of Madison Valley. Conformably overlies Virgelle Sandstone. About 425 m thick. Virgelle Sandstone - Thin- to thick-bedded, medium- to coarse-grained, light-brown to yellowish-brown, trough-cross-bedded quartz sandstone that forms prominent white-weathering ledges. Conformably overlies Telegraph Creek Formation. Does not crop out west of Madison Valley. About 25-50 m thick.' 1290|'Light-gray to dark-brown, thin- to medium-bedded, feldspathic, calcite-cemented sandstone and interbedded dark-gray siltstone and mudstone. Middle marker sequence is a 20-m thick white tuffaceous siltstone and sandstone. Contains thin flaggy sandstones in lower part. Conformably overlies Cody Shale. Does not crop out west of Madison Valley. Thickness about 200 m.' 1291|'Upper part consists of thin-bedded black fissile shale that is interbedded with minor amounts of thin-bedded, brown, commonly bioturbated calcareous, fine-grained sandstone. Lower part consists of black, fissile shale and minor siltstone that weathers dark gray. Does not crop out west of Madison Valley. Conformably overlies Frontier Formation. Thickness about 300 m.' 1292|'Mostly a sequence of alternating black shale and light-gray to yellowish-tan, thin- to very thick bedded, cross-bedded sandstone. In the Madison Range, sandstone ledges as thick as 3 m; ratio of sandstone to shale about 1:3. Shale is locally carbonaceous or coaly. Shale sequences are as thick as about 20 m and commonly contain equally spaced 5- to 10-cm-thick sandstone beds, spaced 15-20 cm apart. Contains several white porcellanite beds; one prominent porcellanite bed in the Madison Range, about 6 m thick, is about 15 m above base of sequence and displays well-developed ball-and-pillow structures. Conformable with underlying Mowry Shale. Formation thickens greatly to the west; thickness in Madison Range about 140-180 m while that in Gravelly Range is greater than 1,500 m.' 1293|'Mostly brownish-gray and greenish-gray tuffaceous mudstone and shale and, in upper part, contains abundant thin sandstone beds. Lower Vaughn Member consists of conspicuous gray, green, yellow, brown, orange, pink, and cream-colored bentonitic mudstone, porcellanite, siltstone, and minor interbedded quartz sandstone, although strikingly varicolored units missing in northern Madison Range. Poorly exposed in most places. Unconformable with underlying Muddy Formation. Thickness 90-180 m.' 1294|'Muddy Sandstone: Thin- to medium-bedded, medium- to coarse-grained, brown to brownish-gray, poorly to moderately indurated, clayey, ledge-forming, salt-and-pepper sandstone; locally contains mud chips as long as 1 cm. In northern Madison Range, formation typically exposed as upper and lower sandstone sequence with central, poorly exposed shaly sequence. Thickness varies widely; in the northern Madison Range is about 20-45 m thick; further south in the Madison Range it is as great as 107 m (Tysdal, 1990); in the Gravelly Range it is about 15 m thick (Hadley, 1969b, 1980). Thermopolis Shale: Composed of an upper sequence (three-quarters of the total thickness) that is black to dark-gray, locally carbonaceous fissile shale and poorly indurated, thin-bedded, silty brown sandstone. Lower one-quarter of unit is thin- to medium-bedded, fine-grained white to tan quartzite that contains black shale interbeds; Ripple marks and Liesegang bands are common in sandstone beds. Unconformable contact with underlying Kootenai Formation. Thickness throughout quadrangle about 70-80 m.' 1295|'Upper 10-15 m is medium-bedded light-gray, micritic, oolitic limestone that contains abundant gastropod fossils in uppermost part. Middle part is variegated red, purple, yellow, and gray shale, mudstone, siltstone, sandstone, and locally nodular freshwater limestone. Formation typically weathers to a reddish soil. Lower 10-40 m is medium- to coarse-grained gray, well-indurated, ledge-forming salt-and-pepper sandstone that contains a basal chert-pebble conglomerate as thick as 1 m. Lower contact is unconformable. Total thickness in the northern Madison Range is about 90 m (Kellogg, 1992, 1993a); in the Gravelly Range it is as thick as 170 m (Hadley, 1980), and in the southeastern part of the map area it as much as about 130 m thick.' 1296|'Morrison Formation: Upper 20-30 m is mostly black and locally purple shale that contains minor intercalated thin- to medium-bedded, rusty-brown and gray quartz sandstone that is lensoidal in places. The lower, thicker part of the Morrison is composed of thin-bedded, gray, yellow, orange, red, and green shale and siltstone interbedded with lesser amounts of gray quartzite and thin-bedded, brown limestone. Unconformable with underlying unit. About 75-100 m thick. Ellis Group includes: Swift Sandstone--brown, medium- to coarse-grained glauconitic sandstone, locally containing abundant chert pebbles, and minor olive-green shale; glauconite commonly weathered to orange limonitic clots. More calcareous in eastern part of map area (Tysdal, 1990). Unconformably overlies Rierdon Limestone. Less than 12 m thick. Rierdon Limestone - Thin- to thick-bedded, yellowish-gray to brownish-gray, fine-grained, oolitic limestone; contains a few limy siltstone interbeds. Bivalves Camptonectes sp. and Gryphaea sp. locally abundant. Forms prominent cliffs near Shell Creek in western Madison Range. Thickness about 8-30 m. Sawtooth Formation - Interbedded limestone and shale, weathers yellowish-brown, and is very poorly exposed. Sawtooth unconformably overlies Dinwoody Formation. Thickness 25-55 m. Woodside Siltstone - Brick-red to orange-red, thin-bedded siltstone and mudstone, interbedded with gypsum and scattered thin beds of gray limestone. Mostly poorly exposed, although forms prominent red soil. Thickness increases markedly to southeast; nonexistent north of Porcupine Creek in the Gallatin Range (Simons and others, 1985) and only a "feather edge" may exist in the northernmost Gravelly Range (Hadley, 1980). Unit previously mapped as Chugwater Formation in the southeastern part of the quadrangle (Simons and others, 1985). Thickness 0 to about 220 m (Tysdal, 1990). Dinwoody Formation - Brown, silty sandstone, siltstone, and thin-bedded brown limestone. Small brachiopods locally abundant. Weathers light yellowish-brown and is poorly exposed. Dinwoody unconformably overlies Shedhorn Sandstone. Thickness ranges between about 20 and 80 m.' 1297|'Upper 20-30 m is mostly black and locally purple shale that contains minor intercalated thin- to medium-bedded, rusty-brown and gray quartz sandstone that is lensoidal in places. The lower, thicker part of the Morrison is composed of thin-bedded, gray, yellow, orange, red, and green shale and siltstone interbedded with lesser amounts of gray quartzite and thin-bedded, brown limestone. Unconformable with underlying unit. About 75-100 m thick.' 1298|'In most places this combined unit is poorly exposed. Total thickness about 185 m. Swift Sandstone - Brown, medium- to coarse-grained glauconitic sandstone, locally containing abundant chert pebbles, and minor olive-green shale; glauconite commonly weathered to orange limonitic clots. More calcareous in eastern part of map area (Tysdal, 1990). Unconformably overlies Rierdon Limestone. Less than 12 m thick. Rierdon Limestone - Thin- to thick-bedded, yellowish-gray to brownish-gray, fine-grained, oolitic limestone; contains a few limy siltstone interbeds. Bivalves Camptonectes sp. and Gryphaea sp. locally abundant. Forms prominent cliffs near Shell Creek in western Madison Range. Thickness about 8-30 m. Sawtooth Formation - interbedded limestone and shale, weathers yellowish-brown, and is very poorly exposed. Sawtooth unconformably overlies Dinwoody Formation. Thickness 25-55 m. Woodside Siltstone - Brick-red to orange-red, thin-bedded siltstone and mudstone, interbedded with gypsum and scattered thin beds of gray limestone. Mostly poorly exposed, although forms prominent red soil. Thickness increases markedly to southeast; nonexistent north of Porcupine Creek in the Gallatin Range (Simons and others, 1985) and only a "feather edge" may exist in the northernmost Gravelly Range (Hadley, 1980). Unit previously mapped as Chugwater Formation in the southeastern part of the quadrangle (Simons and others, 1985). Thickness 0 to about 220 m (Tysdal, 1990). Dinwoody Formation - Brown, silty sandstone, siltstone, and thin-bedded brown limestone. Small brachiopods locally abundant. Weathers light yellowish-brown and is poorly exposed. Dinwoody unconformably overlies Shedhorn Sandstone. Thickness ranges between about 20 and 80 m.' 1299|'Mapped in Garnet Mountain quadrangle by McMannis and Chadwick (1964), who used the term Phosphoria Formation instead of Shedhorn Sandstone. Shedhorn Sandstone - Mostly medium- to massive-bedded, gray, fine- to coarse-grained, very well indurated quartz-rich sandstone that contain white cherty stringers and nodules. Thin, shaly phosphorite beds in middle of formation are rarely exposed. Dolomitic near base. In Gravelly Range, chert is locally massive and filled with voids, so that chert outcrops commonly have a rubbly appearance. Conformably overlies Quadrant Sandstone. Formation about 35-70 m thick, though is about 50 m thick in northern Madison Range. Quadrant Sandstone - Medium- to thick-bedded, white to yellowish-tan, well-sorted, fine- to medium-grained, dolomite-cemented quartzite; cross-beds common. Lower half and at least upper 15 m of formation contain a few medium-bedded, light-yellowish-tan dolostone beds. Crops out prominently. Conformable contact with underlying Amsden Group. Thickness about 75 m. Amsden Group - Varies widely, both in stratigraphy and thickness. In central and southern Madison Range, and in the Gravelly Range east of the Greenhorn Thrust, the unit consists of about 12 to 50 m of brick-red, reddish-brown, and pink calcareous siltstone, silty shale, shale, sandstone, and light-gray limestone and yellowish-gray dolomite. Along western Madison Range, upper part contains medium- to thin-bedded gray to grayish-tan dolostone, locally quartzitic, that grades downward into gray limestone beds (correlated by Kellogg, 1992, with the Middle and Lower Pennsylvanian Devils Pocket Formation and Alaska Bench Limestone of Wardlaw and Pecora, (1985); this sequence overlies maroon siltstone and shale that contain a few thin beds of gray limestone and dolostone (correlated with Lower Pennsylvanian Tyler Formation of Wardlaw and Pecora, 1985). Total thickness of Amsden Group rocks in the western Madison Range is 88 m.' 1300|'Mostly medium- to massive-bedded, gray, fine- to coarse-grained, very well indurated quartz-rich sandstone that contain white cherty stringers and nodules. Thin, shaly phosphorite beds in middle of formation are rarely exposed. Dolomitic near base. In Gravelly Range, chert is locally massive and filled with voids, so that chert outcrops commonly have a rubbly appearance. Conformably overlies Quadrant Sandstone. Formation about 35-70 m thick, though is about 50 m thick in northern Madison Range.' 1301|'Quadrant Sandstone - Medium- to thick-bedded, white to yellowish-tan, well-sorted, fine- to medium-grained, dolomite-cemented quartzite; cross-beds common. Lower half and at least upper 15 m of formation contain a few medium-bedded, light-yellowish-tan dolostone beds. Crops out prominently. Conformable contact with underlying Amsden Group. Thickness about 75 m. Amsden Group - Varies widely, both in stratigraphy and thickness. In central and southern Madison Range, and in the Gravelly Range east of the Greenhorn Thrust, the unit consists of about 12 to 50 m of brick-red, reddish-brown, and pink calcareous siltstone, silty shale, shale, sandstone, and light-gray limestone and yellowish-gray dolomite. Along western Madison Range, upper part contains medium- to thin-bedded gray to grayish-tan dolostone, locally quartzitic, that grades downward into gray limestone beds (correlated by Kellogg, 1992, with the Middle and Lower Pennsylvanian Devils Pocket Formation and Alaska Bench Limestone of Wardlaw and Pecora, (1985); this sequence overlies maroon siltstone and shale that contain a few thin beds of gray limestone and dolostone (correlated with Lower Pennsylvanian Tyler Formation of Wardlaw and Pecora, 1985). Total thickness of Amsden Group rocks in the western Madison Range is 88 m. Snowcrest Range Group - Mapped only in western Madison Range (Kellogg, 1992), although rocks of the group may have been either unrecognized elsewhere or placed in the Amsden Group. The upper 57 m of the Snowcrest Range Group (Upper Mississippian Lombard Limestone) is mostly thin- to medium-bedded gray limestone that becomes more dolomitic and shaly toward base. The Lombard overlies the Kibbey Sandstone, which consists of about 75 m of thin- to medium-bedded yellowish-gray to maroon friable dolomitic sandstone, sandy dolostone, and siltstone.' 1302|'Mapped only west of Greenhorn thrust in Gravelly Range (Hadley, 1969b, 1980); upper, thicker part consists of thin- to thick-bedded, pale olive gray, locally very fossiliferous limestone and minor dark-greenish or olive-gray mudstone and shale. Lower 60 m or so consists of red and greenish-gray shale and calcareous sandstone and minor greenish-gray cherty limestone; sandstone and red color increase downward in sequence. Detailed description of the Big Snowy Group is given by Hadley (1980). Total thickness 135-275 m.' 1303|'Includes: Mission Canyon Limestone - Medium- to massive-bedded, light-gray, gray, and brownish-gray, medium-crystalline limestone and minor dolostone. Weathers light gray. Chert stringers and nodules common; locally fossiliferous; solution breccias in uppermost part. Prominent ridge-forming unit. Conformable with underlying Lodgepole Limestone. Thickness about 240 m. Lodgepole Limestone - Thin- to medium-bedded, gray to brownish-gray, finely crystalline limestone that commonly grades into silty limestone interbeds several centimeters thick. Locally cherty; upper half is profusely fossiliferous. Conformable with underlying Three Forks Formation (erroneously described as unconformable above the Three Forks in Kellogg, 1992, 1993a). Thickness about 180 m.' 1304|'Medium- to massive-bedded, light-gray, gray, and brownish-gray, medium-crystalline limestone and minor dolostone. Weathers light gray. Chert stringers and nodules common; locally fossiliferous; solution breccias in uppermost part. Prominent ridge-forming unit. Conformable with underlying Lodgepole Limestone. Thickness about 240 m.' 1305|'Thin- to medium-bedded, gray to brownish-gray, finely crystalline limestone that commonly grades into silty limestone interbeds several centimeters thick. Locally cherty; upper half is profusely fossiliferous. Conformable with underlying Three Forks Formation (erroneously described as unconformable above the Three Forks in Kellogg, 1992, 1993a). Thickness about 180 m.' 1306|'Three Forks formation - Mostly thin-bedded, yellowish-orange to yellowish-tan siltstone and silty limestone containing a few thin interbeds of brick-red-weathering siltstone. Weathers light yellowish tan. Contains a medium-gray, irregularly bedded, approximately 12-m-thick, rough-weathering limestone about 25 m above base of unit (Logan Gulch Member). Formation poorly exposed; forms slopes and swales. Unconformably overlies Jefferson Dolostone. Thickness 40-60 m. Jefferson Formation - Thin- to thick-bedded, black, brown, dark-gray, and light-gray, petroliferous, medium-crystalline to coarsely crystalline dolostone; colors vary considerably over short stratigraphic intervals. Weathers mostly brown and outcrops are typically knobby and irregular; forms conspicuous brown hoodoos. At least upper 15 m is thick- to massive-bedded, gray dolostone solution breccia (Birdbear Member). Unconformably overlies Bighorn Dolomite(?) in northern Madison Range. Thickness about 110 m.' 1307|'Bighorn Dolomite(?) - Medium-gray, sugary dolostone in 0.2- to 1.0-m-thick beds; weathers very light gray. Tentatively placed in Bighorn Dolomite by Hanson (1952) along ridge in NE 1/4, sec. 22 and NW 1/4, sec. 23, T. 5 S., R. 1 E in northwestern Madison Range; not recognized south of this location. Unconformably overlies Red Lion Formation. Thickness about 3 m at location noted above. Red Lion Formation - Thin-bedded, medium-gray to tan, siliceous dolostone containing conspicuous orange-tan to reddish-tan, cherty stringers as thick as 2 cm; lower 7 m contains intraformational clasts as large as 5 mm. Along west side of Gallatin Range, contains a lower greenish-gray, locally quartzitic shale sequence as thick as 10 m (Dry Creek Shale Member) (McMannis and Chadwick, 1964). Unconformably overlies Pilgrim Formation. Thickness about 40-60 m. Pilgrim Dolomite - Gray, light-gray, and brownish-gray, medium- to massive-bedded, locally oolitic, medium-crystalline dolostone. Weathers light gray and contains irregularly shaped darker gray mottles. Conformably overlies Park Shale. Forms conspicuous crags. Thicknesses about 30 m in northern Madison Range (Kellogg, 1992), about 60 m in the south-central part of the quadrangle (Tysdal, 1990); as much as 120 m in the Gravelly Range, although is missing along east side of range (Hadley, 1969b), and as much as 75 m in the northern Gallatin Range (McMannis and Chadwick, 1964). Park Shale - Greenish-gray to tan, fissile shale. Poorly exposed, slope-forming unit conformably overlies Meagher Limestone. Thickness about 30-50 m in the Madison Range and Gravelly Range, although is missing along eastern side of Gravelly Range (Hadley, 1969b); thickness 50-75 m in northern Gallatin Range (McMannis and Chadwick, 1964).' 1308|'Mapped east of Gallatin River and north of Spanish Peaks fault (McMannis and Chadwick, 1964) Red Lion Formation - Thin-bedded, medium-gray to tan, siliceous dolostone containing conspicuous orange-tan to reddish-tan, cherty stringers as thick as 2 cm; lower 7 m contains intraformational clasts as large as 5 mm. Along west side of Gallatin Range, contains a lower greenish-gray, locally quartzitic shale sequence as thick as 10 m (Dry Creek Shale Member) (McMannis and Chadwick, 1964). Unconformably overlies Pilgrim Formation. Thickness about 40-60 m. Pilgrim Dolomite - Gray, light-gray, and brownish-gray, medium- to massive-bedded, locally oolitic, medium-crystalline dolostone. Weathers light gray and contains irregularly shaped darker gray mottles. Conformably overlies Park Shale. Forms conspicuous crags. Thicknesses about 30 m in northern Madison Range (Kellogg, 1992), about 60 m in the south-central part of the quadrangle (Tysdal, 1990); as much as 120 m in the Gravelly Range, although is missing along east side of range (Hadley, 1969b), and as much as 75 m in the northern Gallatin Range (McMannis and Chadwick, 1964).' 1309|'Mapped south of Cedar Creek and east of Madison Valley (Tysdal, 1990; Kellogg, 1992). Red Lion Formation - Thin-bedded, medium-gray to tan, siliceous dolostone containing conspicuous orange-tan to reddish-tan, cherty stringers as thick as 2 cm; lower 7 m contains intraformational clasts as large as 5 mm. Along west side of Gallatin Range, contains a lower greenish-gray, locally quartzitic shale sequence as thick as 10 m (Dry Creek Shale Member) (McMannis and Chadwick, 1964). Unconformably overlies Pilgrim Formation. Thickness about 40-60 m. Pilgrim Dolomite - Gray, light-gray, and brownish-gray, medium- to massive-bedded, locally oolitic, medium-crystalline dolostone. Weathers light gray and contains irregularly shaped darker gray mottles. Conformably overlies Park Shale. Forms conspicuous crags. Thicknesses about 30 m in northern Madison Range (Kellogg, 1992), about 60 m in the south-central part of the quadrangle (Tysdal, 1990); as much as 120 m in the Gravelly Range, although is missing along east side of range (Hadley, 1969b), and as much as 75 m in the northern Gallatin Range (McMannis and Chadwick, 1964). Park Shale - Greenish-gray to tan, fissile shale. Poorly exposed, slope-forming unit conformably overlies Meagher Limestone. Thickness about 30-50 m in the Madison Range and Gravelly Range, although is missing along eastern side of Gravelly Range (Hadley, 1969b); thickness 50-75 m in northern Gallatin Range (McMannis and Chadwick, 1964).' 1310|'Mapped east of Gallatin River and north of Spanish Peaks fault (McMannis and Chadwick, 1964) Park Shale - Greenish-gray to tan, fissile shale. Poorly exposed, slope-forming unit conformably overlies Meagher Limestone. Thickness about 30-50 m in the Madison Range and Gravelly Range, although is missing along eastern side of Gravelly Range (Hadley, 1969b); thickness 50-75 m in northern Gallatin Range (McMannis and Chadwick, 1964). Wolsey Shale - Mostly thin-bedded, greenish-gray, olive-drab, gray, and grayish-brown micaceous sandstone, siltstone, and shale. Sandstone beds are wavy and bioturbated, contain green and gray- and green-mottled shale interbeds, and generally weather brown; animal trails common; locally glauconitic. Near middle of unit is a 10- to 15-m-thick section of thin-bedded, dark-gray, brown-weathering argillaceous limestone interbedded with lesser amount of sandstone and shale. Upper 5 m is interbedded wavy-laminated, thin-bedded, gray limestone and gray, micaceous siltstone. Conformable with underlying Flathead Sandstone. Forms slopes and swales. Thickness 30-65 m. Flathead Sandstone - Thin- to medium-bedded, medium- to coarse-grained, reddish-brown, tan, and purplish-tan, quartz-rich, feldspathic sandstone; locally weathers to rusty red. Two thin zones of fine-grained, micaceous, greenish-gray argillaceous sandstone near top of formation. Basal part of formation contains rounded pebbles of metamorphic rock. Unconformably overlies Archean crystalline rock. Thickness variable, from 15 to 75 m.' 1311|'Thin- to massive-bedded, light-gray to brownish-gray, finely crystalline limestone. Locally oolitic, especially in upper part. Upper 30 m is thin-bedded, gray limestone that contains conspicuous orange mottles; upper few meters contain fissile gray-green shale. Middle 50 m is medium- to massive-bedded limestone, locally mottled tan, and containing small silicic limestone stringers. Lower 30 m is thin- to medium-bedded, tan-mottled, gray limestone that contains a few intercalated micaceous shale beds in lower part. Forms cliffs. Conformably overlies Wolsey Shale. About 100-150 m thick in the Madison Range (Tysdal, 1990; Kellogg, 1992), 100-110 m thick in the Gravelly Range (Hadley, 1980), but only about 50 m thick in northern Gallatin Range (McMannis and Chadwick, 1964).' 1312|'Wolsey Shale-Mostly thin-bedded, greenish-gray, olive-drab, gray, and grayish-brown micaceous sandstone, siltstone, and shale. Sandstone beds are wavy and bioturbated, contain green and gray- and green-mottled shale interbeds, and generally weather brown; animal trails common; locally glauconitic. Near middle of unit is a 10- to 15-m-thick section of thin-bedded, dark-gray, brown-weathering argillaceous limestone interbedded with lesser amount of sandstone and shale. Upper 5 m is interbedded wavy-laminated, thin-bedded, gray limestone and gray, micaceous siltstone. Conformable with underlying Flathead Sandstone. Forms slopes and swales. Thickness 30-65 m. Flathead Sandstone-Thin- to medium-bedded, medium- to coarse-grained, reddish-brown, tan, and purplish-tan, quartz-rich, feldspathic sandstone; locally weathers to rusty red. Two thin zones of fine-grained, micaceous, greenish-gray argillaceous sandstone near top of formation. Basal part of formation contains rounded pebbles of metamorphic rock. Unconformably overlies Archean crystalline rock. Thickness variable, from 15 to 75 m.' 1313|'Black to dark-greenish-gray, fine- to medium-grained, equigranular, well-indurated diabase in steep, northwest-striking dikes as wide as 30 m (most are considerably thinner). Contains about 30-50 percent euhedral labradorite, 30-60 percent augite, 0-30 percent hornblende (inverted from augite), 0-10 percent potassium feldspar, 5-8 percent opaque minerals, 1-3 percent apatite, 0-5 percent biotite, 0-3 percent quartz, and from trace to 1 percent epidote. Emplaced during two periods: 1455 Ma and 750-780 Ma (Harlan and others, 1990). Low-grade metamorphic rocks of the Ruby Creek area, eastern Gravelly Range (Proterozoic or Archean): Metamorphic grade of rocks in this area is as low as chlorite facies (quartz-albite-epidote-biotite subfacies of Turner and Verhoogen, 1960), but increases in grade southward to almandine-amphibolite facies. The Ruby Creek assemblage is bounded on the north by a north-dipping thrust, which places marble in the hanging wall above the lower grade rocks of the Ruby Creek area. The age of the assemblage is unknown, but may be as young as Proterozoic (J.M. O’Neill, personal commun., 1995). The geology for this area is modified from Vargo (1990) and Hadley (1969a,b).' 1314|'Mapped by Hadley (1969a, b) but not described in detail.' 1315|'Green metamorphosed fine-grained clastic sedimentary rocks, locally showing good cross-bedding and graded beds. Contains quartz, plagioclase, actinolite, and epidote. Weathers to blocky outcrops.' 1316|'Dark brown-gray to black, well-foliated schist containing quartz, plagioclase, biotite, chlorite, magnetite, and muscovite; porphyroblasts of plagioclase abundant.' 1317|'Greenish-gray, greenish-red, gray-red, and dark gray phyllite containing quartz, biotite, chlorite, muscovite, hematite, magnetite, and rare garnet. Contains thin layers of micaceous and actinolite-bearing quartzite and lenticular amphibolite bodies (not shown).' 1318|'Well foliated chlorite-actinolite schist that contains abundant shear-bounded bodies of metagabbro, metadiorite, and amphibolite. Interpreted as sheared amphibolite (J.M. O’Neill, personal commun., 1995) and may be approximately equivalent to biotite chlorite schist (unit P_Abs). Includes a thin staurolite-sillimanite mylonite that bounds southern contact of unit.' 1319|'Approximately as mapped by Hadley (1969b); part of quartz-feldspar gneiss and pegmatite unit of Vargo (1990).' 1320|'Approximately as mapped by Hadley (1969b), who did not describe unit in detail. Not known if unit is intrusive into adjacent units or forms older basement.' 1321|'Alternating layers, typically 2-10 cm thick, of reddish-brown to black magnetite-quartz and quartzite. Contains minor hydrobiotite and magnetite locally oxidized to hematite and goethite. Layers commonly tightly folded.' 1322|'Pink, coarse-grained, massive to slightly foliated biotite monzogranite porphyry. Contains about 20 percent conspicuous euhedral to subhedral potassium-feldspar phenocrysts as long as 1.5 cm; matrix contains about 30 percent quartz, 35 percent plagioclase, 15 percent potassium feldspar, and about 20 percent biotite. Locally sheared, forming well-developed pinkish-gray augen gneiss, as on Indian Ridge in Spanish Peaks.' 1323|'Light-gray to light pinkish gray, generally tan-weathering, medium-grained, hypidiomorphic, weakly to moderately foliated orthogneiss generally ranging in composition from tonalite to monzogranite. Mafic mineral almost exclusively biotite (trace to 15 percent); may contain as much as 5 percent almandine and, rarely, 5 percent hornblende, 2 percent augite, and 2 percent muscovite; also contains traces of zircon, epidote, allanite, and opaque minerals. One unusual occurrence is a pluton near Summit Lake in the Spanish Peaks, the pluton is a discordant body composed of massive to weakly foliated, very-light-gray biotite tonalite.' 1324|'Black (commonly speckled with white feldspar and pink garnet), fine-grained, equigranular, granoblastic, weakly foliated to massive hornblende-augite-almandine metabasite. Composition variable; contains 15-45 percent plagioclase (mostly andesine), 10-60 percent yellowish-green to brown hornblende, 2-20 percent augite, 0-20 percent almandine, 0-5 percent reddish-brown biotite, 1-3 percent opaque minerals, and trace of apatite. In some places relict porphyritic texture is preserved as white clusters of fine-grained plagioclase as long as 1 cm. Mostly occurs as sills as wide as about 40 m concordant to foliation; in some places sills show pinch-and-swell structure and boudinage. Commonly enveloped in medium-grained amphibolite margin as wide as 10 m, indicating post-emplacement metasomatism at amphibolite grade. Equivalent to orthoamphibolite of Vitaliano and Cordua (1979) in Tobacco Root Mountains.' 1326|'Light- to medium-gray, medium-grained, hypidiomorphic, poorly to well foliated hornblende-biotite granodiorite orthogneiss. Contains 30-40 percent plagioclase, 10-20 percent potassium-feldspar, 15-25 percent quartz, 10-20 percent hornblende, and about 10 percent biotite. Most rock is highly strained, forming well-developed foliation; in such places, mafic minerals are concentrated in ribbony layers 2-5 mm thick. Contains numerous, thin (3-10 mm) K-feldspar-quartz migmatitic layers. Cut by at least three periods of sill and dike intrusion; sills and dikes locally so closely spaced that they form an agmatite. Cut by granitic orthogneiss of Summit Lake pluton.' 1327|'Black to dark-greenish-gray, fine- to medium-grained, well foliated to massive, variably serpentinized ultramafic rocks of wide-ranging composition; includes olivine websterite, lherzolite, and olivine clinopyroxenite. Accessory minerals include olive-green spinel, magnetite, and apatite. Commonly contains secondary amphibole (anthophyllite or actinolite) serpentine, talc, dolomite, magnesite, and (or) mica. Occurs in lenses, pods, and small irregularly shaped masses, rarely more than 10 m in diameter. Probably tectonically incorporated into country rock.' 1328|'Mapped where rocks are heterogeneous, generally layered, light- to medium-gray microcline-plagioclase-quartz-biotite gneiss. Commonly migmatitic and blastomylonitic. Some areas not mapped in detail, such as south of Sphinx Mountain in the Madison Range and east of the Gallatin River in the Gallatin Range, which may contain many of the other gneissic rock units.' 1329|'Includes biotite-garnet schist, sillimanite-garnet schist, garnetiferous quartzite, quartzite, highly garnetiferous quartzofeldspathic gneiss, corundum gneiss, gedrite schist, cummingtonite schist, and garnetiferous amphibolite (Vitaliano and Cordua, 1979). May represent refractory component ("restite") after removal of partial melts.' 1330|'Most is gray to black, medium-grained, hypidiomorphic equigranular, moderately-foliated to well-foliated hornblende-plagioclase gneiss and amphibolite; contains as much as 5 percent quartz and traces of zircon, opaque minerals, and apatite; locally garnetiferous. Plagioclase typically An30 and weathers white. Commonly contains white, migmatitic leucosomes of anorthosite as thick as 10 cm. Similar unit in Tobacco Root Mountains interpreted to be either metamorphosed clay-rich dolomite or mafic-extrusive origin (Vitaliano and Cordua, 1979). Amphibolite envelopes around some metabasite intrusive bodies indicate at least some amphibolite was derived from intrusive rocks. Unit may include minor amounts of other Archean units.' 1331|'Black, dark gray, and gray, fine to medium grained biotite-plagioclase-quartz-±hornblende-±microcline schist. Interpreted to be sheared mafic rock that equilibrated at lower amphibolite facies; not studied in detail. Crops out in the Gallatin Canyon-Hellroaring Lake (Spanish Peaks) area.' 1332|'Cummingtonite gneiss is light gray, medium grained, strongly lineated, and vitreous. Contains as much as 90 percent cummingtonite, and variable amounts of quartz, almandine, muscovite, rutile, and opaque minerals. Grades into metaquartzite. Gedrite gneiss is brown to grayish brown, moderately well foliated gneiss that contains as much as 70 percent clove-brown gedrite, in addition to quartz, plagioclase, ±sillimanite, ±biotite, ±cordierite, and traces of magnetite and rutile. Both rock types occur as small lenses and concordant layers in other Archean rocks.' 1333|'White, light-gray, dark-gray, and black, medium-grained, well-foliated and well-layered gneiss. Leucosomes contain plagioclase, quartz, biotite, ±potassium feldspar, ±garnet, and a trace of opaque minerals. Melasomes contain biotite, hornblende, plagioclase, quartz, ±garnet, and trace of opaque minerals. Contains rare quartzite layers and sillimanite-bearing gneiss. Commonly migmatitic. Layers typically 1-20 cm thick. Injected by numerous sills of metabasite. Crops out extensively in Bear Trap Canyon of the Madison River. Similar rocks, also mapped as unit Abh, occur at several other less extensive localities.' 1334|'Light- to medium-gray mottled mylonite with a well-developed L-S tectonite fabric and a fine-grained recrystallized (granoblastic) texture. Composition variable, and ranges from that of quartzofeldspathic gneiss to amphibolite. The mylonite equilibrated at near-peak (lower granulite) metamorphic conditions and forms anastomosing, concordant zones in the northern Madison Range (Kellogg and Mogk, 1991). The margins of the unit are concordant with and grade into relatively unmylonitized gneiss.' 1335|'Gray to dark-brownish-gray, medium-grained, inequigranular, generally well-foliated, commonly micaceous gneiss and schist containing aluminosilicate (mostly sillimanite and rarer kyanite). Unit contains 5-90 percent anhedral quartz having undulatory extinction, 0-30 percent microcline, 0-35 percent plagioclase, 0-30 percent almandine, 0-20 percent muscovite, trace to 15 percent sillimanite or kyanite, 0-10 percent reddish-brown biotite, 0-2 percent opaque minerals, including graphite, and trace of zircon. Commonly rich in quartz and locally grades into quartzite. Several kyanite prospects are on east side of Gravelly Range, in Sec. 6, T. 8 S., R. 1 W. (Nordstrom, 1947).' 1336|'Light- to medium-gray, medium-grained, poorly to well foliated quartz-feldspar gneiss that contains abundant thin schistose layers with both biotite and muscovite. Aluminosilicate-bearing lenses are common and one prominent 20-50-m-thick schistose horizon contains as much as 50 percent coarse-grained biotite, about 20 percent quartz, 0-10 percent gedrite, 5 percent plagioclase, 5 percent sillimanite, 3 percent kyanite (as large blue blades), 3 percent garnet, and 3 percent muscovite. Mapped on north side of Hellroaring Creek valley in Spanish Peaks.' 1337|'White, medium- to coarse-grained, inequigranular, moderately foliated to massive quartzite; locally bright green where trace amounts of chromium-bearing mica (fuchsite) present. In most places, unit is composed entirely of anhedral quartz grains having undulatory extinction, but locally contains as much as 30 percent microcline, 20 percent muscovite, 15 percent sillimanite, 10 percent cummingtonite, 8 percent almandine, 2 percent actinolite, and trace of zircon and opaque minerals. Commonly forms prominent ridges, especially in northwestern Madison Range, where the core of a plunging, nearly isoclinal antiform is composed almost entirely of quartzite. Unit is interlayered in most places with mafic amphibolite.' 1338|'White, coarse-grained, massive to moderately well foliated dolomitic marble that contains as much as 3 percent quartz grains. Weathers orange- brown. Locally hydrothermally altered to commercial deposits of talc on east side of Gravelly Range. A few thin layers of calcitic marble, containing as much as 30 percent serpentine, mapped in northern Madison Range. Extensive, commercially exploited talc deposits occur in hydrothermally altered zones in large marble body on eastern side of the Gravelly Range.' 1339|'Alternating layers of reddish-brown to black magnetite-quartz and quartzite, typically 2-10 cm thick. Contains minor hydrobiotite and magnetite locally oxidized to hematite and goethite. Layers commonly tightly folded. Mapped on eastern side of Gravelly Range by Vargo (1990).' 1340| 1400|'Man-made deposits, typically within Quaternary surficial deposits.' 1401|'Alluvium, fan and terrace gravel, gravel deposits on pediment surfaces, and landslide and travertine deposits (Pleistocene and Holocene); till, glacial lake, and outwash deposits (Pleistocene).' 1402|'Fan and gravel deposits on pediment surfaces (Pliocene); conglomerate, sandstone, mudstone, and volcanic ash beds (Eocene, Oligocene, or Miocene).' 1403|'Rhyolite, quartz latite, latite, and dacite interbedded with lesser amounts of basalt and andesite.' 1404|'Andesite, basalt, and latite flows.' 1405|'Quartz monzodiorite and granodiorite.' 1406|'Rhyolite and dacite flows and tuffs.' 1407|'Basalt and andesite flows, flow breccia, and air-fall tuff.' 1408|'Foliated and non-foliated biotite-muscovite monzogranite, leucomonzogranite, and granodiorite. Includes muscovite-bearing granitic rocks with Tertiary K-Ar ages (Wallace and others, 1992) assigned to Tmg unit of Wallace (1987). Because these age determinations may date cooling and not crystallization, a Cretaceous age is also possible.' 1409|'Foliated and nonfoliated biotite granodiorite, hornblende-biotite granodiorite, tonalite, and quartz diorite.' 1410|'Gabbro, microgabbro, diorite, and lamprophyre.' 1411|'Pyroxenite, leucosyenite, and hornblende syenite.' 1412|'Aplite, alaskite, and pegmatite dikes, sills, and pods.' 1413|'Granite and monzogranite porphyry, biotite monzogranite, biotite-muscovite monzogranite and granodiorite, and quartz monzonite.' 1414|'Foliated and non-foliated biotite-hornblende granodiorite, biotite-hornblende monzogranite, and tonalite.' 1415|'Hornblende quartz diorite, granodiorite, tonalite, quartz monzodiorite, diorite, quartz diorite, leucogranodiorite, and augite-hornblende monzonite.' 1416|'Diorite, gabbro, microgabbro, and diabase.' 1417|'Latite sill in northeastern corner of map.' 1418|'Andesitic, quartz latitic, and basaltic tuff, breccia, and flows.' 1419|'Unit includes, in descending order: Cretaceous Two Medicine Formation, Virgelle Sandstone, Telegraph Creek Formation, Marias River Shale, and Blackleaf Formation in northeast corner of quadrangle. Northwest of Deer Lodge unit consists of Cretaceous Golden Spike Formation, Carter Creek, Jens, and Coberly Formations of Gwinn (1961), Blackleaf Formation, and Kootenai Formation, as well as Jurassic Morrison Formation and Ellis Group.' 1420|'Unit includes, in descending order: Permian Shedhorn Sandstone, Phosphoria Formation, and Park City Formation, Pennsylvanian Quadrant Quartzite, Snowcrest Range Group (Pennsylvanian and Mississippian), Madison Group and related rocks (Mississippian), Three Forks Formation (Mississippian and Devonian) and Jefferson and Maywood Formations (Devonian).' 1421|'Red Lion Formation, Hasmark Formation, Silver Hill Formation, and Flathead Quartzite. Park Shale, Meagher Limestone, and Wolsey Shale are lateral equivalents to the Silver Hill Formation and are present in northeast part of quadrangle.' 1422|'Gabbro, microgabbro, and diorite dikes and sills.' 1423|'Coarse- to medium-grained, reddish or buff quartzite containing thin beds of sandy argillite.' 1424|'Grayish-green micaceous quartzite and impure argillite.' 1425|'Red and green interbedded argillite and siltite, and buff quartzite. Contains distinctive green and red chert beds and clasts. Amount of quartzite increases to the south and southwest.' 1426|'Pink or buff, medium- and coarse-grained feldspathic quartzite. Matrix-supported granules and pebbles present locally. Abundant trough cross-beds.' 1427|'Reddish quartzite and subordinate argillite and siltite in lower and middle parts of section. Upper part is predominantly reddish argillite. Matrix-supported granules and pebbles present in southern part of area, but medium to fine grain size is most common. Abundant planar laminations and ripple cross-laminations. Previously subdivided into three informal members, the middle one being the most quartzitic (Wallace and others, 1986). However, significant quartzite may be present in the lowermost part of the section as well, particularly in the western part of the map area.' 1428|'Shown as a single unit in the southwest part of area where the Shepard Formation is thin, poorly exposed, and only recognized at a few localities. Includes rocks previously mapped as carbonate facies of the lower part of the Mount Shields Formation (Wallace and others, 1986). In western part of area near Burnt Fork Lake both Snowslip and Shepard Formations are present but have yet to be mapped separately. Here they are more quartzitic than to the east in the Philipsburg area.' 1429|'Green microlaminated argillite at base overlain by thin lenticular beds of green dolomitic siltite and fine-grained quartzite. Upper part is red, thinly bedded dolomitic quartzite and siltite. Abundant load casts and ripple marks. Poorly exposed and easily missed when mapping.' 1430|'Green and red argillite near base and increasing amounts of siltite and quartzite up section. Some quartzite beds contain coarse, well-rounded quartz grains and less feldspar than is typical for quartzite in the Belt Supergroup. Upper quartzitic part of section is difficult to distinguish from Mount Shields Formation. Abundant straight-sided mud cracks in lower part of section.' 1431|'Limestone, dolomite, dolomitic and calcareous siltite, and argillite of the Helena Formation in the eastern part of the area and dolomitic siltite, quartzite, argillite, and sedimentary breccia of the Wallace Formation in the western part of the area.' 1432|'Mostly greenish-gray argillite and siltite with interbeds of quartzite and dolomite; red to purple argillite near base. Carbonate content increases up section.' 1433|'Reddish siltite, argillite, and thin beds of quartzite.' 1434|'Dark gray thinly laminated argillite and siltite grading up into light-gray to greenish-gray, thinly bedded siltite and quartzite.' 1435|'Highly recrystallized quartzite and lesser amounts of phyllite and schist. Although assigned to Mount Shields Formation by Wallace and others (1986), these rocks appear to underlie the Middle Belt carbonate and are probably Ravalli Group equivalent. Exposures of Yq in the Anaconda range were assigned to Ravalli Group by Calkins and Emmons (1915).' 1436|'Quartzofeldspathic gneiss, migmatite, schist, quartzite, and calc-silicate rocks. Includes rocks assigned to the Prichard Formation and Neihart Quartzite (lowermost Belt) by Calkins and Emmons (1915) and later assigned to the Mount Shields Formation by Wallace and others (1986). The older assignment is more likely, given their structural position in the uplifted block of the Anaconda Range. Also includes rocks in the southwest part of the map area which may be metamorphic equivalents of the Prichard Formation(?), Ravalli Group, and Middle Belt carbonate.' 1437|'Unit includes, in descending order: Permian Shedhorn Sandstone, Phosphoria Formation, and Park City Formation, Pennsylvanian Quadrant Quartzite, Snowcrest Range Group (Pennsylvanian and Mississippian), Madison Group and related rocks (Mississippian), Three Forks Formation (Mississippian and Devonian) and Jefferson and Maywood Formations (Devonian). These rocks have been silicified and sheared.' 1450|'Silt, sand, and gravel in channels and flood plains of major rivers and streams and in related alluvial fans. May be partly of Pleistocene age. Maximum thickness unknown.' 1451|'Angular fragments of bedrock mixed with soil or heterogeneous boulders and finer grained material derived from glacial deposits on steep valley walls; characterized by irregular, hummocky topography; boggy in places. Many landslides are marked by torn sod, tilted trees, and steep unvegetated slopes that indicate continuing movement.' 1452|'Poorly sorted silty sand and gravel deposited in broad alluvial fans along valley margins. Includes gravel veneer on pediments.' 1453|'Silt and sand in thin-bedded, well-sorted deposits in the Big Hole basin in western part of map area.' 1454|'Unsorted mixture of boulders, cobbles, pebbles, and sand deposited by glaciers. Includes deposits of at least two, and in some places three, episodes of glaciation. Boulder fields in south-central part of map area are characterized by frost riving and frost-heaved boulders in locally boggy and commonly terraced terrane that formed by creep and solifluction on flanking hills. These boulder fields probably formed as a result of weathering in severe cold climate adjacent to Pleistocene glaciers. Rocks in the boulder fields are essentially in place, but structural information is incomplete or lacking.' 1455|'Poorly sorted bouldery gravel and sand deposited by glacial meltwater.' 1456|'Unconsolidated, deeply weathered angular to rounded pebbles, cobbles, and small boulders of quartzite, arkosic sandstone, and granitic rocks in sand matrix; commonly covered by a thin layer of eolian silt; most deposits are located along southeast flank of Highland Mountains. Elsewhere, consists of poorly sorted silt, sand, and bouldery gravel of uncertain age. Present on ridge crests in the vicinity of Fish Creek in northeastern part of map area, near Wise River in north-central part, and in Lemhi Valley. Overlies tuffaceous rocks of late Tertiary age at Fish Creek and in Lemhi Valley and Paleozoic and Mesozoic rocks at Wise River. Present locally overlying Bozeman Group and related valley-fill deposits (Tbz) but not mapped separately. Thickness 0-50 m.' 1457|'Light-gray to yellowish-brown, moderately indurated to well-indurated tuffaceous sandstone and siltstone containing subordinate interbeds of limestone and marl and lenses of pebble and cobble conglomerate composed of locally derived rock fragments. Exposures of related valley fill in Horse Prairie basin and in Salmon River valley include lignitic rocks of Oligocene age (Cavender, 1977; Harrison, 1982, 1985; Ruppel and Lopez, 1984, 1988). Commonly veneered with residual gravel or a thin layer of eolian silt. Thickness ranges from less than 300 m to about 3,000 m in the valley of Grasshopper Creek near the confluence of Grasshopper and Swamp Creeks and as much as 4,600 m in the Bighole basin. Deposits also include Tertiary landslide deposits derived from flanking mountains. Bozeman Group has been divided into Sixmile Creek and Renova Formations (Kuenzi and Fields, 1971), mainly on the basis of mammalian fossil data ; however, formation names are not used here because in most places in Dillon quadrangle the two formations cannot be separated lithologically (Hanneman, 1989). Maximum thickness, in Big Hole basin, exceeds 4,000 m.' 1458|'Grayish-black vesicular basalt or basaltic andesite flows near Sweetwater Creek in south-central part of map area are about 4 Ma (Daniel and Berg, 1981, p. 110); similar basaltic rocks are present in the central part of Ruby Range (James, 1990).' 1459|'Basaltic andesite, andesite, latite, rhyodacite, and rhyolite as flows, ash-flow tuffs, and tuff and flow breccias. Mainly dark-gray or very dark red to medium-gray and pale-reddish brown, partly vesicular to scoriaceous, porphyritic hornblende or pyroxene basaltic andesite and andesite, and pale-red or light-brownish-gray to light-gray and pale-brown, porphyritic, partly welded quartz latite, latite, and rhyodacite. Locally contains thin interbeds of light-olive-gray, biotitic, tuffaceous, coarse-grained sandstone and conglomerate composed of volcanic detritus. Present only in southwestern and west-central parts of map area. Includes rocks in Withington Creek caldera, about 10 km in diameter, at north end of Lemhi Range; caldera contains massive ash-flow tuffs at least 600 m thick that pass laterally across caldera margin into 50-100 m of thinly layered ash-flow tuffs and andesitic flows. Age probably about 40-45 Ma (Staatz, 1979, p. 12-23; Ruppel and Lopez, 1984). Preserved thickness in most places probably less than 200 m. Volcanic rocks east of Idaho-Montana State line are not clearly related to Challis volcanic field of east-central Idaho and are mapped as Tvu.' 1460|'Basaltic andesite, andesite, dacite, and rhyodacite. Light-brownish-gray to brownish-gray (tinted purple in places), fine-grained, locally porphyritic volcanic and associated intrusive rocks. Phenocrysts include sodic plagioclase, sanidine, quartz, and biotite. Partly equivalent to Eocene Lowland Creek Volcanics directly north of map area. Present mainly along east flank of Pioneer Mountains. Age 50-46 Ma (Zen, 1988). Individual flows as much as 50 m in thickness.' 1461|'Includes mainly dikes of dark-gray aphanitic basalt containing sparse phenocrysts of olivine(?) near North Fork of Salmon River in west-central part of map area (Lopez, 1982b) and a finely crystalline diorite dike that intrudes Beaverhead Divide-Miner Lake fault zone directly south of Center Mountain in the Beaverhead Mountains east of Salmon.' 1462|'Dikes, dike swarms, and associated extrusive volcanic rocks. Dark-green to pale-yellowish-brown, gray or yellowish-gray to pale-red, fine-grained, porphyritic volcanic and related intrusive rocks containing phenocrysts of sanidine, plagioclase, alkali feldspar, and quartz, or of plagioclase, biotite, and hornblende in more mafic rocks. Mainly rhyodacite and dacite. Extrusive rocks principally rhyodacite and dacite tuffs and breccias and rhyolite flows, tuffs, and welded tuffs. Present in northwestern and north-central parts of map area. Age about 43 Ma (Desmarais, 1983, p.49-50, 106).' 1463|'Greenish-black basaltic plug intruded along Grasshopper Creek in south-central part of map area. May be related to Tvu.' 1464|'Intrusive rocks in Carmen and Napoleon Ridge stocks and small plutons south of Gibbonsville in Beaverhead Mountains, in western part of map area; rocks intrude Yellowjacket Formation (Middle Proterozoic) and lower part of Medicine Lodge thrust plate. Age about 55-48 Ma; eastern part of Carmen stock contains small areas of more mafic rocks that are about 83-80 Ma (Kilroy, 1981; Ruppel and Lopez, 1988).' 1465|'Complex mixture of porphyritic, fine- to medium-grained, gray to dark-gray granodiorite stocks and dikes. Phenocrysts consist of orthoclase, plagioclase, and rounded glassy quartz. K-Ar age determined on biotite is 51 Ma (Desmarais, 1983).' 1466|'Light gray, medium to coarse grained, equigranular to porphyritic, and nonfoliated. Phenocrysts, where present, consist of orthoclase, microcline, and locally plagioclase. K-Ar ages determined on biotite and muscovite range from 58 to 50 Ma (Desmarais, 1983).' 1467|'Fine- to medium-grained, nonporphyritic granodiorite.' 1468|'Granitic batholithic rocks in the Anaconda Range: Porphyritic and equigranular, medium-grained rocks in stocks and minor dikes. Granitic rocks in the Boulder and Tobacco Root batholith: Light-gray, equigranular to strongly porphyritic, medium-grained to very coarse grained granitic rocks that compose the Moose Creek stock, Hell Canyon pluton, and most of the Butte Quartz Monzonite in central part of Boulder batholith. Slight textural and compositional variants are present. Includes small bodies of dark-gray diorite and gabbro.' 1469|'Gray, medium to coarse grained, equigranular; characterized by distinctive foliation. K-Ar ages determined on biotite and hornblende range from 74 to 60 Ma (Desmarais, 1983).' 1470|'Light-gray rhyolitic dike, in southern Pioneer Mountains, composed of quartz and sanidine phenocrysts in a finely granular groundmass of quartz and potassium feldspar. Age about 46 Ma (Snee, 1982; Pearson and Zen, 1985).' 1471|'Light gray, medium to coarse grained, locally porphyritic. Age about 65-61 Ma (Snee 1982, p. 153-154; Marvin and others, 1983). Includes Clifford Creek Granite of Paleocene and Late Cretaceous age in north-central Pioneer Mountains (Zen, 1988).' 1472|'Pinkish gray, medium to coarse grained; contains euhedral potassium feldspar phenocrysts about 1 cm long. Includes associated tonalite, rhyodacite porphyry, rhyolite porphyry, intrusive breccia, pegmatite, and Grayling Lake Granite in north-central Pioneer Mountains (Zen, 1988). Age about 68-73 Ma (Snee, 1982, p. 134-152).' 1473|'Light-gray, medium-grained hornblende-biotite granodiorite of Uphill Creek Granodiorite (Zen, 1988), the predominant rock exposed in Pioneer batholith; includes plutons, mainly in northwestern part of batholith, of dark-gray to bluish-gray, medium-grained biotite and biotite-hornblende tonalite and quartz diorite. Ages of various plutons range from 68 to 77 Ma, age of granodiorite about 75 Ma (Snee, 1982, p. 104-134; Pearson and Zen, 1985; Pearson and others, 1987, 1988). Includes granitic rocks in McCartney Mountain stock, directly east of Pioneer Mountains, and in stocks north of Pioneer Mountains, in north-central part of map area.' 1474|'Dark greenish gray, medium to coarse grained. Contains as much as 50 percent hornblende in most places, but as much as 80 percent hornblende in northeastern part of Pioneer Mountains. Age about 80 Ma (Snee, 1982, p. 93-104).' 1475|'Bluish-gray, weakly to moderately foliated, fine-grained, porphyritic metadacite and meta-andesite; phenocrysts commonly fractured parallel to foliation; hornblende locally present as elongate crystals parallel to foliation. Exposed in northwestern Pioneer Mountains; interpreted by Berger and others (1983) as Precambrian cratonic rock.' 1476|'Bluish gray, strongly foliated, medium to coarse grained; contains porphyroblasts as much as 2 cm long of plagioclase, smoky quartz, and sparse microcline aligned parallel to foliation. Accessory minerals include sphene, magnetite, zircon, and apatite. Original rock may have been coarse-grained granodiorite or porphyritic dacite or andesite. Exposed only in northwestern part of Pioneer Mountains and interpreted by Berger and others (1983) as Precambrian cratonic rock.' 1477|'Dark bluish gray to bluish gray, medium to fine grained, strongly foliated; contains porphyroblasts of plagioclase and smoky quartz. Accessory minerals include epidote, sphene, magnetite, and zircon. Exposed in northwestern Pioneer Mountains; interpreted by Berger and others (1983) as Precambrian cratonic rock.' 1478|'Leucocratic, fine- to coarse-grained granite; occurs as small, irregular pods and stocks in Butte Quartz Monzonite. Only largest bodies are shown, many being too small to be shown.' 1480|'Medium- to dark-gray, equigranular, fine- to medium-grained rocks of Rader Creek and Donald plutons intruded along southeast margin of Boulder batholith. Also includes Tobacco Root batholith, a zoned pluton that is mostly a medium-gray, coarse-grained, porphyritic granodiorite in map area (O’ Neill, 1983), and related stocks in the Tobacco Root Mountains.' 1481|'Principally monzogranite, granodiorite, and related granitic intrusive rocks in small stocks and plugs in east-central and southern parts of map area. Includes porphyritic, high-potassium dacite in a 16-km-long intrusive sheet and as a 1-km-diameter plug that is hydrothermally altered and mineralized 15 km west of Dillon.' 1482|'Comprises an upper lava-flow and breccia unit of intermediate composition and a lower silicic pyroclastic unit. The upper unit is high-potassium dacite and minor andesite and dacite as volcanic breccia, lava flows, a volcanic dome, and minor volcanic-pebble conglomerate, volcaniclastic sandstone, tuff, and dikes. The lower unit comprises pale-colored pyroclastic flow deposits and tuff that are nonporphyritic to slightly porphyritic and, in most places, zeolitic.' 1483|'Light-gray to dark-gray and grayish-red to greenish-gray dominantly andesitic to latitic welded tuff and tuff breccia, tuff, lapilli tuff, lava flows, flow breccia, and poorly sorted volcanic sandstone and conglomerate in northeastern part of map area.' 1484|'Moderately indurated massive boulder, cobble, and pebble conglomerate having a quartz-sand matrix and cemented with calcite, and some interbedded sandstone and fresh-water limestone; in most places these rocks consist of rounded to subangular fragments of Proterozoic quartzite and Paleozoic carbonate rocks; however, near Bannock the sequence can be divided into an upper unit composed mainly of quartzite clasts that is separated from underlying carbonate-rich unit by volcanic rocks (Kvu) not assigned to the Beaverhead; as much as 350 m thick. Exposed only in south-central part of map area.' 1485|'Thick succession of poorly known, partly metamorphosed sandstone and shale that underlie McCartney Mountain in east-central part of quadrangle.' 1486|'Includes poorly known rocks of Blackleaf Formation and overlying strata. Blackleaf Formation of this report formerly was designated as part of Colorado Group, undivided, or as Aspen Formation. Upper part (Upper Cretaceous) - Greenish-gray or light yellowish-gray to grayish-white, tuffaceous subgraywacke, tuffaceous sandstone, quartz sandstone, and tuff, locally interlayered with conglomerate lenses. Interbedded with grayish-green, greenish-blue, and dark-gray silty mudstone, shale, and siltstone. Beds near top of section locally contain plant fragments and pollen indicating mid-Campanian to Maastrichtian age (Zen, 1988). Thickness uncertain, but possibly as thick as 1,200 m. Exposed only along east flank of Pioneer Mountains northward to area of Granulated Mountain north of Wise River. Lower part (Upper Cretaceous) - Sandstone, conglomerate, shale, mudstone, and porcelaneous mudstone, siltstone, limestone, and bentonite. Sandstone is yellowish gray, light olive gray, light to medium bluish gray to pale yellowish brown, brownish gray and medium to dark gray; mostly medium to coarse, rounded to angular grains, but finer grained in places; medium to thick bedded; prominent trough cross-stratification in many beds; commonly calcareous and stained with limonite, and contains disseminated carbonaceous material. Conglomerate commonly grades laterally into coarse-grained sandstone and contains rounded fragments as much as 30 cm in diameter predominantly of quartzite, chert, porcelaneous mudstone, and, less commonly, limestone, in a sandstone matrix. Mudstone, shale, and siltstone are light olive green and bluish gray to dark gray, calcareous, bentonitic, and commonly porcellanitic; commonly burrowed and bioturbated, and locally contains abundant carbonaceous material and leaf and wood impressions. Limestone, in rare, scattered, discontinuous beds 1 m or less thick, is medium gray to greenish gray, fine grained, silty, commonly bioturbated, and grades laterally into calcareous mudstone. In part lithologically equivalent to Frontier Formation near Lima Peaks in southern Beaverhead County (Dyman and others, 1989). Measured thickness as much as 420 m, but top of formation not present in this region (Dyman, 1985a, 1985b, 1985c; Dyman and Tysdal, 1989; Tysdal and others, 1989). Blackleaf Formation (Upper and Lower Cretaceous)-Separable into two members. Flood Member separable into three informal lithofacies units as described by Dyman (1985a, 1985b, 1985c), Dyman and Nichols (1988), and Dyman and Tysdal (1989). Total thickness is 940 m at Rock Creek in Pioneer Mountains and about 460 m in Snowcrest Range. Vaughn Member - Mudstone, porcelaneous mudstone, siltstone, volcanic-rich sandstone, conglomerate, and limestone. Predominantly mudstone and siltstone, which are light olive gray, light greenish gray, dusky red, greenish gray, and medium gray, partly thinly laminated, commonly porcelaneous in middle and upper parts of unit, partly calcareous, hard and dense, locally bentonitic, and locally pyritic; contains locally abundant lithophysae in tuffaceous beds. Sandstone and conglomerate occur in thin, laterally discontinuous channel fills. Sandstone and conglomerate are light olive gray and greenish or bluish gray to dark gray, mostly fine to medium grained, consisting of subrounded to angular grains; partly calcareous, partly pyritic, thin to medium bedded; conglomerate zones contain rounded to angular fragments as much as 3 cm in diameter of mudstone, shale, siltstone, quartzite, and chert. Silty, micritic limestone is rare and typically occurs in thin argillaceous lenses and grades into highly calcareous mudstone. Thickness 340-356 m in eastern Pioneer Mountains; thins eastward to 80 m in Snowcrest Range. Flood Member - Clastic lithofacies-Interbedded with subordinate siltstone, mudstone, shale, and conglomerate. Sandstone is light olive gray and light greenish gray to dark yellowish brown and pale red, fine to coarse grained and locally conglomeratic; grains are rounded to angular; medium to thick bedded, ripple or horizontally laminated or trough cross stratified; commonly calcareous, micaceous, and contains abundant carbonaceous material and wood fragments. Siltstone, mudstone, and shale are yellowish-gray and light olive gray to yellowish brown and dark gray, partly calcareous, commonly bioturbated, and contain carbonaceous material and wood fragments. Thickness about 75 m in eastern Pioneer Mountains, thinning eastward to 4-15 m in and near Snowcrest Range. Mudstone-shale lithofacies - Mudstone, shale, siltstone, and greatly subordinate sandstone. Mudstone, shale, and siltstone are medium gray to dark gray, partly thinly laminated, calcareous, and contain abundant calcareous nodules as much as 5 cm in diameter in discrete zones; commonly bioturbated, limonite stained, and commonly contain carbonaceous material, wood fragments, and coal. Sandstone is pale blue to medium light gray, very fine to fine grained, thin bedded and parallel laminated, and calcareous. Grains are rounded to subangular. Thickness about 20-50 m in eastern Pioneer Mountains, thickens eastward to about 75 m in and near Snowcrest Range. Transitional clastic lithofacies - Sandstone, siltstone, mudstone, and subordinate limestone, shale, and conglomerate. Sandstone is medium light gray, light olive-gray, pale yellowish brown, and medium gray, and generally very fine to medium grained. Grains are rounded to subangular grains. Is in part horizontally laminated, ripple laminated, or trough cross-stratified, and in places coarse grained and conglomeratic, containing fragments of limestone and mudstone as much as 2.5 cm in diameter; calcareous, partly bioturbated, and contains carbonaceous material. Mudstone, shale, and siltstone are medium light gray and light olive gray to medium bluish gray, medium dark gray, and greenish black; calcareous, and locally contains calcareous nodules as much as 1 cm in diameter; partly thinly laminated, thin bedded, commonly bioturbated, and contains carbonaceous material. Limestone is medium light gray, light bluish gray, yellowish gray to medium gray, and medium bluish gray, micritic, thin bedded, typically nodular with nodules as much as 10 cm in diameter, carbonaceous, partly silty to sandy, and grades laterally into siltstone or sandstone. Thickness about 80-85 m near Pioneer Mountains, thinning eastward to about 15 m near Snowcrest Range.' 1487|'Mudstone, siltstone, limestone, and sandstone. Upper part of formation is principally limestone with subordinate interbedded mudstone, siltstone, and sandstone; limestone beds also occur in middle part of formation (Myers, 1952; Schwartz, 1983; Pearson and Zen, 1985). Limestone is medium light gray to medium dark gray, finely to coarsely crystalline, thin to medium bedded, and fossiliferous; abundant fragments of gastropods in some beds. Mudstone and siltstone, principally in middle and lower parts of formation, are light olive gray, grayish green, and medium light gray to pale red, grayish red, and medium dark gray, partly thinly laminated, partly fissile, calcareous, and contain carbonaceous material and fragments of fossil wood. Sandstone occurs in subordinate interbeds throughout formation but is predominant in lower part; typically it is light olive gray and medium light gray to medium dark gray, fine to coarse grained, thin to thick bedded, and calcareous; rounded to subangular quartz grains and grains of dark chert are abundant; basal sandstone commonly is conglomeratic and contains abundant pebbles and cobbles of chert. Thickness 200-400 m.' 1488|'Includes Jurassic Morrison Formation and the Triassic Thaynes Limestone, Woodside Formation, and Dinwoody Formation, undivided.' 1489|'Grayish-red, yellowish-gray, and greenish-gray shale and siltstone, and minor interbedded fine- to medium-grained sandstone. Maximum thickness about 60-65 m in Argenta and Wise River areas and about 10 m in Blacktail Mountains and Snowcrest Range, where these rocks are mapped with Kootenai Formation; absent elsewhere in Dillon region.' 1490|'Thaynes Limestone (Lower Triassic) - Light-gray limestone, sandstone, and siltstone. Maximum thickness about 120 m in south-central and southeastern parts of map area; absent elsewhere in Dillon region. Woodside Formation (Lower Triassic) - Reddish-brown, reddish-purple, and medium-gray calcareous mudstone, siltstone, and limestone. Maximum thickness about 30 m in south-central part of map area and in Snowcrest Range; absent elsewhere in Dillon region. Dinwoody Formation (Lower Triassic) - Grayish-green, yellowish-gray, and grayish-brown, calcareous, thinly laminated siltstone; fine-grained sandstone; shale; gray, pale-red and light-brownish-gray-weathering, thin-bedded limestone. Maximum thickness, in south-central part of map area, is about 250 m but most commonly 100-160 m.' 1491|'Includes the Phosphoria Formation (Permian) and Quadrant Formation (Pennsylvanian and Upper Mississippian). Phosphoria Formation: Divisible into upper and lower units. Upper unit is yellowish-brown to brownish-gray and dark-gray, partly glauconitic chert, cherty fine-grained sandstone, and quartzitic sandstone. About 40-45 m thick. Lower unit, in most places, includes, from top to bottom, (1) dark-gray, carbonaceous, phosphatic mudstone and shale and phosphate rock, and oil shale on Retort Mountain and Dalys Spur; (2) a unit of light-gray to grayish-brown, cherty, fine-grained quartzitic sandstone and cherty or sandy dolomite; (3) a yellowish-brown and medium-bluish-gray chert, yellowish-brown to dark-gray, cherty, carbonaceous and phosphatic mudstone and phosphate rock; and (4) a basal unit of light-gray to yellowish-gray, pale-reddish-brown, fine-grained dolomitic sandstone, siltstone, chert, and cherty limestone containing minor interbeds of vitreous quartzite. Total thickness of formation ranges from 50 to 180 m; it thins northward mainly by disappearance of lower part of lower unit. Quadrant Formation: Light-gray to pale-yellowish-brown, fine- to medium-grained, quartzitic sandstone and vitreous quartzite; contains well-sorted, subrounded to well-rounded quartz grains; commonly cross-stratified, locally hematitic. Locally includes thin interbeds of light-gray to medium-gray, micritic, silty or sandy dolomite or limestone; in some places lower 15-55 m of formation is mainly dolomite or limestone containing thin interbeds of dolomitic or calcareous, fine-grained sandstone. Thickness ranges from 210 m in Blacktail Mountains to 115 m near Wise River, thickness is 200-400 m in Pioneer Mountains, about 90 m in the Ruby Range, and 150 m in Snowcrest Range (Saperstone and Ethridge, 1984).' 1492|'Divisible into upper and lower units. Upper unit is yellowish-brown to brownish-gray and dark-gray, partly glauconitic chert, cherty fine-grained sandstone, and quartzitic sandstone. About 40-45 m thick. Lower unit, in most places, includes, from top to bottom, (1) dark-gray, carbonaceous, phosphatic mudstone and shale and phosphate rock, and oil shale on Retort Mountain and Dalys Spur; (2) a unit of light-gray to grayish-brown, cherty, fine-grained quartzitic sandstone and cherty or sandy dolomite; (3) a yellowish-brown and medium-bluish-gray chert, yellowish-brown to dark-gray, cherty, carbonaceous and phosphatic mudstone and phosphate rock; and (4) a basal unit of light-gray to yellowish-gray, pale-reddish-brown, fine-grained dolomitic sandstone, siltstone, chert, and cherty limestone containing minor interbeds of vitreous quartzite. Total thickness of formation ranges from 50 to 180 m; it thins northward mainly by disappearance of lower part of lower unit.' 1493|'Light-gray to pale-yellowish-brown, fine- to medium-grained, quartzitic sandstone and vitreous quartzite; contains well-sorted, subrounded to well-rounded quartz grains; commonly cross-stratified, locally hematitic. Locally includes thin interbeds of light-gray to medium-gray, micritic, silty or sandy dolomite or limestone; in some places lower 15-55 m of formation is mainly dolomite or limestone containing thin interbeds of dolomitic or calcareous, fine-grained sandstone. Thickness ranges from 210 m in Blacktail Mountains to 115 m near Wise River, thickness is 200-400 m in Pioneer Mountains, about 90 m in the Ruby Range, and 150 m in Snowcrest Range (Saperstone and Ethridge, 1984).' 1494|'Snowcrest Range Group - Formerly mapped as Amsden Formation and (or) Big Snowy Group in this region, but now defined as Snowcrest Range Group, which contains Conover Ranch Formation, Lombard Limestone, and Kibbey Sandstone (Wardlaw and Pecora, 1985; Tysdal and others, 1987). Conover Ranch Formation (Lower Pennsylvanian and Upper Mississippian) - Pale-reddish-brown to pale-reddish-purple, thin-bedded, calcareous mudstone and minor interbeds of limestone, calcareous sandstone and siltstone, limestone-pebble conglomerate, and phosphatic claystone. About 13-33 m thick. Mainly of late Chesterian age but includes a few thin beds of Pennsylvanian age at top of formation. Lombard Limestone (Upper Mississippian) - Light-olive-gray, thin- to thick-bedded fossiliferous limestone and thin interbeds of silty limestone, siltstone, and shale, about 85-125 m thick, underlain by olive-gray and medium-red to pale-reddish-purple calcareous mudstone about 40 m thick. Chesterian to late Meramecian age. Kibbey Sandstone (Upper Mississippian) - Pale-red to pale-yellow, thin- to medium-bedded siltstone, sandstone, and claystone, and interbedded limestone solution breccia and evaporite solution breccia in middle part of formation. About 5-45 m thick. Late Meramecian age. Madison Group - Limestone, dolomitic limestone, and shale included in Mission Canyon Limestone and Lodgepole Limestone. Mission Canyon Limestone (Upper and Lower Mississippian) - Olive-gray to yellowish-gray and dark-yellowish-brown, light-gray-weathering, medium- and thick-bedded to massive, generally fine- to medium-grained (but ranges from very fine grained to coarse-grained) limestone. Locally cherty. Includes a few beds of dolomitic limestone in places. Upper part of formation includes pale-red to grayish-orange limestone solution breccia. Thickness ranges from 240 m in Blacktail Mountains to as much as 420 m in Ruby Range and Pioneer Mountains. Lodgepole Limestone (Lower Mississippian) - Medium- to dark-gray, fine- to medium-grained, thin- to thick-bedded limestone, overlying medium- to dark-gray and brownish-gray, thin-bedded, laminated, argillaceous limestone, containing thin interbeds and bedding partings of dark-gray shaly limestone and calcareous shale. In Blacktail Mountains, Ruby Range, and Highland Mountains the formation is composed of Woodhurst Member (upper) and Paine Member (lower). Thickness is 180-230 m in Ruby Range and Blacktail Mountains, and 100-200 m in Pioneer Mountains region.' 1495|'Three Forks Shale (Mississippian? and Upper Devonian) - Composed of, at top, Sappington Member of yellowish-orange calcareous or argillaceous siltstone and fine-grained sandstone; underlain by Trident Member of dark-gray shale and interbedded fossiliferous dark-gray limestone and yellowish-gray to yellowish-orange argillaceous limestone; and at base the Logan Gulch Member of brownish- to purplish-gray, finely crystalline limestone, yellowish- and brownish-gray to purplish-gray, thin- to thick-bedded limestone and dolomite solution breccia interbedded with greenish-gray shale near base. Thickness about 20-50 m in Ruby Range and Blacktail Mountains, 0-50 m in Pioneer Mountains region. Jefferson Formation (Upper and Middle Devonian) - Moderate- to dark-yellowish-brown to dark-gray, medium- to thick-bedded, locally thinly laminated, fine- to medium-grained, fetid dolomite and interbedded yellowish-brown to yellowish-orange, thin-bedded, fine-grained dolomite and silty, shaly dolomite; locally includes minor beds of shale and solution breccia. Thickness ranges from about 30-35 m in Blacktail Mountains and 80 m in Ruby Range to 150-200 m in Pioneer Mountains region.' 1496|'White to grayish-white, vitreous quartzite, locally containing dark, heavy minerals in planar layers and cross-beds; interlayered near base with greenish-black, silty, locally sandy argillite (Lewis, 1990). Argillite contains numerous trace fossils that are definitely of Paleozoic age, most probably Ordovician (M.W. Reynolds, written commun., 1987).' 1497|'Contains Snowy Range Formation, Pilgrim Formation, Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone (Ruppel and Lopez, 1984; Ruppel, 1985). In northern part of Pioneer Mountains, unit made up of Hasmark and Silver Hill Formations (Zen, 1988). Snowy Range Formation (Upper Cambrian) - The name "Red Lion Formation" used in some areas. Both formations divided into Sage Dolomite Member and underlying Dry Creek Shale Member. Sage Member is yellowish-brown to light-gray, very fine grained to fine-grained, thin- to thick-bedded dolomite, mottled and ribboned grayish red in lower part; locally contains thinly laminated algal beds and dome-shaped to columnar stromatolites in upper part. Dry Creek Member is olive-gray to reddish-gray, fissile, finely micaceous shale. Thickness about 20-50 m; absent in Blacktail Mountains, southern Tobacco Root Mountains, and locally absent in Pioneer Mountains. Pilgrim Formation (Upper Cambrian) - Pale-yellowish-brown to light-gray, fine- to medium- to thick-bedded dolomite; upper part is sandy and includes interbeds of fine- to medium-grained sandstone. About 90-120 m thick in eastern and central parts of Dillon region; thins to 12-13 m in western part, near Jackson. Park Shale (Middle Cambrian) - Pale-olive or grayish-green, finely micaceous, fissile shale and minor thin interbeds of yellowish-brown, medium-grained dolomite and yellowish-gray, thinly platy, calcareous, fine-grained sandstone, siltstone, and mudstone. About 45-60 m thick, pinches out to the west. Meagher Limestone (Middle Cambrian) - Medium-gray, medium-light-gray and yellowish-brown, fine- to medium-grained limestone, dolomitic limestone, and dolomite that partly is irregularly mottled yellowish gray, light gray, or pale red. Mainly limestone and dolomitic limestone in Tobacco Root Mountains and mainly dolomitic limestone and dolomite farther south and west, in Dillon region. Thickness about 75-150 m in Tobacco Root Mountains, 170-230 m in Ruby Range and Highland Mountains, and 166 m in Blacktail Mountains; thins westward to 12-13 m near Jackson. Wolsey Shale (Middle Cambrian) - Olive-green, grayish-red and reddish-brown, micaceous, glauconitic siltstone and olive-green, fissile, micaceous shale interbedded with grayish-red and yellowish-gray, glauconitic, argillaceous, fine-grained, platy sandstone and medium-gray, fine-grained limestone. Thickness about 15-75 m in Tobacco Root Mountains, 80 m in Highland Mountains, 18-30 m in Ruby Range, 20 m in Blacktail Mountains, and 85-120 m near Jackson. Flathead Sandstone (Middle Cambrian) - Pale-orange, light-yellowish-gray, and yellowish-brown, thin- to medium-bedded, mostly fine- to medium-grained, but locally coarse-grained, quartzitic sandstone and arkosic sandstone, locally conglomeratic. Upper part of formation commonly includes interbeds of glauconitic, fine-grained sandstone, and olive-green and pale-red mudstone and shale. Basal part of formation locally includes interbeds of yellowish-gray to pale-brown siltstone, mudstone, and shale. Thickness 0-43 m in Tobacco Root Mountains, 9-16 m in Ruby Range and Blacktail Mountains, and 20-30 m in Pioneer and Highland Mountains. Hasmark Formation (Upper and Middle Cambrian) - Light-gray, thin- to thick-bedded dolomite; locally contains oolites, pisolites, pellets, and algal structures. About 235-300 m thick. Correlative of Pilgrim Formation, Park Shale, and Meagher Limestone. Silver Hill Formation (Middle Cambrian) - Light-yellowish-brown calcareous shale, limestone, and dolomite and olive-green shale and mudstone; includes minor interbeds of thin-bedded sandstone. Thickness 0-100 m. Correlative of Park Shale, Meagher Limestone, Wolsey Shale, and Flathead Sandstone. Locally, also includes fine-grained clastic rocks of probable Middle Proterozoic Belt Supergroup age.' 1499|'Includes Gunsight Formation, Apple Creek Formation, and Big Creek Formation in Dillon quadrangle. Gunsight Formation - Light-brownish-gray, medium-light-gray to grayish-red-purple, thin- to medium-bedded, fine- to medium-grained feldspathic quartzite; many beds laminated or cross-laminated. Minimum thickness about 1,800 m, but no unfaulted sections known. Apple Creek Formation - Grayish-green, thin-bedded siltite and very fine grained feldspathic quartzite that contains irregular streaks and lenses of light-gray, ferrodolomite-cemented fine-grained sandstone; laminated or cross-laminated, commonly displaying mud cracks and ripple marks; locally pyritic. Lower part of formation includes subordinate beds of grayish-red-purple and dark-gray siltite and argillite. Thickness about 760-900 m. Big Creek Formation - Pale-greenish-gray too light-gray, mostly thick-bedded or massive, fine-grained feldspathic quartzite. Many beds laminated or cross-laminated. Thickness about 3,100 m.' 1500|'Medium-gray to medium-dark-gray, mostly thin- to medium-bedded, very fine grained to fine-grained, feldspathic, finely biotitic quartzite and subordinate interbedded dark-greenish-gray to medium-dark-gray, thin- to medium-bedded siltite and argillite. Probable thickness more than 8,000 m, but the base of the formation has not been found and the upper part is everywhere bounded by Late Cretaceous Medicine Lodge thrust plate in east-central Idaho and adjacent southwestern Montana (Lopez, 1981,1982a).' 1502|'Pioneer Mountains and the east flank of the Beaverhead Mountains: Unit includes Garnet Range Formation, McNamara Formation, Bonner Quartzite, and Mount Shields Formation and probable metamorphosed equivalents. Garnet Range Formation-Grayish-red, medium-red to pale-red, and light-brown, fine- to coarse-grained feldspathic, limonitic sandstone in thin and platy beds separated by thin beds and bedding partings of grayish-red to dusky-red siltite and fissile argillite. Bedding surfaces commonly spangled with white mica. In part thinly laminated or cross-laminated. Thickness 0-800 m. Known to occur in vicinity of Warm Spring Creek near Jackson, on Big Hole Divide, and in Argenta mining district in south-central part of map area. McNamara Formation-Grayish-red to pale-red, thin-bedded, platy, medium-grained, partly thinly laminated or cross-laminated feldspathic quartzite and quartzitic sandstone; thin interbeds and bedding partings of grayish-red argillite and interbedded greenish-gray, fine-grained quartzite and light-greenish-gray siliceous argillite; muscovite common on bedding surfaces; locally mud cracked and ripple marked. Thickness uncertain, possibly as much as 2,300 m. Known to occur in Warm Spring Creek area, as isolated outcrops near Jackson, and in northern part of Pioneer Mountains. Bonner Quartzite and Mount Shields Formation-Grayish-pink, grayish-red, and pale-red to light-gray and medium-light-gray, medium- to coarse-grained, but locally fine grained, thin- to thick-bedded, commonly cross-laminated, micaceous feldspathic quartzite and subordinate grayish-green to grayish-red siltite and argillite, most commonly as thin bedding partings and chips in quartzite. Includes conglomerate beds and lenses as much as 3 m thick, mainly in northern part of Wise River valley and east of French Creek. Thickness unknown. Occurs in central part of map area from Horse Prairie Creek northward through Pioneer Mountains to French Creek and Little Granulated Mountain. In most places unit is probably the middle and upper parts of Mount Shields Formation, but probably mostly the overlying Bonner Quartzite in area between French Creek and Little Granulated Mountain. In Beaverhead Mountains, quartzitic rocks included in Mount Shields Formation differ from those farther east, and probably are mostly lower part of Mount Shields Formation. Upper part of sequence is mainly light- brownish-gray, greenish-gray to light-gray, pale-red, or less commonly grayish-red-purple or dark-reddish-gray, fine- to medium-grained, thin-bedded to massive, locally ripple-marked, laminated and cross-laminated feldspathic quartzite; contains well-sorted, well-rounded glassy quartz grains and pink quartz and reddish chert grains in some places, and abundant dark-gray heavy minerals disseminated throughout the rock or concentrated on laminae and cross-laminae. Muscovite common on bedding surfaces; bedding partings and thin interbeds of dark-gray to greenish-gray and grayish-red argillite and siltite are present locally. Thickness unknown but probably more than 1,500 m. Lower part of the Mount Shields in Beaverhead Mountains consists of three zones: (1) Upper zone of 10 to 50-m-thick units of interlayered thin- and thick-bedded to massive, light-brownish-gray to light-gray, fine- to medium-grained, laminated and cross-laminated feldspathic quartzite and quartzitic sandstone interlayered with thin beds of dark-gray argillite; bedding surfaces and partings are commonly calcareous or dolomitic; probably about 150-200 m thick. (2) Middle zone, as much as 500-700 m thick, of light-brownish-gray to pale-purple, fine- to medium-grained, thick-bedded to massive, in part coarsely laminated and cross-laminated, feldspathic, partly calcareous or dolomitic quartzite and quartzitic sandstone containing bedding partings and sparse thin beds of dark-gray argillite; sparse floating pebbles and pebbly zones are common on bedding surfaces, and dark-gray heavy minerals are disseminated throughout rock and concentrated on laminae and cross laminae. (3) Lower zone, about 150-200 m thick, of light-brownish- or greenish-gray to light-gray, pebbly and conglomeratic, fine-grained to very coarse grained, cross-laminated calcareous or dolomitic quartzite, quartzitic sandstone, and sparse lenticular beds of sandy dolomite and sparse bedding partings and thin interbeds of dark-gray argillite; includes conglomerate lenses, as much as 2 m thick, of subangular to subrounded pebbles 1-4 cm in diameter of white quartz and less common quartzite, granite, and feldspar pebbles in sand matrix and on bedding surfaces. Lower part of Mount Shields Formation in Beaverhead Mountains exposed from vicinity of Ajax Lake and Lena Lake south to Hamby Lake; best exposed in vicinity of Ajax and Rock Island Lakes. Estimated thickness of lower part about 1,000 m; base not exposed. Anaconda Range and in the northern Beaverhead Mountains: Mount Shields Formation-Grayish-white to pink, fine- to coarse-grained, feldspathic quartzite locally interlayered with quartz-pebble conglomerate. Quartzite beds, generally less than 1 m thick, commonly become fine upward and are locally capped by siltite and argillite. The distinctive feldspathic quartzite beds of this formation grade downward into interlayered dark-gray argillite and feldspathic quartzite; argillite becomes predominant in lower part of formation, Upper contact of formation is not exposed. Thickness about 500 m. Shepard Formation-Tan, calcareous and dolomitic siltite, fine-grained quartzite, and minor conglomerate. Thickness about 200 m. Snowslip Formation-Tan to pinkish-white, fine- to medium-grained quartzite interbedded with dark-gray, thin-bedded, irregularly laminated siltite, silty argillite, and argillite. Contains sparse, thin (less than 20 cm thick) interbeds of clean, white, medium-grained quartzite. Thickness exceeds 450 m. Highland Mountains and in the northern Tobacco Root Mountains: Interlayered quartzite and siltite that pinches out to the east and west within Highland Mountains. Upper part consists of white to pinkish-white, fine- to medium-grained quartzite interlayered with minor tan siltite. Middle part consists of interlayered white to tan, thick-bedded, locally platy and cross-bedded quartzite interlayered with laminated, very fine grained, argillaceous quartzite. Quartzite grades laterally into subangular, pebble to cobble intraformational conglomerate in central part of Highland Mountains. Lower part consists of medium-gray, laminated, very fine-grained sandstone and siltstone showing even parallel lamination. Maximum thickness about 300 m.' 1503|'Metasediments in the Pioneer Mountains and the east flank of the Beaverhead Mountains: Interlayered gray to greenish-gray and brownish-gray, fine-grained biotite-muscovite metaquartzite; locally contains garnet and sillimanite. Previously interpreted by L.W. Snee in Berger and others (1983) to be pre-Belt in age. Metamorphosed Belt rocks in the Anaconda Range and in the northern the Beaverhead Mountains: Quartz-feldspar-biotite gneiss and minor schist. Gneiss is medium gray to tan, micaceous, and well foliated, and contains thin interlayers of dark-gray to black schist. Locally, schist is predominant rock type and consists of dark-gray to black mica schist enclosing thin stringers of quartz and feldspar. These rocks can be traced laterally into unmetamorphosed sedimentary rocks.' 1504|'Light-gray, tan, and white, slightly feldspathic, medium- to coarse-grained, thick- to thin-bedded quartzite. Contains a distinctive quartz-pebble to quartz-cobble conglomerate, as much as 5 m thick, in middle part. Unconformably overlies conglomerate (Ycg). Maximum thickness 300 m. Quartzite is best exposed at Grace Lake, located 1.5 km south of Black Lion Mountain.' 1505|'Light- to medium-gray, matrix-supported pebble conglomerate, pebbly quartzite, and poorly sorted, argillaceous quartzite and sandstone grit. Most of unit is very similar to parts of LaHood Formation of Belt Supergroup in Highland Mountains to the east. Zen (1988) named this unit Black Lion Conglomerate and inferred a Cambrian age.' 1506|'White, yellowish-white, and pinkish-white, medium- to coarse-grained, weakly feldspathic, evenly parallel-laminated to cross-bedded quartzite common in the upper 20 m of the section. Quartzite is underlain by, and locally interbedded with, yellow-tan to greenish-tan, parallel to cross-laminated argillaceous sandstone and siltstone, minor laminated argillite, and rare brownish-gray limestone. Base not exposed. Equivalent to the sequence of Swamp Creek of Zen (1988) and to Belt Supergroup rocks of Frazer and Waldrop (1972) exposed 3-5 km directly south of Wise River. Upper quartzite is similar to quartzite of Grace Lake, and underlying argillaceous rocks are similar to the laminated Greyson Shale in Highland Mountains to the east. Maximum exposed thickness near 50 m.' 1510|'Greyson Shale rocks in the Anaconda Range and in the northern Beaverhead Mountains: Quartzite is equivalent to Spokane Formation in Highland Mountains to the east. Quartzite of Queener Mountain-Light-gray to white, fine- to medium-grained quartzite interlayered with minor dark-gray argillite and siltite. Quartzite grades upward into dark-gray argillite interlayered with minor dark-gray, fine-grained quartzite, and local, thin beds of clean, white, medium- to coarse-grained quartzite. Thickness is nearly 600 m. Quartzite is best exposed at Queener Mountain at the head of Seymour Creek, directly north of the quadrangle boundary. Unit is stratigraphically equivalent to Spokane Formation of Ravalli Group in Helena embayment of Belt Basin. It was mapped as Ravalli Formation by Emmons and Calkins (1913); however, Elliot and others (1985) included this unit in Missoula Group rocks. Greyson Shale-Dark-gray to dark-greenish-gray, well-laminated argillite and silty argillite. Beds, generally less than 5 cm thick, commonly contain uneven, parallel lamellae of white to tan, very fine grained sandstone and siltstone. Contains thin, dark-gray quartzite lenses in the upper part. Thickness in excess of 1,000 m. Unit is stratigraphically equivalent and lithologically similar to Greyson Shale of the Helena embayment of the Belt basin. Unit was mapped as the Prichard Formation by Emmons and Calkins (1913) and included with Missoula Group rocks by Elliot and others (1985). Spokane and Greyson Formation rocks in the Highland Mountains and in the northern Tobacco Root Mountains: (Unit named "Spokane and Greyson Formations (Middle Proterozoic)", but has same map label.) Spokane Formation consists of grayish-tan to tan, structureless to laminated and commonly cross-bedded, argillaceous siltite and minor dark-grayish-green silty argillite interlayered with 0.2- to 1-cm-thick, discontinuous, very fine grained sandy lenses. Conspicuous white to pinkish-white, fine- to coarse-grained, grain-supported quartzite beds 1-2 m thick are common in upper part of formation and are thickest and most abundant in central part of Highland Mountains. Greyson Formation, underlying the Spokane, is thickest in central part of Highland Mountains, and consists of an upper dark-gray to black, laminated and platy to massive argillite and silty argillite that contains numerous soft-sediment slump structures. Basal part consists of light- to dark-gray silty argillite and argillaceous siltite having uneven to even parallel laminations and locally interlayered with tan, discontinuous silty lenses, commonly less than 1 cm thick; these basal siltite beds locally are interlayered with 0.2- to 0.5-m-thick, laterally discontinuous beds of matrix­supported quartzite composed of well-rounded grains, and locally dense, tan to white, cross-bedded quartzite. Thickness of combined formations ranges from less than 400 m in the west to more than 900 m in central part of Highland Mountains.' 1512|'Helena and Empire Formation rocks in the Highland Mountains and in the northern Tobacco Root Mountains: Helena and Empire Formations are thickest in central part of Highland Mountains, pinch out to the west, and thin to the east. The Empire generally underlies the Helena, but the formations are locally complexly interlayered and grade laterally into one another. Helena Formation typically consists of tan, pinkish-gray to grayish-green, laminated to thin-bedded, calcareous, and commonly vuggy siltite; locally shows water-expulsion features, mud cracks, and very finely laminated, upwardly convex pods that may be algal mounds. In central part of range, upper part of the Helena consists of calcareous siltite and argillite that locally show well-developed water-expulsion structures and rip-up conglomerate lenses; lower part consists of upward-fining cycles of very fine grained sandstone showing wavy parallel lamination overlain by argillaceous siltite interlayered with very fine grained sandstone lenses. Empire Formation consists of dark-gray to black, very fine grained, argillaceous siltite containing elongate pods of tan, silty limestone that is locally interlayered with thick-bedded, purplish-gray siltstone. Maximum thickness of unit about 350 m. Helena and Empire Formation rocks in the Anaconda Range and in the northern Beaverhead Mountains: Helena Formation consists of dark- to medium-gray, laminated, locally silty, calcareous argillite, argillaceous limestone, and dolomite. Bedding is uneven, wavy, and laterally discontinuous. In eastern part of range, basal part of the Helena locally contains abundant interbeds of fine- to medium-grained, gray to tan quartzite. Underlying Empire Formation consists of dark-gray to black, calcareous siltite containing irregular pods of white to light-gray limestone. Combined thickness of these formations exceeds 1,000 m. Middle Belt carbonate rocks appear to be present beneath rocks of Medicine Lodge thrust plate in Beaverhead Mountains north of Salmon, Idaho. Exposures directly north of North Fork consist of alternating sequences of calc-silicate rock, characterized by a basal upward-fining, fine-grained and laminated sandstone, 0.1 to about 2 m or more in thickness, capped by 0.1- to 1-m thick units of calcareous mud, silt, and sand in beds about 1 cm thick. The calcareous deposits are metamorphosed, are medium to dark green, and contain abundant porphyroblasts of actinolite-tremolite. In a few places the calcareous units become very thinly laminated. These calc-silicate units are interlayered with laminated to cross-bedded tan quartzites. Pinch-and-swell, centimeter-thick couplets of tan sand and brown to dark-gray argillite are common in upper part of sequence. These rocks, originally mapped as Yellowjacket Formation by Lopez (1982b), resemble the lower Yellowjacket Formation at its type locality at Yellowjacket Creek as well as similar exposures at Hayden Creek, where the unit exceeds 1,600 m (Lopez, 1981, p. 16-24). The cyclical clastic to carbonate cycles present in these rocks are also similar to sedimentary cycles of Wallace Formation of middle Belt carbonate to the north in Montana (D. Winston, personal commun., 1990) and are here tentatively correlated with these rocks rather than with the Yellowjacket. Rocks exposed directly south of Lost Trail Pass consist of at least three south-southwest-dipping rock units. Upper part of upright section consists mainly of cyclical deposits of clastic to calc-silicate rocks similar to those rocks to the south. These rocks are underlain by laminated, light-colored quartzite, very poorly exposed only on the east along Continental Divide, which in turn overlie thinly laminated siltite-argillite. The calc-silicate rocks are correlated with rocks directly to the south; these rocks were originally mapped as the Wallace Formation by Desmarais (1983). The underlying quartzite and laminated siltite-argillite are similar in sequence and lithology to the lower Belt quartzite and underlying siltite-argillite beneath the Helena Formation in the Anaconda Range (Ysg). Thickness of these units is not known.' 1514|'The Newland Formation consists of dark-gray to pinkish-gray argillite, silty argillite, and minor marl. Thin, discontinuous lenses of finely crystalline to medium-crystalline, medium-gray limestone are present in the upper part. The argillaceous rocks show a distinctive blocky bedding character: nonlaminated, structureless beds, 1-3 cm thick, of argillite are separated by ruler-straight, single to multiple lamellae of tan silt and very fine grained sandstone. Outcrops and float from the Newland are characterized by tabular, centimeter-thick beds having extremely planar parting surfaces. The Newland is everywhere underlain by a mappable sequence of argillite, siltite, and minor sandstone that lacks both the tabular, blocky beds of the Newland and the argillaceous grit found only in underlying LaHood Formation. The LaHood Formation consists of coarse sedimentary conglomerate and breccia; in Highland Mountains, cobble to boulder conglomerate grades laterally into quartz-pebble conglomerate, into coarse, argillaceous, lithic grit, and into arkose. The arkose grades into interlayered shale, siltstone, sandstone, and subordinate pebble conglomerate and argillaceous grit. Newland and LaHood Formations, like the overlying units, are thinnest on the west-less than about 700 m-and thickest in central part of Highland Mountains, more than 1,500 m.' 1515|'Quartzite, siltite, and argillite present only in Highland Mountains. Upper part of unit consists of massive white quartzite beds separated by 1- to 5-m-thick, upward-fining sequences of intraformational conglomerate, sandstone, siltstone, and argillite. Lower part consists of about 250 m of quartzite that is overlain by interbedded clean to argillaceous quartzite, siltite, and argillite. Basal contact is gradational into LaHood pebble and cobble conglomerate and sandstone; unit thins and pinches out to the west and is locally interlayered with Newland Formation on the east. Maximum thickness is more than 500 m.' 1516|'Dark-greenish-gray to black, fine- to medium-grained diabase dikes. Most dikes were intruded along pre-existing, northwest-trending faults in pre-Belt crystalline rocks between 1.4 and 1.1 Ga (Wooden and others, 1978).' 1517|'Light-colored, medium- to coarse-grained, generally equigranular microcline-plagioclase gneiss interlayered with dark-gray to black, fine-grained amphibolite, minor migmatitic and augen gneiss, and rare garnet-bearing calc-silicate schist and fine-grained calc-silicate gneiss. Exposed on north flank of Sheep Mountain. Age about 1.8-1.6 Ga (Arth and others, 1986; Zen, 1988).' 1518|'Dark-gray, medium- to coarse-grained, biotite-rich gneiss. Contains wispy lenses, seams, and augen of quartz and feldspar. In part, rock is very similar in composition and texture to mylonitic biotite gneiss (X(A)m) in Highland Mountains. Locally cut by numerous quartz-rich pegmatite dikes and large quartz veins. Exposed east of Bloody Dick Creek. Age about 1.8 Ga (R.E. Zartman, oral commun., 1987).' 1519|'Dense swarm of leucocratic, granitic aplite and pegmatite sills and minor dikes, and augen-bearing orthogneiss; interlayered mainly with mylonitic biotite gneiss and quartz-feldspar-biotite gneiss.' 1520|'Interlayered granitic to granodioritic, mylonitic orthogneiss and nonfoliated, leucocratic, medium- to coarse-grained granite sills.' 1521|'Dark-gray to black, medium-grained to very coarse grained, strongly folded, garnet-rich biotite gneiss; locally contains abundant thin ptygmatic quartz-feldspar veins.' 1522|'Dark-gray to black, medium- to coarse-grained, biotite-rich gneiss and minor schist. Contains conspicuous porphyroclasts of quartz and plagioclase that are commonly anhedral and broken, are sheared and drawn out in plane of foliation, or are augen; locally interlayered with granitic, augen-bearing orthogneiss.' 1523|'A diverse sequence chiefly composed of garnet-rich gneiss and schist; also contains discontinuous lenses of marble, quartzite, iron-formation, and amphibolite, Ranges in thickness from 0 to 100 m.' 1524|'Gray to tan, fine- to medium-grained quartz-feldspar-biotite gneiss in which compositional banding defines a distinct, platy foliation; locally interlayered with seams of finely layered mylonitic leucogneiss or sills of fine- to coarse-grained granite.' 1525|'Leucocratic, massive, medium- to coarse-grained, strongly deformed and lineated quartz-feldspar gneiss that contains sparse biotite or muscovite. Locally contains well-developed quartz-feldspar augen ranging in diameter from less than 1 cm to more than 6 cm.' 1526|'Black to dark-greenish-brown, coarse-grained rocks present as small lenses and irregularly shaped pods commonly less than 30 m in length or width but locally more than 1.5 km long. Consists of orthopyroxene, olivine, spinel, hornblende, and magnetite; locally partly altered to serpentine and talc (Desmarais, 1981).' 1527|'Complexly interlayered granitic, tonalitic, quartz-feldspar-biotite and hornblende-plagioclase gneiss (Vitaliano and Cordua, 1979). In Ruby Range the gneiss appears to underlie main metasedimentary succession (James, 1990); in Tobacco Root Mountains the gneiss is complexly interlayered with, and overlies, these metasedimentary rocks. In core of Armstead anticline, unit includes minor metasedimentary rocks (Lowell, 1965). Rb-Sr age determined from these rocks is 2.76 Ga (James and Hedge, 1980).' 1528|'Dark-gray to greenish-black, medium- to coarse-grained, hornblende-rich gneiss; forms lenses ranging from less than 1 m to more than 500 m in thickness. Rock composed mainly of hornblende and plagioclase.' 1529|'Light-colored, coarse-grained quartz-muscovite schist. Subordinate minerals include sillimanite or, locally, kyanite and minor garnet, anthophyllite, and biotite. Aluminous schist is closely associated with quartzite and marble.' 1530|'Composed mainly of quartz-feldspar-biotite gneiss, granitic gneiss, and amphibolite.' 1531|'Medium-gray to dark-brown, coarsely crystalline, carbonate-rich rocks present as layers that range from less than 1 m to more than 500 m thick. Rock composed mainly of calcite and dolomite, and contains subordinate amounts of quartz, diopside, tremolite, and forsterite.' 1532|'White to pinkish-gray, coarse-grained quartzite in lenses less than 2 cm thick to persistent beds 30-100 m thick. Commonly associated with marble and iron-formation. Thickest beds are present in southern Tobacco Root Mountains.' 1533|'Dark-gray rock consisting of alternate quartz-rich and magnetite-rich layers and lenses. All layers consist of essential magnetite, quartz, orthopyroxene and clinopyroxene, and garnet; individual layers typically 1-2 cm thick, but mappable units are as much as 30 m thick (James, 1981). Commonly associated with quartzite and aluminous schist. Present in southern Tobacco Root Mountains and Ruby Range.' 1534|'Well-foliated medium- to dark-gray biotite-hornblende-garnet gneiss, migmatite, augen gneiss, and aluminous schist. Structurally lowest sequence in Ruby Range (Karasevich and others, 1981).' 1550|'Gravel, sand, silt, and clay in flood plains and low terraces along present drainages. Includes alluvial fans.' 1551|'Includes large rock slumps, flow, and slides. Generally consists of angular blocks of bedrock in a finer grained matrix; mixed at places with glacial debris. Most abundant on slopes topographically below trace of Lewis thrust fault on east and south sides of Glacier National Park (Carrara, 1990).' 1552|'Includes till (in ground, end, and ablation moraines), outwash, and other fluvioglacial deposits from both continental and mountain glaciers. Includes eskers at a few places in Glacier National Park (Carrara, 1990).' 1553|'Conglomerate, sandstone, siltstone, marlstone, oil shale, and coal deposited in the Kishenehn basin. About 3,800 ft thick in the valley of the Middle Fork of the Flathead River in central part of map area (Whipple, 1992).' 1554|'Mainly dark-gray mudstone of the Marias River Shale as described by Mudge and Earhart (1983). May include the Horsethief Sandstone and St. Mary River Formation at northeast edge of Glacier National Park. At least 1,500 ft thick. Mudge and Earhart (1983) description of Marias River Shale: Mainly dark-gray, marine mudstone, ranges from 365 to 395 m thick. Divided into four members by Cobban, Erdmann, Lemke, and Maughan (1959b, 1976), in descending order: Kevin, Ferdig, Cone, and Floweree. The Kevin is dark-gray, calcareous mudstone with some thin, very fine grained sandstone in upper part. Characteristically contains many thin, micaceous bentonite beds and zones of calcareous and ferruginous limestone concretions. Bentonite beds thicker and far more numerous in southern outcrops. Pelecypods and ammonites common. The Kevin Member ranges from about 226 m thick in the east to as much as 326 m in the west (Mudge, 1972). The eastern facies of the Ferdig Member contains gray, noncalcareous mudstone with many very thin, iron-stained sandstone beds, concretions of yellow-weathering limestone and red-weathering ferruginous dolostone, and some very thin bentonite beds, about 50 m thick. The western facies of the Ferdig, exposed in the North and South Forks of the Sun River, is about 150 m thick, and resembles the Cardium Formation of southern Alberta, Canada (Mudge, 1972). Contains nodular sandstone, sandy shale, and even-bedded sandstone in the middle and upper parts of the western facies; thick-bedded, light-gray sandstone in upper part. Organic burrows and trails common. The lower part is like that exposed in the eastern facies. The Cone Member contains abundant, very thin, medium-gray, calcareous siltstone and crystalline limestone beds in the upper part and dark-gray noncalcareous fissile shale with some limestone concretions in the lower part (Cobban, Erdmann, Lemke, and Maughan, 1976); contains several bentonite beds throughout. The upper beds commonly have petroliferous odor on a freshly broken surface. Contains a characteristic fauna which includes Mytiloides labiatus (Cobban, Erdmann, Lemke, and Maughan, 1976). Ranges from 15 to 30 m thick. The Floweree Member is noncalcareous, dark-gray, nonfossiliferous shale (Cobban, Erdmann, Lemke, Maughan, 1976; Mudge, 1972). Locally contains basal siltstone with chert-pebble conglomerate. The shale has metallic luster and yellowish-brown stains on bedding and fracture planes (Mudge, 1972). Ranges from 9 to 12 m thick.' 1555|'Predominantly gray to olive mudstone and gray sandstone interbedded with some limestone. Includes the Blackleaf, Kootenai, and Mount Pablo Formations as mapped and described by Mudge and Earhart (1983). Thickness about 2,200 ft. Mudge and Earhart (1983) description of Blackleaf, Kootenai, and Mount Pablo Formations: BLACKLEAF FORMATION: Gray, marine mudstone and interbedded sandstone. Divided into three members by Cobban, Erdmann, Lemke, and Maughan (1959b, 1976), in descending order: Vaughan, Taft Hill, and Flood. The formation ranges from about 200 m thick in southern outcrop to about 490 m in the west and about 260 m in the north. The nonmarine Vaughan Member consists of alternating gray to olive-drab mudstones and bentonitic mudstone with many thin interbeds of light-gray, locally cross-bedded sandstone. Contains less sandstone to the north and lower beds are laterally equivalent to upper part of the marine Taft Hill strata to the south. Locally, beds of conglomerate fill small channels at base of some sandstone units (Mudge, 1972). In the Sun River area, the upper part of member contains tuffaceous debris, one bed contains accretionary lapilli (Mudge, 1972). Member locally contains wood and leaf fragments and, in the vicinity of Teton Pass, contains beds of coal and carbonaceous shale. Ranges from 90 m thick in the eastern outcrops to possibly 213 m in the north. In the south, the marine Taft Hill Member consists of thinly bedded, gray, fossiliferous sandstone, locally cross-bedded and ripple marked (Mudge, 1972), interbedded with dark-gray mudstone containing some very thin bentonite beds. These units grade northward into nonmarine lithologies of the Vaughan. The Taft Hill is as much as 183 m thick in west-central part of the area, thins to 0 m to the north and 58 m in the southeast outcrop. The marine Flood Member in the south consists of two sandstone units with a distinctive intervening shale unit. Lower sandstone absent to the north where upper sandstone unit is thicker and shale unit thinner, as compared to southern exposures. Gray, thinly cross-bedded, very fine grained, finely micaceous and moderately well sorted sandstones (Mudge, 1972); commonly weather grayish brown. Locally large sandstone nodules in lower part. The shale unit, transitional with upper sandstone unit, is distinct, very dark gray fissile shale with a metallic luster on bedding surfaces. Thin sandstone lentils and nodules of limestone and claystone common, locally phosphatic nodules present. Organic trails and burrows abundant in transition zone. The Flood ranges from 45 m thick in eastern outcrops to 165 m in western. About 40 m thick in the north part of mapped area (Rice and Cobban, 1977). KOOTENAI FORMATION: Nonmarine, gray-green and maroon mudstone with numerous lenticular, poorly sorted, greenish-gray sandstone beds, locally cross-bedded and contain lenticular basal conglomerates (Mudge and Sheppard, 1968; Mudge, 1972). Brown to brownish-gray limestone with thin to thick lenses of coquina with pelecypods and gastropods is near top of formation. Brown, iron-stained limestone nodules common in mudstone beds. The Sunburst Sandstone Member of Rice and Cobban (1977) is at base of formation in the southern outcrops and absent to the north. The Sunburst consists of many thin beds of hard, noncalcareous, poorly sorted quartz sandstone with few scattered grains of chert and feldspar. The Kootenai ranges from 198 to more than 305 m thick. MOUNT PABLO FORMATION: Nonmarine, formerly referred to as the western facies of the Morrison Formation in the Sun River Canyon area (Mudge, 1972). Consists of limestone, mudstone, and sandstone in upper part, variegated mudstone interbedded with sandstone in middle part, and sandstone and conglomerate in lower part (Mudge and Rice, 1980). Everywhere overlain unconformably by the Kootenai Formation (Kk). The lower sandstone and conglomeratic sandstone is the Cut Bank Sandstone Member, an important hydrocarbon reservoir rock near Cut Bank, Mont. The upper and middle parts of the formation are mostly gray to gray-green mudstone with some maroon and orange-red mudstone and thin beds of fine-grained sandstone. Mostly reddish-brown mudstone with some sandstone in southwestern area (Mudge, 1972). Widespread, light-gray, lacustrine limestone, as much as 10 m thick, near top of the formation. The Cut Bank Sandstone Member in places contains coarse-grained, cross-bedded sandstone in the upper part, composed of nearly equal amounts of quartz and black chert. Grades upward into very fine grained sandstone as much as 10 m thick in the north. The lower conglomeratic sandstone unit is prominent, cross-bedded, poorly sorted, very coarse grained sandstone that grades upward into very fine sandstone. In many places has dominantly chert-pebble conglomerate at its base, to the south conglomeratic lenses common at various horizons. Commonly about 8 m thick, the lower sandstone unit is as much as 30 m thick. The Mount Pablo Formation (KJm) ranges from 0 to about 90 m thick, unconformably overlies the Morrison Formation except in northeastern outcrops where overlies the Swift Formation.' 1556|'Includes siltstone, sandstone, and shale accompanied by minor limestone and thin conglomerate at base of some members as mapped and described by Mudge and Earhart (1983). About 1,500 ft thick. Mudge and Earhart (1983) description of Mount Pablo Formation (Lower Cretaceous), Morrison Formation (Upper Jurassic), and Ellis Group (Upper and Middle Jurassic), undivided unit: The Morrison Formation is not mapped separately and therefore is described below. The most complete sections of the Morrison are in east-central and southern outcrop areas. Mainly grayish-green, tuffaceous siltstone with interbedded sandstone, limestone, and some cherty siderite in the eastern part of the Sun River Canyon area (Mudge, 1972). Maroon and tints of pinkish-gray beds common in the upper part. Cherty siderite occurs as lenses in the middle part, limestone occurs as beds or nodules in the lower part. The Morrison is about 61 m thick in the Sun River area (Mudge, 1972), about 82 m thick in the Wolf Creek area (Schmidt, 1972a). Mostly eroded prior to deposition of the Mount Pablo Formation north of the Sun River area. In most places, the Morrison is less than 30 m thick. Mudge and Earhart (1983) description of ELLIS GROUP: Divided into three formations by Cobban (1945), in descending order: Swift, Rierdon, and Sawtooth. In the Wolf Creek area the Rierdon Formation is absent, the combined thickness of the Swift and Sawtooth Formations is about 65 m. The three formations have an aggregate thickness of about 87 m in the Sun River area, about 205 m thick west of that area, and more than 188 m in the northern part of mapped area (Mudge and Earhart, 1978). The Upper and Middle Jurassic Swift Formation was divided into upper and lower unnamed members by Cobban (1945). Ranges from 30 to 36 m thick in the southeast to more than 60 m in the northwest. In the northeast, part of the upper member was eroded prior to sedimentation of the Cretaceous Mount Pablo Formation. The upper member is thin-bedded, gray to gray-brown, very fine to fine-grained sandstone. As much as 30 m thick in the Sun River area (Mudge, 1972), less than 3 m thick in the northeast outcrop. The lower member is dark-gray shale with some interbeds of sandstone. A thin bed of poorly indurated glauconitic sandstone with water-worn belemnites and locally chert pebbles, at base of the member except in the northern outcrops. The lower member averages about 15 m thick in the south and about 21 m thick in the north. The Swift unconformably overlies the Rierdon Formation. The Middle Jurassic Rierdon Formation contains calcareous gray-brown siltstone and claystone in the upper part and calcareous, dark-gray, laminated shale and claystone in the lower part. Many thin beds of argillaceous limestone scattered throughout formation. Barite nodules, numerous pelecypods and some ammonites common. Phosphatic nodules common in the lower part of northern exposures. About 44 m thick in the Sun River area, as much as 56 m thick to the north. The Middle Jurassic Sawtooth Formation is divided into three unnamed members by Cobban (1945), the lower member is absent in the northern outcrop area. Ranges in thickness from 15 to 69 m, thickening to the north. The upper siltstone member is a prominent unit, many thin beds of grayish-brown to yellowish-gray siltstone with a few thin beds of shale, increasingly sandy northward. Lenses of phosphatic pellets common in the western and northern outcrops. Pelecypods common and ammonites rare. Member 6-13 m thick in the south (Mudge, 1972), thickens northward to about 18 m (Imlay, 1962). The shale member is dark-gray shale with some siltstone, sandstone, and a few beds of limestone. Thickens northward from 5 m in the Sun River area to about 77 m near Mount Patrick Pass. Some beds locally pyritic. North of the Teton River contains black, phosphatic pellets and lies unconformably on Mississippian carbonate rocks. The lower sandstone member in the southern part of the area rests unconformably on Mississippian rocks. In most places, hard, fine-grained, and light-gray sandstone beds, conglomeratic in the basal part. Locally consists of two beds of sandstone separated by dark-gray shale. The conglomerate consists of pebbles and cobbles and locally boulders of Mississippian carbonate and chert (Mudge, 1972). The sandstone member ranges from 0-6 m thick, in most places 0.6-2 m thick (Mudge, 1972). Mudge and Earhart (1983) description of MOUNT PABLO FORMATION: Nonmarine, formerly referred to as the western facies of the Morrison Formation in the Sun River Canyon area (Mudge, 1972). Consists of limestone, mudstone, and sandstone in upper part, variegated mudstone interbedded with sandstone in middle part, and sandstone and conglomerate in lower part (Mudge and Rice, 1980). Everywhere overlain unconformably by the Kootenai Formation (Kk). The lower sandstone and conglomeratic sandstone is the Cut Bank Sandstone Member, an important hydrocarbon reservoir rock near Cut Bank, Mont. The upper and middle parts of the formation are mostly gray to gray-green mudstone with some maroon and orange-red mudstone and thin beds of fine-grained sandstone. Mostly reddish-brown mudstone with some sandstone in southwestern area (Mudge, 1972). Widespread, light-gray, lacustrine limestone, as much as 10 m thick, near top of the formation. The Cut Bank Sandstone Member in places contains coarse-grained, cross-bedded sandstone in the upper part, composed of nearly equal amounts of quartz and black chert. Grades upward into very fine grained sandstone as much as 10 m thick in the north. The lower conglomeratic sandstone unit is prominent, cross-bedded, poorly sorted, very coarse grained sandstone that grades upward into very fine sandstone. In many places has dominantly chert-pebble conglomerate at its base, to the south conglomeratic lenses common at various horizons. Commonly about 8 m thick, the lower sandstone unit is as much as 30 m thick. The Mount Pablo Formation (KJm) ranges from 0 to about 90 m thick, unconformably overlies the Morrison Formation except in northeastern outcrops where overlies the Swift Formation.' 1558|'Upper part is the Castle Reef Dolomite and lower part is the Allan Mountain Limestone as mapped and described by Mudge and Earhart (1983). About 1,200 ft thick Mudge and Earhart (1983) description of MADISON GROUP: Divided into the Castle Reef Dolomite and the underlying Allan Mountain Limestone (Mudge, Sando, and Dutro, 1962), equivalent in age to the Mission Canyon and Lodgepole Limestones in Central Montana. The Madison ranges from about 275 m to 550 m thick, much of the variation in thickness is a result of pre-Jurassic erosion (Mudge, 1972). The Upper and Lower Mississippian Castle Reef Dolomite, ranges from about 230 m thick in the eastern outcrops to about 305 m in the west, is divided into an upper member, the Sun River Member, and a lower unnamed member (Mudge, Sando, and Dutro, 1962). The Sun River Member, 76-137 m thick, consists mostly of thick beds of fine- to medium-crystalline dolomite (Mudge, 1972), and is main hydrocarbon reservoir rock on the Sweetgrass Arch (Chamberlain, 1955). In many places oil residues common in cavities, pores, fractures, and on bedding planes in the upper part of the member. The lower member of the Castle Reef consists of thin to thick beds of fine to coarsely crystalline, light-gray dolomite, calcitic dolomite, and dolomitic limestone, dolomite content increases westward. Beds of coarsely crystalline encrinites at various horizons in the Castle Reef, increasingly abundant to the north and west. Lenses and nodules of dark-gray chert common in lower and middle parts, light-gray chert nodules common in upper part. In places, sand-filled joints and bedding planes in upper part (Mudge, 1972). Corals and brachiopods common in the formation. The Lower Mississippian Allan Mountain Limestone, ranges from about 165 to 200 m thick, contains three widespread unnamed members. The upper member is mainly gray, medium- to thick-bedded, fossiliferous limestone with some beds of dolomitic and magnesium limestone (Mudge, Sando, and Dutro, 1962). In places encrinite beds occur in middle and upper parts of member. Member ranges from 42 to 106 m thick. The middle member contains abundant, irregular-shaped lenses and nodules of very dark chert in sparsely fossiliferous, medium-bedded, dark-gray limestone and dolomitic limestone, ranges from 45 to 58 m thick. Chert is dispersed throughout member at 15-25 cm intervals. The lower member consists of very thinly bedded, dark-gray, argillaceous limestone and dolomitic limestone with dark-gray shale partings (Mudge, Sando, and Dutro, 1962). The lower part of lower member contains alternating beds of dark-gray to gray-brown limestone and very calcareous shale, locally potential hydrocarbon source rocks (Mudge, Rice, Earhart, and Claypool, 1978); ranges from 50 m to 67 m thick.' 1559|'Mainly limestone and dolomite in upper part; in lower part mudstone increases downward. Units mapped and described by Mudge and Earhart (1983). About 1,700 ft thick. Mudge and Earhart (1983) description of THREE FORKS, JEFFERSON, AND MAYWOOD FORMATIONS, UNDIVIDED: Widespread in the Lewis, Hoadley, and Eldorado thrust plates in western part of mapped area and in numerous thrust plates in the Sawtooth Range. Range from 290 m thick in the southeast exposures to 457 m northwestward at Slategoat Mountain (Mudge, 1972) and northward to about 519 m southwest of Swift Reservoir. Natural gas produced from Birdbear Member of the Jefferson on the Sweetgrass Arch, and from equivalent rocks and the Three Forks Formation in southern foothills of Alberta. In most places the Upper Devonian Three Forks almost entirely an evaporite-solution breccia, consists of angular fragments (mostly less than 1 m across) of pale-yellowish-brown dolomite and dolomitic limestone (Mudge, 1972). Grayish-green mudstone and massive gray limestone bed, as much as 30 m thick, overlie breccia in the southern outcrop (Mudge, 1972). Elsewhere beds above the breccia are thinly bedded, yellowish-gray, silty limestone and siltstone and black shale. Black shale mostly about 30 cm thick, locally as much as 1 m thick, observed at north end of Sawtooth Ridge, in Deep Creek (Mudge, 1972), on north side of Crown Mountain, in tributary of the Dearborn River, south of Steamboat Mountain, on South Fork Teton River across from Rierdon Gulch, on a mountain on south side of the upper reaches of that fork, on northeast side of Family Peak, on southeast side of Mount Drewyer, and at head of the north fork of Whitetail Creek. The presence of black shale at these widely scattered localities indicates that it is widespread, but in places is absent as a result of tectonic deformation and in places is covered by thick talus deposits. Fossils recorded in black shale by Gutschick, Suttner, and Switek (1962) and Sandberg (1965). The Three Forks Formation as much as 60 m thick, locally about 15 m thick in eastern outcrops (Mudge, 1972). Thickens westward to 180 m at Slategoat Mountain. The Upper Devonian Jefferson Formation contains the Birdbear Member at the top (Sandberg,1965) and a lower unnamed member. Ranges from 225 to 245 m thick in the eastern outcrop, thins westward to about 190 m at Slategoat Mountain. The Birdbear Member consists mostly of pale-yellowish-brown, very fine to finely crystalline, thin dolomite beds, characteristically pinches and swells, weathers light yellowish gray. Brachiopods most common fossil (Mudge, 1972). The Birdbear ranges from about 16 m thick in the north to 45 m in the southeast, about 72 m in the southwest. The lower member is mostly limestone in northern and southwestern exposures, mostly dolomite in southeastern exposures. Commonly gray-brown and locally light-gray fetid beds, about 30 cm thick. One or more thin, evaporite-solution breccias in lower part in southern outcrops; thicker and more numerous throughout member in northern outcrops. Biostromes of Amphipora widespread in upper part and locally in lower part, corals and brachiopods occur at several horizons. The lower member ranges from 200 to 210 m thick in the central part of the Sawtooth Range, thinning to about 120 m to the west at Slategoat Mountain and to 130 m to the southeast near Gibson Dam (Mudge, 1972). The Upper and Middle Devonian Maywood Formation is divisible into two unnamed members (Mudge, 1972). Ranges from 16 to 85 m thick. The upper member is mostly thinly bedded, finely crystalline, somewhat fossiliferous limestone and dolomitic limestone, forms resistant ledges. Mostly grayish-brown, mottled to pale yellowish orange to yellowish gray on the weathered beds (Mudge, 1972). Locally thin, evaporite-solution breccia in lower part. The upper member thickens westward from about 8 m in the southeastern outcrops, 16 m in the northeastern, to about 50 m in the western outcrops. The lower member is mostly greenish-gray mudstone with thin beds of dolomite, dolomitic limestone, and breccia in the lower part. Maroon mudstone common in the upper part of member in the western outcrops. In places, coarse-grained sandstone at base. North of the Sun River, the lower part of member contains channel fills as much as 6 m deep; the Cambrian- Devonian unconformity is at the base of channel fill deposits, The lower member thickens westward from about 8 m in the Sun River area (Mudge, 1972), about 4 m west of Swift Reservoir, to about 63 m at Slategoat Mountain.' 1560|'Alternating units of carbonate and shale above a basal sandstone. In Montana disturbed belt, includes (in descending order): Devils Glen Dolomite (Upper Cambrian), Switchback Shale (Upper and Middle Cambrian), and Steamboat Limestone, Pentagon Shale, Pagoda Limestone, Dearborn Limestone, Damnation Limestone, Gordon Shale, and Flathead Sandstone (Middle Cambrian) (Mudge and Earhart. 1983). Only lower three or four formations exposed unconformably above Belt rocks in southern part of map area. Maximum exposed thickness about 1,000 ft. Mudge and Earhart (1983) description of CAMBRIAN ROCKS: Includes, in descending order: Devils Glen Dolomite, Switchback Shale, Steamboat Limestone, Pentagon Shale, Pagoda Limestone, Dearborn Limestone, Damnation Limestone, Gordon Shale, and Flathead Sandstone. Limestone formations contain considerable mudstone in the east, mostly limestone in the west. All formations well exposed in the western part of mapped area. Only upper three formations exposed in some thrust blocks in the Sawtooth Range. In the western part of the mapped area, Cambrian rocks thin northward from about 535 m in the southwest to about 440 m at Pagoda Mountain, and to 495 m at Pentagon Mountain (Deiss, 1939). In the southern part of the mapped area, the Dearborn River area, they are 680 m thick. The Upper Cambrian Devils Glen Dolomite is thick-bedded, light-gray dolomite, in most places forms prominent, light-gray cliff. Trilobites recorded in the upper part in the Sun River Canyon area (Mudge, 1972). Ranges from about 32 m thick at Pagoda Mountain (Deiss, 1939) in the west to as much as 167 m at Nineteen Mountain (Mudge, 1972) in the southeast, and about 60 m thick to the north; variation in thickness very likely a result of pre-Devonian erosion. The Upper and Middle Cambrian Switchback Shale is mostly noncalcareous, greenish-gray, thinly laminated, clayey shale with thin local interbeds of dolomite, limestone, sandstone, and conglomerate. Conglomerates locally contain brachiopods (Mudge, 1972). The Switchback varies considerably in thickness from about 35 m at Pentagon Mountain to 23 m at Pagoda Mountain (Deiss, 1939) to 77 m at Nineteen Mountain, and to 82 m at Arsenic Mountain (Mudge, 1972). The Middle Cambrian Steamboat Limestone consists of two parts in the western outcrop, a lower shaly interval and a much thicker upper limestone and dolomite interval (Deiss, 1939; Mudge, 1972), whereas in the eastern outcrop, in the Sawtooth Range, is about equal parts of alternating sequences of limestone and dolomite, and calcareous shales. Carbonates in both areas consist of hard, nodular, dark-gray to yellowish-brown, thinly bedded, finely crystalline beds of limestone and dolomite with nodules and lentils of dark-yellowish-orange, limey siltstone. Thin beds of intraformational conglomerate present locally. Mudstone units mainly grayish-green, noncalcareous, clayey shale with interbedded calcareous siltstone and claystone. Trilobites and some brachiopods locally in both limestone and shale beds. The Steamboat Limestone is about 92 m thick at its type locality in Dearborn Canyon, 66 m at Pentagon Mountain, 81 m at Pagoda Mountain (Deiss, 1939), and 67 m at Nineteen Mountain (Mudge, 1972). The Middle Cambrian Pentagon Shale is present only in the northern part of mapped area, occurs as clastic wedge that extends south from Pentagon Mountain about 22 km (Deiss, 1939), and north about 7 km. The Pentagon consists of calcareous, gray to tan-gray, thick-bedded, platy, fossiliferous shale, contains some platy, blue-gray, argillaceous limestone in upper part (Deiss, 1939). Ranges from 0 to 88 m thick. The Middle Cambrian Pagoda Limestone consists of an upper limestone member and a lower shale member. Upper member is finely crystalline, pale-yellowish-brown, very thin bedded limestone and thin- to thick-bedded dolomitic limestone and dolomite, the most prominent cliff-forming unit in the Cambrian sequence. Intraformational conglomerate locally in the middle part of the Pagoda. The lower member consists of beds of grayish-green, thinly laminated to nodular, clay shale with some gray-brown limestone and minor sandstone. Fossils locally present in limestone beds. The Pagoda, in general, thickens to the south and west; varies from about 53 m thick along the Continental Divide in the central part of mapped area to about 107 m at Pagoda Mountain, and 111 m near Prairie Reef Lookout Station (Deiss, 1939). The Middle Cambrian Dearborn Limestone consists of an upper, finely crystalline, pale-yellowish-brown to gray, very thinly and irregularly bedded limestone unit and a lower mudstone unit of gray to gray-green, clayey shale with some sandy shale that contains trilobites. Ranges from about 67 to 106 m thick, averages about 90 m (Deiss, 1939). The Middle Cambrian Damnation Limestone consists of thinly bedded, finely crystalline, medium- to dark-gray dolomitic limestone and limestone with laminae of grayish-orange to yellowish-gray siltstone. Mottled, grayish-orange limestone beds. Trilobites present in lower part of formation (Deiss, 1939). The Damnation is 31 m thick on the Continental Divide near Cliff Mountain, thickens northward to 58 m at Pentagon Mountain, southward to 45 m at Prairie Reef (Deiss, 1939), and about 51 m thick in the Dearborn Canyon area (Deiss, 1939). The Middle Cambrian Gordon Shale is mainly a dark-gray to gray-brown, very thinly laminated shale with greenish tint. Contains many thin beds of sandstone and some beds of limestone. In places, limestone contains glauconite, algal structures, fossil fragments, limestone chips, and quartz grains (Mudge, 1972). Organic burrows and trails common in lower sandstone beds. Trilobites numerous in upper part (Deiss, 1939). Thickens southward from about 43 m at Pentagon Mountain to about 90 m along the Continental Divide near Cliff Mountain (Deiss, 1939), from that area it thins southward to 60 m at Prairie Reef (Deiss, 1939). The Middle Cambrian Flathead Sandstone consists of thin- to thick-bedded, noncalcareous, yellowish-gray, poorly sorted, poorly indurated, fine- to coarse-grained, cross-bedded quartzose sandstone with scattered quartz pebbles. Commonly contains organic trails and burrows, speckled with brown hematite. Ranges from about 13 to 35 m thick, rests unconformably on rocks of the Belt Supergroup.' 1562|'Dioritic to gabbroic rocks that commonly show alteration of mafic minerals to calcite, epidote, clinozoisite, and hematite or limonite. Sills range in thickness from a few tens to a few hundreds of feet. Thicker sills tend to have contact-metamorphic zones around them. Sills commonly persist for many miles and in most places maintain approximately the same stratigraphic position. Locally, they cut across the section at a low angle or, rarely, cut steeply across section to form dikes. Intruded in map area probably at about 1,000 Ma, or 800 Ma, or both (Harrison, 1972).' 1563|'Gray to olive, micaceous siltite to quartzarenite interlaminated or interbedded with gray-green, micaceous, argillitic siltite. Occurs only in small exposures at south edge of map area beneath Cambrian unconformity. Maximum thickness of remnant about 500 ft.' 1564|'Predominantly grayish-green, interbedded and interlaminated argillite and siltite that contain thin chert laminae and chips. Oolites, stromatolites, quartzarenite, and stratabound cooper [sic] minerals present at places. Relatively thin red-bed sequences locally interbedded in the green strata. Small-scale sedimentary features include ripple marks, shrinkage cracks, scours, and cross-beds. Rests sharply on Bonner Quartzite. About 4,000 ft thick.' 1565|'Red to pink, micaceous, arkosic, cross-bedded, fine- to medium-grained quartzite containing red argillite interclasts. Tabular and trough cross-beds and climbing ripple marks common. Interbeds of red, laminated argillite and pink, planar-laminated siltite scattered throughout unit. Rests in sharp contact on Mount Shields Formation. Thickness ranges from about 800 to 1,200 ft.' 1566|'Consists of a series of informal members similar to those seen to the west in the Kalispell 1°x2° quadrangle (Harrison and others, 1992) where six informal members are described; sequence in the Cut Bank quadrangle is similar, but member 5 is missing. Type section (Childers, 1963), about 2,550 ft thick, is between the Blacktail and Roosevelt faults in southeast corner of Glacier National Park on Mount Shields. Childers did not use the informal members subsequently found to be widespread in the formation. Maximum thickness in map area about 2,900 ft. Uppermost unit in map area (member 6) is a thinly laminated, black argillite and white or green siltite that commonly displays small-scale slump folds and shrinkage cracks. Thickness about 200 ft. Conformably below is member 4, which consists of blocky, green, dolomitic, silty argillite that shows parallel-laminated graded couplets. Foot-thick carbonate beds are scattered throughout unit, as are rare salt casts. Thickness about 900 ft. Grades downward into member 3, which is predominantly alternating red and green beds of interlaminated argillite and siltite. Mud chips, mud cracks, ripple marks, and fluid-escape structures are common. Salt casts are abundant, particularly in red beds. Thickness about 500 ft. Grades by interlayering, through a few tens of feet, to member 2 below. Member 2 is pale-red, flat-laminated, coarse-grained siltite to fine-grained quartzarenite that is blocky, feldspathic, and dolomitic. Dolomite is in cement or in streaks and pods parallel to bedding. Minor layers of red or green argillite. Red or buff stromatolites and oolites in a zone of one or more beds at top. Thickness about 700 ft. Grades downward into member 1. Member 1 is red to maroon, feldspathic quartzarenite interbedded with red siltite. Siltite increases in abundance to east and north. Red argillite beds and partings common between coarser-grained beds. Cross stratification, mud chips, and heavy-mineral streaks common. Rare stromatolites. Thickness about 600 ft in southern part of map area.' 1567|'Consists of a variety of rock types, most of which are carbonate bearing or carbonate rich. The most characteristic lithology is green to gray, dolomitic argillite and siltite in even-parallel to wavy laminae that show graded couplets. This rock weathers into distinctive orange-brown platy slabs. Common interbeds a few to a few tens of feet thick include gray dolomitic limestone, stromatolites and oolites, laminated green argillite, gray argillitic siltite, and white quartzarenite. All rock types may contain pyrite cubes, small-scale wiggly calcite segregations (molar tooth structure), horizontal carbonate pods, ripple marks, or mud cracks. Base of unit is marked at places by shallow channels of quartzarenite, or in northwestern part of map area by an unconformity that separates the Shepard from the underlying Purcell Lava. Shepard thickness ranges from about 700 ft in Glacier National Park to about 2,700 ft in the Swan Range in southwestern part of map area (fig. 2).' 1569|'Type section of the Snowslip Formation (Childers, 1963) is on the south spur of Mount Shields in the southern part of Glacier National Park. Detailed study of the Snowslip by Whipple and Johnson (1988) in the park has led to recognition of six informal members that can be correlated over at least the 1,000 mi2 of the park. The Snowslip generally consists of two alternating intervals that are each one hundred to several hundred feet thick. One interval consists of thinly laminated, red to purple argillite and siltite interbedded with thinly laminated green argillite and siltite. The other interval consists of couplets of greenish-gray siltite and olive argillite. Within both intervals are beds of stromatolites, arenite, and carbonate as layers or cement; certain characteristics of these carbonate and arenite interbeds help to distinguish the various members of the Snowslip (Whipple and Johnson, 1988). Small-scale sedimentary structures include mud cracks, ripple marks, mud-chip breccias, fluid-escape structures, cross lamination (particularly in the arenite beds), and flat-pebble conglomerate. Sharply overlies Helena Formation. The Snowslip and its members generally thin to the north and thicken to the west across depositional strike. Ranges in thickness from about 1,200 ft in the north to about 4,000 ft in the Swan Range to the south (fig 2).' 1570|'The most carbonate-rich formation in the Belt Supergroup. Contains abundant beds of dolomite, stromatolitic or oolitic limestone, and molar-tooth limestone and dolomite, and lesser amounts of quartzarenite and black argillite. In Glacier National Park contains an interval of stromatolitic limestone about 100 ft thick (the conophyton zone of Rezak, 1957) that separates the upper and middle parts of the Helena. Commonly displays sedimentary cycles, 6-50 ft thick, that in eastern exposures have a clastic bed at the base, stromatolitic and oolitic beds in the middle, and dolomite at the top (Eby, 1977). Abundance of stromatolites and oolites decreases rapidly to the west and south where cycles are still well defined but change to a clastic bed at the base, a middle unit of silty dolomite containing pods and ribbons of calcite, and an Lipper unit of dense, conchoidal-fracturing dolomite. Rests on laminated green argillite beds of the Empire Formation. Formation changes thickness rapidly across the various structural blocks in the map area (fig. 2), ranging from about 1,200 ft in southeastern Glacier National Park to about 7,000 ft in the Swan Range.' 1571|'Thinly laminated, dark-green and light-green dolomitic argillite and silty argillite or siltite. Laminae mostly wavy and discontinuous although some are even parallel. Fluid-escape structures are characteristic, and horizontal pods of white or pink calcite are particularly abundant in upper part. Ripple marks, syneresis cracks, and mud chips common in places. Lower part contains white dolomitic quartzarenite beds as thick as 10 ft. Pyrite cubes common in more carbonate-rich strata, and a few exposures display stratabound copper minerals. A few purple interlaminated argillite and siltite beds commonly occur near base and at places near middle of unit. Thickness about 500 ft.' 1572|'Three lithologic units are found in most areas. The upper member has beds a few feet to a few tens of feet thick of laminated, green argillite and siltite that alternate with much thicker beds of purple, laminated argillite and siltite. Dolomitic cement is common as are ripple marks, mud chips, desiccation cracks, fluid-escape structures, ball-and-pillow structures, and flute casts. Beds a few inches to a few feet thick of white, rounded grains forming medium-grained, crossbedded quartzite that displays mud chips and balls on cross strata are abundant in northern part of map area but decrease rapidly in both number and thickness in southern and western parts. The middle member is predominantly pink to purple-gray, very fine grained, feldspathic quartzite or coarse siltite that has planar lamination or long tabular cross lamination. Interbeds of purple argillite are common. The lower member is similar to the upper member but has more purple argillitic beds, is more dolomitic, and has scattered iron-carbonate specks and cement. The Spokane changes facies eastward and northward where abundance of distinctive white, rounded-grain quartzite has been used arbitrarily to define the laterally equivalent Grinnell Formation. Maximum thickness of the Spokane is about 4,500 ft in the Swan Range, but the unit thins to about 1,700 ft in northern exposures (fig. 2).' 1573|'Term used in Glacier National Park for the quartzarenite-rich lithofacies equivalent to the Spokane Formation. White, rounded-grain, crossbedded quartzarenite containing red argillite chips and pellets forms about 60 percent of the formation and almost 100 percent of the upper part. Amount of quartzarenite decreases rapidly westward and southward (fig. 2). Red to purple laminated siltite, silty argillite, and argillite form interbeds that make up most of the remainder of the unit. Argillitic beds generally have even-parallel lamination, and couplets of argillite and siltite display ripple marks, mud cracks, and fluid-escape structures. Grades by interlayering into the Appekunny Formation. Thickness ranges from about 1,700 to 2,600 ft.' 1574|'Predominantly bright-green to olive argillite interbedded and interlaminated with laminated olive siltite and gray pyritic quartzarenite that weathers brown. Lower part contains maroon argillite and siltite. Can be divided into five informal members in Glacier National Park (Whipple, 1992). Various members’ display mud cracks, mud-chip breccia, load structures, fluid-escape structures, and a few scour-and-fill structures. Rests unconformably on Altyn Formation in Glacier National Park but apparently intertongues with Prichard Formation transition member to southwest where unit mapped as Appekunny along west side of Flathead Range has beds characteristic of both Appekunny and Prichard. Maximum thickness about 2,000 ft.' 1577|'Generally consists of three units: a basal unit of medium-gray blocky-weathering siltite and minor quartzite, a middle unit of interlaminated light- and dark-gray siltite and argillite, and an upper unit similar to the middle but containing interbeds of light-olive-gray siltite and quartzite similar to those in the overlying Appekunny Formation. These three units are present nearly everywhere, but their relative proportions vary from place to place. Laminae are wavy to lenticular; scour-and-fill structures, fluid-escape structures, and syneresis cracks are common. Iron sulfide weathers to give rusty aspect to many outcrops. Randomly oriented biotite porphyroblasts common, particularly near top, in all but easternmost exposures. Calcareous siltite and argillite beds, some in the middle but most in the upper unit, make up a small part of the member. Contains some stromatolitic beds in Glacier National Park. Probably intertongues with Altyn Formation to east. Basal contact is sharp. Thickness about 2,000 ft.' 1600|'Unconsolidated stream-laid sand, gravel, and silt, bouldery, poorly to moderately well sorted. Includes alluvial fan, slope wash, colluvial and glacial outwash deposits of Bull Lake and younger in age. Thickness of deposits as much as 10 m.' 1601|'Mostly rock debris, locally coarse angular rock fragments in silt or clay matrix. Forms hummocky topography. Produced by rockfall- and rockslide-avalanches, slump, and earthflow. Thickness possibly as much as 300 m.' 1602|'Drift, heterogeneous mixture of rock fragments in silty clay matrix. Forms hummocky topography. Includes deposits from alpine and continental glaciations; as much as 100 m thick except in Swan River valley where thickness of mostly sand and gravel deposits exceed 300 m.' 1603|'Light-gray to dark-gray and gray-brown, thin- to thick-bedded silty clay, clay, and some sand; deposited in glacial lakes. Thickness of deposits as much as 30 m.' 1604|'Coarse stream-laid gravel on remnants of at least four bench surfaces as much as 250 m above modern stream alluvium. Maximum thickness is about 10 m.' 1605|'Gray, yellowish-gray, gray-brown, sandy silt, silt, clay, shale, marl, and some poorly sorted conglomerate; locally thin coal and carbonates. Locally includes wood and leaf fragments, insects, fish, gastropods, pelecypods, and ostracods. In Landers Fork, Blackfoot River includes tuff. Thickness possibly as much as 125 m.' 1606|'Map unit includes the Paleocene and Upper Cretaceous Willow Creek Formation (Viele, 1960; Schmidt, 1972b) in southeast corner of map where only the lower part of the Willow Creek is present and where it consists of grayish-green, olive-drab, and light-gray mudstone and sandstone. Some coarse to very coarse grained beds of volcanic sandstone. The upper part of the St. Mary River consists of light- to dark-red, purplish-red, brown, gray, greenish-gray mudstone and sandstone, and some beds of volcanic sandstone and conglomerate. A thin bed of white ash-fall tuff occurs near middle part. The formation is as much as 790 m thick. The upper part of the St. Mary River Formation, west of Augusta, is mostly grayish-green, sandy mudstone with moderate-light-red and purple interbeds. It contains numerous zones of dark-brown limestone nodules. Near Augusta, strata equivalent to the lower part of the St. Mary River are present and consist of grayish-green, olive-drab, and light-gray mudstone, sandy mudstone, and sandstone with some gray argillaceous limestone and carbonaceous shale. Sandstones are fine to medium grained and crossbedded. They are as much as 12 m thick. Thin argillaceous limestone beds as much as 0.3 m thick are locally present. A carbonaceous shale bed as much as 1.0 m thick is beneath a 1.0 m oyster bed near the base of the formation. Some reptile bone fragments and pelecypods are near the base. In the Augusta area the formation thickness is as much as 430 m.' 1607|'Horsethief Sandstone is mostly gray to gray-brown, fine- to medium-grained crossbedded sandstone. The upper 6-12 m of the formation commonly contain lentils of titaniferous magnetite sandstone. Southeast of Augusta the Horsethief contains volcanic-rich sandstone and conglomerates (Viele and Harris, 1965). The Horsethief is absent in the southeast part of the quadrangle where its position is marked by an Ostrea glabra bed (Viele, 1960); locally the sandstone contains pelecypods. The Horsethief is as much as 50 m thick. The Bearpaw-Horsethief transition unit of Cobban (1955) beneath the Horsethief, consists of dark-gray mudstone interbedded with light- to medium-gray mudstone and fine- to medium-grained sandstone that is thin bedded in the lower part becoming thicker bedded in the upper part. Near Sun River a 0.3 m coal bed occurs about 6 m below the top. Thickness of the transition unit as much as 60 m.' 1608|'Two Medicine Formation: Gray-green and gray mudstone with minor sandstone in upper and middle parts with gray-green, olive-drab, and gray sandstone and mudstone in lower part. Upper and middle parts locally contain reddish-gray, red-brown, and purple interbeds of mudstone. The sandstones are fine to coarse grained and locally conglomeratic. Carbonaceous shale and locally a coal bed are present in the lower part. Petrified wood common about 30 m above base; vertebrate bones common in the upper 150 m. Pelecypods locally present at various horizons. Thickness about 670 m. Virgelle Sandstone: consists of moderately thick, light-gray, fine-grained sandstone beds of which some are locally iron impregnated and crossbedded. At the top of the Virgelle, a titaniferous magnetite sandstone bed, as much as 6 m thick, forms a prominent rimrock south, west, and northwest of Choteau, as well as numerous other localities in the outcrop area. The Virgelle rarely contains pelecypods; wood fragments are locally in upper part. Thickness ranges from about 40 m to 60 m. Telegraph Creek Formation: A transitional unit between the underlying Marias River Shale and the overlying Virgelle Sandstone. Consists mainly of beds of gray mudstone and fine-grained sandstone. The sandstone beds are thinly bedded in the lower part becoming thicker bedded in the upper part. Pelecypods and ammonites are common. Thickness ranges from about 90 m in the eastern outcrop to about 165 m in the western outcrop. Marias River Shale: Mostly a dark-gray mudstone that is divided into four members (listed in descending order), the Kevin, Ferdig, Cone, and Floweree, by Cobban, Erdmann, Lemke, and Maughan (1959, 1976). Total thickness ranges from 365 m to 395 m. Kevin Member Dark-gray mudstone with some very thin sandstone beds, numerous bentonite beds, and many light-gray calcareous concretions. Pelecypods and ammonites common. Ranges in thickness from 245 m to 305 m. Ferdig Member Gray, noncalcareous siltstone and shale with many thin, iron-stained sandstone lenses, concretions of yellow-weathering limestone, red-weathering ferruginous dolostone, and some very thin bentonite beds. Contains abundant organic trails and burrows, but pelecypods and ammonites are rare. In the western outcrop, along the tributaries of the Sun River, the Ferdig is mostly sandstone (Mudge, 1972). Ranges in thickness from 50 m to 105 m. Cone Member Abundant, very thin, medium-gray calcareous siltstone and crystalline limestone beds in the upper part and dark-gray, noncalcareous shale in the lower part. Contains several bentonite beds throughout. Pelecypods and ammonites are common, especially in the upper part. The member ranges in thickness from 18 m to 30 m. Floweree Member Dark-gray, noncalcareous fissile to thin-bedded shale with medium-gray siltstone in lower part that locally contains lenses of chert-pebble conglomerate. Pelecypods and ammonites are rare. Thickness about 10 m. Blackleaf Formation: Blackleaf Formation mostly sandstone and mudstone, and includes some fissile shale. Formation divided into three members (listed in descending order), Vaughn, Taft Hill, and Flood. The total thickness about 200 m in the eastern outcrop and 490 m in the western outcrop. Vaughn Member: Nonmarine, light-gray, gray-green, and green tuffaceous and bentonitic mudstone and sandstone. Dark-gray carbonaceous mudstone locally in the upper part in the southeastern outcrop. The sandstone units are fine to coarse grained, locally crossbedded, and in places contain pebble and cobble conglomerate channel-fill deposits as much as 6 m thick (Mudge and Sheppard, 1968). The member contains wood and leaf fragments, and in the vicinity of Teton Pass contains coal and carbonaceous shale. Thickness from about 90 m in the eastern outcrop to about 150 m in the western outcrop. Taft Hill Member: Marine, gray, thinly bedded, fine-grained sandstone units interbedded with dark-gray mudstone; the sandstone is locally crossbedded and ripple marked. In the northwest part of the quadrangle the upper part of the member is replaced by nonmarine strata of Vaughn lithologies. Locally the Taft Hill contains numerous pelecypods. Thickness ranges from about 58 m in the southeast to about 183 m in the west. Flood Member: Two marine, gray, sandstone units separated by as much as 150 m of dark-gray fissile shale. The sandstones are very fine grained, thin to moderately thick bedded, and locally crossbedded and ripple marked. Organic burrows and trails common. Thickness from about 45 m in the eastern outcrop to about 165 m thick in the western outcrop. Kootenai Formation: Consists of nonmarine, gray-green, and dark-reddish-brown mudstone interbedded with lenticular thin to thick sandstone units. Numerous heavily iron-stained, spheroidal nodules of dark-grayish-red sandy limestone are in the mudstone. The sandstone is fine to coarse grained, grayish-green, with chert and quartz grains and locally magnetite. Pebble and cobble conglomerate, as much as 15 m thick, fill narrow channels locally at the base of some sandstone units. In most places a distinctive hard, dense, brown coquinoid limestone containing abundant pelecypods and some vertebrate fragments at or near the top. Thickness ranges from 198 m to 245 m. Mount Pablo Formation: Nonmarine. Exposures widespread in the eastern mountains but locally absent in the adjacent foothills. Rests unconformably on the Morrison Formation. In the western drainages of the Sun River area was included as part of the western facies of the Morrison Formation by Mudge (1972). Ranges from a dominantly sandstone sequence with interbedded bright-reddish-brown mudstone to a dominantly reddish brown mudstone sequence with some sandstone. Dense, dark-gray to light-gray limestone beds as much as 9 m thick are present in the upper part of the formation. In many places, coarse sandstone and thin beds of conglomerate occur as channel-fill deposits. The sandstones are medium to very coarse grained, crossbedded, and contain wood fragments. Thickness about 60 m. Morrison Formation (Upper Jurassic) Nonmarine. A nearly complete section is present in the eastern and southern outcrop, but much of the formation was eroded prior to deposition of the Mount Pablo Formation in the western outcrop. Consists of tuffaceous, grayish-green, olive-green, and olive-gray claystone to siltstone with pink, maroon, purple, and yellowish-gray mudstones in the upper part. A thin, dark-gray carbonaceous shale present near the top in southeastern outcrop. Fine-grained clayey sandstone locally present. Abundant polished quartzite pebbles and limestone nodules characteristic of the Morrison locally. Cherty siderite lenses and nodules locally common about 35 m above the base of the formation. A thin, dark-gray-brown to gray limestone present in the lower part of the formation and locally in the middle part. Gastropods, pelecypods, plant fragments, ostracods, and vertebrate bones are sparse in the middle and lower parts of the formation. Thickness from 60 m to 82 m. Swift Formation (Upper and Middle Jurassic) Marine. Consists of thinly bedded gray to gray-brown fine-grained sandstone in the upper part and dark-gray to olive-drab mudstone with many thin beds of sandstone in the lower part. The upper beds are locally ripple marked and contain minute cross laminations and wood fragments. A thin glauconitic sandstone, with water-worn belemnites, present at the base of the formation. Formation rests unconformably on the Rierdon Formation everywhere except in the southeast corner of the map where it rests on the Sawtooth Formation. Thickness about 35 m. Rierdon Formation: Marine. Widely distributed except in the southeast corner of the map area. Consists mostly of gray, calcareous mudstone with thin interbedded argillaceous limestone. Barite nodules common in the upper and middle parts. Pelecypods and ammonites common throughout. Thickness from 33 m to 60 m. Sawtooth Formation: Marine. The upper member is gray-brown to yellowish-brown, calcareous, thin-bedded siltstone that contains pelecypods and ammonites. About 8 m thick. The middle member is dark-gray, silty to clayey fissile shale with local thin beds of fine-grained sandstone and conglomerate. Locally rests unconformably on Mississippian rocks. From 5 m to 77 m thick. The lower member is mostly thin bedded, fine-grained gray to yellowish-brown sandstone with a basal conglomerate of Mississippian rock fragments. Dark-gray, silty, thinly laminated shale locally interbedded in the sandstone. Pelecypods are locally abundant. Thickness of the lower member as much as 16 m.' 1609|'Gray-green and gray mudstone with minor sandstone in upper and middle parts with gray-green, olive-drab, and gray sandstone and mudstone in lower part. Upper and middle parts locally contain reddish-gray, red-brown, and purple interbeds of mudstone. The sandstones are fine to coarse grained and locally conglomeratic. Carbonaceous shale and locally a coal bed are present in the lower part. Petrified wood common about 30 m above base; vertebrate bones common in the upper 150 m. Pelecypods locally present at various horizons. Thickness about 670 m.' 1610|'South of Augusta. The upper and middle parts are green, grayish-green, gray, brownish-gray, and maroon volcanic sandstone, mudstone, and conglomerate; brown, pink, and white ash-fall and ash-flow tuffs; and interbedded trachyte and latite flows (Viele, 1960; Viele and Harris, 1965; Schmidt and Strong, 1972; and Schmidt, 1972a, b, c). Lower 150 m is gray-green, olive-drab, and gray sandstone and mudstone. Thickness ranges from 640 m too possibly as much as 1,500 m.' 1611|'Virgelle Sandstone: consists of moderately thick, light-gray, fine-grained sandstone beds of which some are locally iron impregnated and crossbedded. At the top of the Virgelle, a titaniferous magnetite sandstone bed, as much as 6 m thick, forms a prominent rimrock south, west, and northwest of Choteau, as well as numerous other localities in the outcrop area. The Virgelle rarely contains pelecypods; wood fragments are locally in upper part. Thickness ranges from about 40 m to 60 m. Telegraph Creek Formation: A transitional unit between the underlying Marias River Shale and the overlying Virgelle Sandstone. Consists mainly of beds of gray mudstone and fine-grained sandstone. The sandstone beds are thinly bedded in the lower part becoming thicker bedded in the upper part. Pelecypods and ammonites are common. Thickness ranges from about 90 m in the eastern outcrop to about 165 m in the western outcrop.' 1612|'Mostly a dark-gray mudstone that is divided into four members (listed in descending order), the Kevin, Ferdig, Cone, and Floweree, by Cobban, Erdmann, Lemke, and Maughan (1959, 1976). Total thickness ranges from 365 m to 395 m. Kevin Member Dark-gray mudstone with some very thin sandstone beds, numerous bentonite beds, and many light-gray calcareous concretions. Pelecypods and ammonites common. Ranges in thickness from 245 m to 305 m. Ferdig Member Gray, noncalcareous siltstone and shale with many thin, iron-stained sandstone lenses, concretions of yellow-weathering limestone, red-weathering ferruginous dolostone, and some very thin bentonite beds. Contains abundant organic trails and burrows, but pelecypods and ammonites are rare. In the western outcrop, along the tributaries of the Sun River, the Ferdig is mostly sandstone (Mudge, 1972). Ranges in thickness from 50 m to 105 m. Cone Member Abundant, very thin, medium-gray calcareous siltstone and crystalline limestone beds in the upper part and dark-gray, noncalcareous shale in the lower part. Contains several bentonite beds throughout. Pelecypods and ammonites are common, especially in the upper part. The member ranges in thickness from 18 m to 30 m. Floweree Member Dark-gray, noncalcareous fissile to thin-bedded shale with medium-gray siltstone in lower part that locally contains lenses of chert-pebble conglomerate. Pelecypods and ammonites are rare. Thickness about 10 m.' 1613|'Blackleaf Formation: Mostly sandstone and mudstone, and includes some fissile shale. Formation divided into three members (listed in descending order), Vaughn, Taft Hill, and Flood. The total thickness about 200 m in the eastern outcrop and 490 m in the western outcrop. Vaughn Member: Nonmarine, light-gray, gray-green, and green tuffaceous and bentonitic mudstone and sandstone. Dark-gray carbonaceous mudstone locally in the upper part in the southeastern outcrop. The sandstone units are fine to coarse grained, locally crossbedded, and in places contain pebble and cobble conglomerate channel-fill deposits as much as 6 m thick (Mudge and Sheppard, 1968). The member contains wood and leaf fragments, and in the vicinity of Teton Pass contains coal and carbonaceous shale. Thickness from about 90 m in the eastern outcrop to about 150 m in the western outcrop. Taft Hill Member: Marine, gray, thinly bedded, fine-grained sandstone units interbedded with dark-gray mudstone; the sandstone is locally crossbedded and ripple marked. In the northwest part of the quadrangle the upper part of the member is replaced by nonmarine strata of Vaughn lithologies. Locally the Taft Hill contains numerous pelecypods. Thickness ranges from about 58 m in the southeast to about 183 m in the west. Flood Member: Two marine, gray, sandstone units separated by as much as 150 m of dark-gray fissile shale. The sandstones are very fine grained, thin to moderately thick bedded, and locally crossbedded and ripple marked. Organic burrows and trails common. Thickness from about 45 m in the eastern outcrop to about 165 m thick in the western outcrop.' 1614|'All or part of the following formations are present in this unit. Castle Reef Dolomite (Upper and Lower Mississippian) Divided into two members. Sun River Member at the top, which consists of thin to thick beds of medium to finely crystalline light-gray dolomite and locally some interbedded calcitic dolomite. Many beds contain thick lenses of encrinite and scattered brachiopods and corals. The Sun River is from less than 1 m to 137 m thick. The lower member is thick-bedded, fine to coarsely crystalline, light- to medium-gray dolomite, calcitic dolomite, dolomitic limestone, and limestone. The coarsely crystalline beds are encrinite and are more numerous in the northern and western outcrop. The lower member contains brachiopods and corals, locally abundant in the lower part. The lower member is 114 m to 145 m thick. Both members thin eastward, mainly as a result of pre-Jurassic erosion. Allan Mountain Limestone (Lower Mississippian) Ranges in thickness from 163 m to 200 m, and contains three unnamed members. The upper member consists of gray, fine-grained, thin to thick beds of limestone, magnesian limestone, and dolomitic limestone. Nodules and lentils of gray- to gray-brown chert are common (Mudge, 1972). It has a large and varied fauna, mostly brachiopods and corals, and locally, lenses and beds of encrinite. Upper member 60 to 90 m thick. The middle member consists of dark-gray, fine-grained, thin to medium beds of limestone with some dolomitic limestone. Characteristically contains nodules and irregular-shaped to even-bedded lenses of dark-gray chert, of which some have a fibrous appearance (Mudge, 1972). Contains sparse brachiopods and corals (Mudge, Sando, and Dutro, 1962). The middle member about 45 m thick. The lower member mostly dark-gray, very thin bedded, argillaceous dolomitic limestone with many calcareous shale partings. Contains dense, gray, moderately thick limestone interbedded with dark-gray mudstone. The mudstone has abundant brachiopods and corals. Thickness of lower member 60 m to 89 m. Three Forks Formation (Upper Devonian) The uppermost unit is a black shale bed of Early Mississippian age unnamed in this quadrangle but called the Sappington Member of the Three Forks elsewhere in Montana and is correlative to the Exshaw Shale in Alberta (Mudge, 1972). In most places it overlies thinly bedded, gray-brown to yellowish-gray limestone. The limestone commonly overlies an evaporite-solution breccia, but locally, in the Sun River area, overlies a thick bed of limestone and in one place a gray-green mudstone (Mudge, 1972). Fossils abound in the dark-gray shale and limestone beds. The rest of the Three Forks consists mostly of a pale-yellowish-brown to yellowish-gray evaporite-solution breccia consisting of angular fragments of dolomite and dolomitic limestone. Thickness from 15 m to 180 m. Jefferson Formation (Upper Devonian) Consists of two members: the Birdbear and a lower member. The Jefferson thickens eastward from about 190 m in the western outcrop to 247 m in the east. The Birdbear Member consists mostly of pale-yellowish-brown to brownish-gray, finely crystalline dolomite beds that pinch and swell. Brachiopods are commonly present. Ranges in thickness from 45 m to about 72 m. The lower member is mostly dolomite in the eastern outcrop, whereas it is mostly limestone in the western outcrop (Mudge, 1972). Consists of distinctive gray-brown beds (mostly less than 1/2 m thick) that characteristically have a fetid odor on the broken surface. Many beds have a sucrosic texture. The lower part commonly contains one or more thin beds of evaporite-solution breccia. Dark-gray chert lenses common in the lower part in the eastern outcrop. Corals, brachiopods, and stromatoporoids common throughout and Amphipora biostromes are widespread in the upper part. Thickness from 128 m to 198 m. Maywood Formation (Upper and Middle Devonian) Contains an upper limestone member and a lower mudstone member. The upper member is mainly dark grayish-brown and yellowish-gray, thinly bedded, finely crystalline limestone and dolomitic limestone that thickens eastward from about 21 m in the west to 48 m in the east. Contains brachiopods and corals. The lower member is mostly greenish gray dolomitic mudstone that in the western outcrops is interbedded with maroon mudstone. Contains thin beds of crystalline yellowish-gray to olive-gray dolomite and dolomitic limestone. A widespread dolomite with some breccia present in the middle part of the member. Charophytes and conodonts are locally present in one of the dolomite beds. The lower member ranges in thickness from about 8 m in the eastern outcrop to 63 m in the western outcrop. Devils Glen Dolomite (Upper Cambrian) A distinctive, thick-bedded, light-gray, finely to very finely crystalline dolomite. Thickness from 54 m to 172 m (Deiss, 1939). Switchback Shale (Upper and Middle Cambrian) Mostly noncalcareous, greenish-gray, thinly laminated clay shale with local thin interbeds of dolomite, limestone, sandstone, and conglomerate. Thickness from 21 m to 92 m. Steamboat Limestone (Middle Cambrian) Differs in lithology between western and eastern outcrops. In the west consists of a lower shaly mudstone interval and a much thicker upper limestone interval (Deiss, 1939). In the eastern exposures is about equal parts of alternating sequences of limestone and calcareous shale. The carbonate units in both areas consist mostly of nodular, hard, dark-yellowish-brown, thinly bedded limestone and dolomite with nodules and lentils of dark-yellowish-orange siltstone. The mudstone units are mainly grayish-green noncalcareous shale with interbeds of calcareous siltstone and claystone. Trilobites, locally abundant, and some brachiopods occur in limestone lenses in the shales and locally near the top of the limestone units. Thickness from 65 m to 80 m. Pentagon Shale (Middle Cambrian) A clastic wedge that consists of very fossiliferous, calcareous, gray to tan-gray, thick-bedded platy shale that contains some platy, blue-gray argillaceous limestone in the upper part (Deiss, 1939). Thickness ranges from less than 1 m to 88 m (Deiss, 1939). Pagoda Limestone (Middle Cambrian) Forms prominent light-gray cliffs in the Cambrian sequence. The upper part consists of yellowish-gray to light-yellowish-brown, thin- to thick-bedded dolomitic limestone and some dolomite overlying very thin bedded limestone. The lower part consists of grayish-green, thinly laminated to nodular clay shale with some gray-brown limestone and minor sandstone. An intraformational conglomerate locally present in the middle and lower parts. Trilobites and brachiopods locally numerous in the shale (Deiss, 1939). Formation thickens to the south and west, and ranges in thickness from 28 m to about 120 m. Dearborn Limestone (Middle Cambrian) Composed of an upper thick limestone unit and a lower thin shale unit. The limestone unit is finely crystalline, yellowish-brown to gray, thin to thick bedded and irregularly bedded. The lower unit consists of gray to gray-green clayey shale with some sandy shale and thin interbedded limestone. Trilobites present in the lower part of the shale unit. Thickness from 67 m to 106 m. Damnation Limestone (Middle Cambrian) Consists of medium- to dark-gray, thin- to thick-bedded, finely crystalline dolomitic limestone and limestone with laminae of grayish-orange to yellowish-gray siltstone that thicken and thin. Locally they are oolitic and contain trilobites, brachiopods, and organic trails and burrows. Thickness from 30 m to 68 m. Gordon Shale (Middle Cambrian) Mainly a dark-gray to gray-brown, very thinly laminated shale with a greenish tint and locally maroonish-gray beds. Contains many thin beds of sandstone and some beds of limestone, especially in the middle and upper parts. In places the limestones contain glauconite, algal structures, limestone chips, grains of quartz, and fossil fragments. The upper part contains numerous fossils (Deiss, 1939), whereas the lower beds locally contain organic trails and burrows, some by trilobites. Thickness from 42 m to 90 m. Flathead Sandstone (Middle Cambrian) Consists of thin- to thick-bedded and crossbedded, noncalcareous yellowish-gray, poorly sorted, poorly indurated, fine- to coarse-grained quartzose sandstone with scattered quartz pebbles. Beds are characteristically speckled by disseminated hematite. Interbeds of gray, purple, or maroon mudstone locally present. Thickness from 13 m to 35 m.' 1615|'Mississippian rocks are the main cliff former in the eastern part of the mountains and are assigned to the Madison Group. The total thickness of the Mississippian rocks ranges from 275 m to 520 m. The Madison is divided into two formations, the Castle Reef Dolomite and the Allan Mountain Limestone, by Mudge, Sando, and Dutro (1962). Castle Reef Dolomite (Upper and Lower Mississippian) Divided into two members. Sun River Member at the top, which consists of thin to thick beds of medium to finely crystalline light-gray dolomite and locally some interbedded calcitic dolomite. Many beds contain thick lenses of encrinite and scattered brachiopods and corals. The Sun River is from less than 1 m to 137 m thick. The lower member is thick-bedded, fine to coarsely crystalline, light- to medium-gray dolomite, calcitic dolomite, dolomitic limestone, and limestone. The coarsely crystalline beds are encrinite and are more numerous in the northern and western outcrop. The lower member contains brachiopods and corals, locally abundant in the lower part. The lower member is 114 m to 145 m thick. Both members thin eastward, mainly as a result of pre-Jurassic erosion. Allan Mountain Limestone (Lower Mississippian) Ranges in thickness from 163 m to 200 m, and contains three unnamed members. The upper member consists of gray, fine-grained, thin to thick beds of limestone, magnesian limestone, and dolomitic limestone. Nodules and lentils of gray- to gray-brown chert are common (Mudge, 1972). It has a large and varied fauna, mostly brachiopods and corals, and locally, lenses and beds of encrinite. Upper member 60 to 90 m thick. The middle member consists of dark-gray, fine-grained, thin to medium beds of limestone with some dolomitic limestone. Characteristically contains nodules and irregular-shaped to even-bedded lenses of dark-gray chert, of which some have a fibrous appearance (Mudge, 1972). Contains sparse brachiopods and corals (Mudge, Sando, and Dutro, 1962). The middle member about 45 m thick. The lower member mostly dark-gray, very thin bedded, argillaceous dolomitic limestone with many calcareous shale partings. Contains dense, gray, moderately thick limestone interbedded with dark-gray mudstone. The mudstone has abundant brachiopods and corals. Thickness of lower member 60 m to 89 m.' 1616|'Consists of limestone, dolomite, and some shale and mudstone; they range in thickness from about 300 m in the eastern outcrop to about 458 m in the western outcrop, and are divided into the Three Forks, Jefferson, and Maywood Formations. Three Forks Formation (Upper Devonian) The uppermost unit is a black shale bed of Early Mississippian age unnamed in this quadrangle but called the Sappington Member of the Three Forks elsewhere in Montana and is correlative to the Exshaw Shale in Alberta (Mudge, 1972). In most places it overlies thinly bedded, gray-brown to yellowish-gray limestone. The limestone commonly overlies an evaporite-solution breccia, but locally, in the Sun River area, overlies a thick bed of limestone and in one place a gray-green mudstone (Mudge, 1972). Fossils abound in the dark-gray shale and limestone beds. The rest of the Three Forks consists mostly of a pale-yellowish-brown to yellowish-gray evaporite-solution breccia consisting of angular fragments of dolomite and dolomitic limestone. Thickness from 15 m to 180 m. Jefferson Formation (Upper Devonian) Consists of two members: the Birdbear and a lower member. The Jefferson thickens eastward from about 190 m in the western outcrop to 247 m in the east. The Birdbear Member consists mostly of pale-yellowish-brown to brownish-gray, finely crystalline dolomite beds that pinch and swell. Brachiopods are commonly present. Ranges in thickness from 45 m to about 72 m. The lower member is mostly dolomite in the eastern outcrop, whereas it is mostly limestone in the western outcrop (Mudge, 1972). Consists of distinctive gray-brown beds (mostly less than 1/2 m thick) that characteristically have a fetid odor on the broken surface. Many beds have a sucrosic texture. The lower part commonly contains one or more thin beds of evaporite-solution breccia. Dark-gray chert lenses common in the lower part in the eastern outcrop. Corals, brachiopods, and stromatoporoids common throughout and Amphipora biostromes are widespread in the upper part. Thickness from 128 m to 198 m. Maywood Formation (Upper and Middle Devonian) Contains an upper limestone member and a lower mudstone member. The upper member is mainly dark grayish-brown and yellowish-gray, thinly bedded, finely crystalline limestone and dolomitic limestone that thickens eastward from about 21 m in the west to 48 m in the east. Contains brachiopods and corals. The lower member is mostly greenish gray dolomitic mudstone that in the western outcrops is interbedded with maroon mudstone. Contains thin beds of crystalline yellowish-gray to olive-gray dolomite and dolomitic limestone. A widespread dolomite with some breccia present in the middle part of the member. Charophytes and conodonts are locally present in one of the dolomite beds. The lower member ranges in thickness from about 8 m in the eastern outcrop to 63 m in the western outcrop.' 1617|'Complete sections of these rocks are only in the western outcrop. Deiss (1939) divided these rocks into nine formations: Devils Glen Dolomite (top), Switchback Shale, Steamboat Limestone, Pentagon Shale, Pagoda Limestone, Dearborn Limestone, Damnation Limestone, Gordon Shale, and Flathead Sandstone (bottom). The Pentagon Shale occurs only in the vicinity of Pentagon Mountain where Deiss (1939) noted it to extend about 25 km to the south and about 7 km north of the mountain. The Devils Glen, Switchback, and Steamboat are in thrust blocks in the eastern outcrops. The limestone formations contain considerable mudstone in the east, but they are mostly limestone in the west. All formations locally contain trilobites and some brachiopods. The Cambrian rocks thin northwestward from 681 m near the Dearborn River to 439 m at Pagoda Mountain (Deiss, 1939). Devils Glen Dolomite (Upper Cambrian) A distinctive, thick-bedded, light-gray, finely to very finely crystalline dolomite. Thickness from 54 m to 172 m (Deiss, 1939). Switchback Shale (Upper and Middle Cambrian) Mostly noncalcareous, greenish-gray, thinly laminated clay shale with local thin interbeds of dolomite, limestone, sandstone, and conglomerate. Thickness from 21 m to 92 m. Steamboat Limestone (Middle Cambrian) Differs in lithology between western and eastern outcrops. In the west consists of a lower shaly mudstone interval and a much thicker upper limestone interval (Deiss, 1939). In the eastern exposures is about equal parts of alternating sequences of limestone and calcareous shale. The carbonate units in both areas consist mostly of nodular, hard, dark-yellowish-brown, thinly bedded limestone and dolomite with nodules and lentils of dark-yellowish-orange siltstone. The mudstone units are mainly grayish-green noncalcareous shale with interbeds of calcareous siltstone and claystone. Trilobites, locally abundant, and some brachiopods occur in limestone lenses in the shales and locally near the top of the limestone units. Thickness from 65 m to 80 m. Pentagon Shale (Middle Cambrian) A clastic wedge that consists of very fossiliferous, calcareous, gray to tan-gray, thick-bedded platy shale that contains some platy, blue-gray argillaceous limestone in the upper part (Deiss, 1939). Thickness ranges from less than 1 m to 88 m (Deiss, 1939). Pagoda Limestone (Middle Cambrian) Forms prominent light-gray cliffs in the Cambrian sequence. The upper part consists of yellowish-gray to light-yellowish-brown, thin- to thick-bedded dolomitic limestone and some dolomite overlying very thin bedded limestone. The lower part consists of grayish-green, thinly laminated to nodular clay shale with some gray-brown limestone and minor sandstone. An intraformational conglomerate locally present in the middle and lower parts. Trilobites and brachiopods locally numerous in the shale (Deiss, 1939). Formation thickens to the south and west, and ranges in thickness from 28 m to about 120 m. Dearborn Limestone (Middle Cambrian) Composed of an upper thick limestone unit and a lower thin shale unit. The limestone unit is finely crystalline, yellowish-brown to gray, thin to thick bedded and irregularly bedded. The lower unit consists of gray to gray-green clayey shale with some sandy shale and thin interbedded limestone. Trilobites present in the lower part of the shale unit. Thickness from 67 m to 106 m. Damnation Limestone (Middle Cambrian) Consists of medium- to dark-gray, thin- to thick-bedded, finely crystalline dolomitic limestone and limestone with laminae of grayish-orange to yellowish-gray siltstone that thicken and thin. Locally they are oolitic and contain trilobites, brachiopods, and organic trails and burrows. Thickness from 30 m to 68 m. Gordon Shale (Middle Cambrian) Mainly a dark-gray to gray-brown, very thinly laminated shale with a greenish tint and locally maroonish-gray beds. Contains many thin beds of sandstone and some beds of limestone, especially in the middle and upper parts. In places the limestones contain glauconite, algal structures, limestone chips, grains of quartz, and fossil fragments. The upper part contains numerous fossils (Deiss, 1939), whereas the lower beds locally contain organic trails and burrows, some by trilobites. Thickness from 42 m to 90 m. Flathead Sandstone (Middle Cambrian) Consists of thin- to thick-bedded and crossbedded, noncalcareous yellowish-gray, poorly sorted, poorly indurated, fine- to coarse-grained quartzose sandstone with scattered quartz pebbles. Beds are characteristically speckled by disseminated hematite. Interbeds of gray, purple, or maroon mudstone locally present. Thickness from 13 m to 35 m.' 1618|'PROTEROZOIC Y STRATIFIED ROCKS The Proterozoic stratified rocks in the quadrangle are assigned to the Belt Supergroup and consist of quartzite, siltite, argillite, limestone, and dolomite that are metamorphosed to the chlorite subfacies. The Belt Supergroup is divided upper, middle, and lower parts. The upper part is the Missoula Group which consists mostly of fine-grained clastic rocks and minor limestone and dolomite above the Helena Formation. The middle part is the Helena Formation which consists mostly of limestone and dolomite. The lower part, the Ravalli Group, contains mostly siltite and argillite with minor quartzite, limestone, and dolomite. Thickness of the Belt Supergroup in the quadrangle ranges from 2,000 m in the eastern part to more than 10,000 m in the western part. Consists of pale-olive to medium-gray and moderate brown, poorly sorted, very fine to fine-grained, micaceous, thin even beds of sandstone and siltstone. Locally they are speckled by hematite, crossbedded and ripple marked. The formation varies in thickness due to pre-Middle Cambrian erosion and ranges from less than 1 m to 490 m, attaining its greatest thickness in the southern outcrops.' 1619| 1620|'In the central and southern parts of the area the McNamara is divisible into two unnamed members, an upper dominantly reddish-brown quartzite and a lower dominantly grayish-green siltite (Somers, 1966; Mudge and Earhart, 1978). Quartzite in the upper member is interbedded with siltite and minor argillite. Most of the unit is thin-bedded, fine-grained, micaceous, minutely crossbedded, and locally ripple marked. The lower member contains thin beds of argillite and fine- to medium-grained quartzite and locally some reddish-gray siltite. Ripple marks, minute cross-bedding, and load casts are common. The upper part of the lower member contains thin beds of glauconitic sandstone, some barite nodules, and vuggy reddish chalcedony, particularly in the eastern outcrop area (Mudge, 1972; Mudge and Earhart, 1978). In the northern part of the area, the formation thickens and is dominantly grayish-green siltite. The formation varies in thickness, due in part to pre-Middle Cambrian erosion, and ranges from 47 m to 1,650 m. Where overlain by the Garnet Range Formation its minimum thickness is 640 m; it thickens northward.' 1621|'Mapped only in the upper reaches of Smith and Elk Creeks.' 1622|'Consists mostly of pink, pale-red, and pinkish-gray, fine- to medium-grained, poorly sorted beds of quartzite that range in thickness from 31 cm to 76 cm. The quartzite is mostly feldspathic and locally includes fragments of red argillite. Many beds are crossbedded and ripple marked. Thickness ranges from 213 m to 580 m.' 1623|'Mostly bright reddish brown, thinly laminated, micaceous siltite, argillite, and thin- to thick-bedded quartzite. Sedimentary features include minute cross-laminations, ripple marks, mud-crack fillings, and mud chips. A grayish-green siltite unit with local interbedded dark-gray fissile shale widespread in the upper part of the formation. Salt-crystal casts widespread in the upper part of the formation, beneath the grayish-green unit. The quartzite beds are mostly fine to medium grained and more common in the middle and lower parts of the formation. A thick (155 m to 305 m) quartzite unit is present in the upper-middle part of the formation in the southern outcrop. It contains poorly sorted, fine- to coarse-grained, pinkish-gray to reddish-brown quartzite beds less than 1 m thick that are separated by thin beds of reddish-brown siltite and argillite. The color, grain size, and sedimentary features of the quartzite beds are similar to those in the Bonner Quartzite. The lower part of the formation in the eastern outcrop commonly contains thin beds of glauconitic quartzite. In the central and northern parts of the area the formation is more argillitic and contains beds of stromatolitic and oolitic limestone. The formation thickens south and west, from 555 m in the eastern outcrop to 2,180 m in the west at Swan Range and to 1,860 m in the south.' 1624|'Consists mostly of greenish-gray to grayish-yellow micaceous siltite and some silty limestone and argillite. Beds of maroon siltite and argillite widespread in the middle part and locally in the upper part. Thin glauconitic quartzite lentils widespread in the upper part of the formation in the eastern outcrop, but sparse elsewhere. Ripple marks, minute cross lamination, load casts, and mud cracks also common in the eastern outcrop. An edgewise conglomerate present near the base of the formation in the east, but elsewhere a stromatolitic limestone bed occurs at the same horizon. Other beds of stromatolitic limestone also present in the lower part of the formation in the Mission Range (Harrison and others, 1969). The formation thickens westward and southward, from 249 m in the eastern outcrop to about 900 m in the west in the Swan Range and about 715 m in the south.' 1625|'Consists of pale-red to reddish-brown beds and interbedded greenish-gray beds of argillite and siltite with some thin beds of very fine to fine-grained quartzite. Thin beds of stromatolitic and oolitic limestone and flat pebble conglomerate locally occur at various horizons. Crossbedded, minute laminae, ripple marks, and mud cracks common features; raindrop impressions mud-chip conglomerates less common. Thin beds of poorly sorted, fine- to coarse-grained quartzite and gritstone common near the lower contact. The formation thickens west and south, from about 215 m in the eastern outcrop to 1, 660 m in the Swan Range and 1,100 m in the southern part of the area.' 1626|'In most places the Helena is divisible into three units. The upper unit consists of beds of limestone interbedded with dolomite, siltite, and argillite. Beds of stromatolites, oolites, and edgewise conglomerates widespread. The middle unit, comprising most of the formation, consists of light- to medium-gray, thin to thick-bedded silty limestone, dolomite, and calcitic dolomite that weathers to a yellowish gray to grayish orange. Commonly, vertical ribbons, blobs, horizontal mats, lenses, and pods differentially weather to form crenulating patterns ("molar tooth structure," O Connor, 1967). Stromatolites, oolites, and edgewise conglomerates locally present at various horizons, especially in the eastern outcrop. The lower unit consists mostly of calcareous or dolomitic siltite with some beds of dolomite and quartzite. The siltite is gray, greenish-gray, and locally dark red near the lower contact. The quartzite that occurs at the base is light gray, thinly bedded, calcareous, medium grained, and poorly sorted. The Helena thickens westward and southward, from about 205 m in the central part of the eastern outcrop to about 3,000 m in the Mission Range (Harrison and others, 1969) to 1,662 m in the south (Mudge and others, 1974).' 1627|'Undivided only in the eastern and part of the southern outcrop area. In those areas the unit is pale-red, maroon, green, and gray siliceous argillite and siltite, with minor thin beds of poorly sorted, fine-grained quartzite. Locally in the eastern outcrop also contains some thin beds of dolomite, edgewise conglomerate, and stromatolite beds.' 1628|'A transitional unit between the Helena and Spokane Formations. Mostly greenish gray argillite and siltite with interbeds of quartzite, dolomite, and locally stromatolitic and oolitic carbonate rock. The quartzites are poorly sorted, ranging from very fine to medium grained, and locally carbonate cemented. The amount of carbonate appears to increase upward. Red to purple beds of predominantly argillite occur in the lower part of the formation. The formation varies in thickness from less than 1 m too as much as 610 m (Mudge and others, 1974, 1978).' 1629|'Mostly pale-purplish-red and grayish-red siltite and argillite interbedded with lithologically similar greenish-gray beds. The southeastern and northwestern outcrops contain light-gray, very fine to medium-grained, thin beds of quartzite that locally contain minute cross-beds and ripple marks. The formation as much as 1,500 m thick in the Swan Range (Johns, 1970) and as much as 915 m thick in the southeast (Schmidt and Strong, 1972).' 1630|'The oldest unit exposed in and near the map area; therefore, its base is not exposed. Consists of light-gray to greenish-gray, thinly bedded siltite with some quartzite, grading down into dark-gray, greenish-gray, very thinly laminated argillite and siltite in the lower part. Its sedimentary features include ripple marks, mudcracks, and locally salt crystal casts (Schmidt and Strong, 1972). Thickness of the formation is as much as 762 m (Schmidt and Strong, 1972).' 1632|'Volcanic neck or plug and sills, gray to light-gray, aphanitic groundmass of greenish-gray feldspar with phenocrysts of light-gray feldspar, hornblende, and biotite. Present only at and near Haystack Butte in southeastern part of map area.' 1633|'Light-gray, fine-grained, equigranular dikes and sills composed of andesine, sanidine, hornblende, augite, biotite, magnetite, and apatite. Hornblende needles, 3-4 mm long, characterize the rock (Schmidt, 1972b, c; Schmidt and Strong, 1972). Dated as 46.3 m.y. by the K-Ar method (R. G. Schmidt, 1978). Widespread in southeast corner of map area.' 1634|'Stocks, dikes, and sills, of light-gray, abundant, 1-cm and larger phenocrysts of orthoclase that weather out as euhedral crystals. Dated at 58.3 m.y. by the K-Ar method (U.S. Geological Survey, 1971; Schmidt, 1978). Present only in southeast corner of map area.' 1635|'Dikes, sills, and irregular-shaped bodies. Includes trachybasalt and biotite trachybasalt of Schmidt (1972b, c, 1978). Dark-gray, aphanitic groundmass with hornblende and some biotite phenocrysts, weathers brown. Present only in the southeast corner of map area.' 1636|'Volcanic and volcanic-sedimentary rocks. Trachyandesite and trachybasalt flows (Schmidt, 1972a), volcanic conglomerate, breccia, and lacustrine deposits of volcanic sandstone, siltstone, and mudstone (Schmidt, 1972a, 1978). Present only in southeast corner of map area.' 1637|'Sills, dark grayish brown, aphanitic groundmass of feldspar with phenocrysts of plagioclase, potassium feldspar, pyroxene, and quartz (Mudge, 1972). Widespread near the North Fork Sun River.' 1638|'Dikes and sills, light-gray to white; finely porphyritic with abundant 2-3 mm phenocrysts of quartz and oligoclase (Schmidt, 1972c, 1978). Present only in southeast corner of the map area.' 1639|'Sills, light-grayish-orange; coarsely porphyritic with abundant 1 cm and larger phenocrysts of potassium feldspar and oligoclase-andesine in a fine-grained groundmass (Schmidt, 1972c, 1978).' 1640|'Sill, gray, grayish-purple; massive and porphyritic with abundant phenocrysts of labradorite in an aphanitic groundmass (Schmidt, 1972c, 1978). Present only in southeast corner of map area.' 1641|'Includes rhyodacite porphyry and diorite as mapped by Schmidt and Strong (1972) and Schmidt (1978). The rhyodacite is greenish gray with phenocrysts of oligoclase and hornblende mainly in a groundmass of quartz and feldspar. The diorite is greenish gray, fine grained, and equigranular (Schmidt and Strong, 1972). Present only in southeast corner of map area.' 1642|'Mostly diorite and quartz diorite, locally minor diorite-gabbro and monzonite. Dark gray, weathers grayish brown. Dated at 750±25 m.y. by K-Ar method (J. D. Obradovich, oral commun., 1966). Widespread throughout map area.' 1650|'Gravel, sand, silt, and clay along active channels of rivers, creeks, and tributaries.' 1651|'Locally derived slope-wash deposits mainly of sand, silt, and clay. Commonly grades into Qal. Locally contains well-rounded cobbles reworked from alluvial terrace gravel. Typically thin veneer concealing bedrock, but locally as thick as 30 ft.' 1652|'Colluvium and alluvium are combined where mapping is insufficient to differentiate between those deposits that grade into one another.' 1653|'Gravel, sand, silt, and clay deposited in fans being formed by tributary streams along major valley margins. Display characteristic fan-shaped map pattern and convex upward profile. Typically grade into Qal. Thickness ranges from very thin at toe to as much as 50 ft at head of fans.' 1654|'Gravel near shore, grading into clay and marl in center. Thickness 1-3 ft (Skipp and others, 1999).' 1655|'Accumulations of locally derived boulder- to pebble-size angular detritus at heads of drainages on both sides of the crest of the Bridger Range.' 1656|'Unconsolidated mixture of soil and blocks of bedrock transported down steep slopes by mass wasting. Characteristic hummocky surface with concentric swales and ridges near down-slope limits. Common along steep slopes beneath resistant rocks but may have developed wherever steep slope and high moisture content produce unstable conditions.' 1657|'Angular and subangular cobble and smaller gravel, sand, and silt derived from local bedrock. Surfaces of deposits are smooth pediments sloping away from the Crazy Mountains and Beartooth Mountains. About 10 to 30 ft thick.' 1658|'Unsorted mixture of clay to boulder size sediment transported and deposited by alpine glaciers. Has characteristic hummocky surface form. Occur in valleys near the Beartooth Mountains in the southwest corner of the map area. Clasts are predominantly Archean metamorphic rocks with lesser amounts of quartzite, dolomite, and limestone. Thickness unknown.' 1659|'Gravel, sand, silt, and clay underlying terraces about 20 to 600 ft above present altitude of modern streams and rivers. Equivalent to one or more of Qat1 through Qat5. Thickness 10 to 40 ft.' 1660|'Gravel underlying terraces about 10 to 20 ft above present altitude of rivers. Mostly cobbles and pebbles with minor amounts of sand and silt. Clasts are mainly granitic igneous rocks, granitic gneiss, schist, and quartzite, with much less limestone and sandstone. 10 to 40 ft thick.' 1661|'Gravel underlying terraces about 20 to 40 ft above Qal. Mostly cobbles and pebbles with minor amounts of sand and silt. Clasts are mainly granitic igneous rocks, granitic gneiss, schist, and quartzite, with much less limestone and sandstone. 10 to 40 ft thick.' 1662|'Gravel underlying terraces about 50 to 90 ft above present altitude of rivers. Mostly cobbles and pebbles with minor amounts of sand and silt. Clasts are mainly granitic igneous rocks, granitic gneiss, schist, and quartzite, with much less limestone and sandstone. 10 to 30 ft thick.' 1663|'Gravel underlying terraces about 200 to 300 ft above present altitude of rivers. These terraces locally exhibit a relatively steep gradient toward the Yellowstone River Valley and may actually include several levels of terraces that are difficult to distinguish. Cobble- and pebble-size clasts are mainly granite, granitic gneiss, schist, and quartzite. Thickness up to about 20 ft.' 1664|'Gravel underlying terraces about 400 to 600 ft above present altitude of rivers. Occur mainly as small discontinuous erosional remnants. Cobble- and pebble-size clasts are mainly granite, granitic gneiss, schist, and quartzite. Local calcite cement, especially at base. Thickness ranges from a very thin remnant to about 20 ft' 1665|'Poorly stratified variously consolidated tuffaceous siltstone, claystone, sandstone, and conglomerate (Roberts, 1964g).' 1666|'Applies to areas within the Absaroka-Beartooth Study Area and the southwestern corner of the Livingston quadrangle where formations within the volcanic sequences were not mapped. Largely composed of units in the Absaroka Volcanic Supergroup. Total thickness unknown.' 1667|'A small intrusive body in Archean metamorphic rocks near the southwestern corner of the quadrangle.' 1668|'Described by Harlan (1986) as typically sodium rich, silica undersaturated, and strongly alkaline (Wilson and Elliott, 1997).' 1669|'An olivine-augite-biotite diorite dike; and a composite sill that consists of upper and lower layers of biotite-augite diorite and a middle layer of syenodiorite (Skipp and others, 1999).' 1670|'Sills intruded into the Fort Union Formation that consist mainly of potassium feldspar with abundant biotite, and locally minor pyroxene. The most prominent of these is the sill that forms Sheep Mountain. Both the syenite intrusive and associated resistant hornfels cap this ridge.' 1671|'Gray to grayish-yellow, fine- to medium-grained, cross bedded sandstone. Interbedded with brownish-gray carbonaceous shale and siltstone and minor thin coal beds. Sandstone beds are ledge forming and commonly support growths of pine trees. About 400 ft of this member is present in the northeastern part of the map area, but thickens dramatically westward. The top of this member was not observed in the map area.' 1672|'Predominantly dark-gray to olive shale, locally yellowish-gray claystone, thin interbedded sandstones and siltstone, yellowish gray. Typically forms smooth grassy slopes below the Tongue River Member. Pinches out in the northeastern part of map area; thickness 0 to 250 ft.' 1673|'Yellowish-gray, fine- to medium-grained, ledge-forming sandstone, cross-bedded in part. Interbedded with gray to greenish-gray claystone, siltstone, and minor carbonaceous shale. Supports growths of pine trees. About 400-600 ft thick in the eastern part of the map area but thickens westward where it could not be mapped separately.' 1674|'Cliff-forming massive sandstone, siltstone, mudstone, and lesser amounts of conglomerate. Conglomerate contains clasts of reworked Cretaceous and older rocks; contains fossil spores, plants, wood, freshwater mollusks, and vertebrates (Roberts, 1964d). Thickness increases toward axis of Crazy Mountain Basin and is probably at least 10,000 ft. Skipp and others (1999) show the Fort Union about 8,000 ft thick in a cross-section on the west side of the Crazy Mountains.' 1675|'Interbedded light-brownish gray, cliff and ledge-forming, fine-grained, thin- to thick-bedded sandstone, and gray, pale greenish gray and pale purple gray mudstones. Sandstone beds support growths of pine trees. Includes basal beds commonly mapped as Lennep Formation that are typical of basal Hell Creek to the east (personal communication, Susan Vuke and Edie Wilde). The Hell Creek and the upper part of the Livingston Group are laterally equivalent and are eastern and western facies respectively. Hell Creek is recognized east of the Shields River north of the Yellowstone River, and east of Springdale south of the Yellowstone River. Total thickness of the formation is about 900 to 1,100 ft.' 1676|'Dark-gray to medium-gray fine-grained diorite and diorite porphyry with phenocrysts of plagioclase, hornblende, pyroxene, and locally biotite. Occurs as stocks, sills, and dikes in the southeastern corner of the quadrangle. Radiometric age is 74-77 Ma (du Bray and Harlan, 1993 and du Bray and others, 1994).' 1677|'This unit includes all the volcanic rocks erupted from the Sliderock stratovolcano (du Bray and others, 1994). Mostly andesite breccia (lahars) gray, pale-purple gray, pale-greenish gray. Andesite in clasts is porphyritic with phenocrysts of chalky plagioclase, hornblende and pyroxene; matrix is similar but lighter in color and slightly finer grained. Very resistant, forming cliffs and very rugged topography especially near the vent zone in the Sliderock Mountain area (on Big Timber quadrangle, Lopez, 2000). In distal areas, as near town of Springdale, clasts are less angular but otherwise similar to breccias elsewhere. The unit contains minor flows of porphyritic andesite and basaltic andesite, with phenocrysts of plagioclase, hornblende and pyroxene. Thickness is at least 1000 ft (du Bray and others, 1994).' 1678|'Descriptions of formations below are from Roberts, 1964c. Includes the Hoppers Formation, Billman Creek Formation, Miner Creek Formation, and the Cokedale Formation.' 1679|'Andesitic sandstone with interbedded claystone and siltstone; massive crossbedded yellowish-gray sandstone member at the base.' 1680|'Olive-gray, brownish-gray, and grayish-red claystone with interbedded sandstone, siltstone, and conglomerate; contains fresh-water mollusks and dinosaur bones.' 1681|'Andesitic siltstone and sandstone with beds of tuff and bentonite in lower part; contains fossil spores, plants, wood, and dinosaur bones.' 1682|'Andesitic siltstone and sandstone with claystone, tuff, bentonite and coal; lower part carbonaceous with some thin coal beds; sandstone locally conglomeratic; contains fossil spores, plants, wood, dinosaur bones, and fresh-water mollusks. Sedan Formation (Upper Cretaceous)-Shown only for the Sedan 15'' quadrangle. The Sedan Formation is equivalent to the Miner Creek and Cokedale formations of the Livingston Group. Descriptions for the members of the Sedan Formation from Skipp and others, 1999.' 1683|'Ridge-forming, olive-gray, greenish-gray, volcaniclastic sandstone, conglomerate, and minor mudstone and devitrified tuff. Unit contains rare dinosaur bone fragments, freshwater and brackish-water mollusks, and, in northern part of quadrangle, trace fossils of Ophiomorpha. Thickness about 350-500 ft.' 1684|'Greenish-gray and brownish-gray, volcaniclastic, mudstone, siltstone, sandstone, and minor interbedded conglomerate and altered vitric tuff. Member forms valleys and low barren hills and has a gradational contact with the middle sandstone member. Unit contains freshwater mollusks, wood, and dinosaur bones. Thickness ranges from 900-1000 ft.' 1685|'Olive-green, olive-gray, and dark-greenish-gray, volcaniclastic sandstone, conglomerate, mudflow conglomerate, and minor siltstone and mudstone. Unit forms series of ridges and disconformably overlies ash-flow tuff member; basal contact is placed at top of highest ash-flow tuff or ash-flow tuff conglomerate. Thickness is about 1000-1430 ft.' 1686|'Pale-yellowish-green, light-greenish-gray, grayish-red, and pale-yellowish-brown, welded to nonwelded tuff and ash-flow tuff conglomerate, interbedded with volcaniclastic conglomerate, sandstone, mudstone, and porcellanite. Unit forms resistant knobs and ridges and disconformably overlies lower sandstone member. Thickness about 200-500 ft.' 1687|'Dark-olive-gray, greenish-gray, and yellowish-gray, volcaniclastic sandstone, siltstone, mudstone, altered crystal lithic tuff, minor volcaniclastic granule and pebble conglomerate, minor grayish-red-purple hornblende dacite, and minor lignitic coal. Unit forms a low ridge and has an abrupt contact with underlying Eagle Sandstone throughout most of the Sedan quadrangle. Thickness 500-1000 ft.' 1688|'Interbedded gray sandstone, mudstone, and shale. Sandstone is feldspathic and lithic. Used in southeast part of map area where this thick interval lies between the Telegraph Creek Formation and Hell Creek formations. Not divided because the intervening Claggett Shale was not observed, although Richards (1957) believed rocks equivalent to the Claggett Shale are present in this area. Thickness is about 1000 to 1400 ft.' 1689|'Light-brownish-gray to very-pale orange, very-fine to fine-grained, cross-bedded sandstone, burrowed to bioturbated in part; interbedded with shale. Thickness about 150 ft.' 1690|'Eagle Sandstone: Light-brownish-gray to very-pale orange, very-fine to fine-grained, cross-bedded sandstone, burrowed to bioturbated in part; interbedded with shale. Thickness about 150 ft. Telegraph Creek Formation: Shale and sandy shale, brownish-gray to medium-dark-gray with thin interbedded sandstone. Dusky-red concretions common near base. Sandstone beds thicker and more abundant upward, grading into Eagle Sandstone. Contact with Eagle is placed at the base of the first cliff-forming sandstone. Maximum thickness about 250 ft.' 1691|'Shale and sandy shale, brownish-gray to medium-dark-gray with thin interbedded sandstone. Dusky-red concretions common near base. Sandstone beds thicker and more abundant upward, grading into Eagle Sandstone. Contact with Eagle is placed at the base of the first cliff-forming sandstone. Maximum thickness about 250 ft.' 1692|'Shale, olive-gray and dark-brownish gray, fissile, and contains abundant thin bentonite beds. Upper half calcareous, containing few very thin bentonite beds, and near top contains thin beds of very calcareous laminated sandstone, siltstone, and sandy limestone. Concretions medium-light-gray to pale-yellowish-brown and from a few inches to one or two ft in diameter are commonly present. Inoceramus prisms common. Upper contact placed at change from calcareous shales to non-calcareous shales of Telegraph Creek. Zone of dusky-red concretions in the Telegraph Creek, just above contact also helps establish its position. Basal contact not exposed, thickness unknown.' 1693|'Gray shale and mudstone interbedded with grayish-green and olive-gray sandstone and siltstone and minor brown-weathering nodular limestone and yellowish-gray bentonite. Entire formation is locally fossiliferous and contains fish scales, starfish, ammonites and other mollusks. Thickness is about 1000 ft (Skipp and others, 1999). Division of Cody Shale into members as listed below for the southwestern part of the Livingston quadrangle from Roberts, 1965.' 1694|'Dark-gray to brown marine shale and siltstone with interbedded very fine-grained sandstone. Thickness 500 to 845 ft (Roberts, 1965, p. B59).' 1695|'Persistent thin-bedded, marine, glauconitic, greenish-gray, fine-grained sandstone. Thickness 90 to 120 ft (Roberts, 1965, p. B59).' 1696|'The lower shale member of the Cody is consists of dark-gray to dark-brown marine shale with interbedded siltstone. The Frontier Formation is as described below. Thickness 400 to 590 ft (Roberts, 1965, p. B59). Frontier Formation: Light brownish gray, fine-grained thick-bedded to massive, salt and pepper sandstone. Contains three sandstone intervals interbedded with dark-gray, fissile shale.' 1697|' (Roberts, 1965, p. B59). Cody Shale: Gray shale and mudstone interbedded with grayish-green and olive-gray sandstone and siltstone and minor brown-weathering nodular limestone and yellowish-gray bentonite. Entire formation is locally fossiliferous and contains fish scales, starfish, ammonites and other mollusks. Thickness is about 1000 ft (Skipp and others, 1999). Division of Cody Shale into members as listed below for the southwestern part of the Livingston quadrangle from Roberts, 1965. Frontier Formation: Light brownish gray, fine-grained thick-bedded to massive, salt and pepper sandstone. Contains three sandstone intervals interbedded with dark-gray, fissile shale. Maximum mapped thickness about 550 ft.' 1698|'Light brownish gray, fine-grained thick-bedded to massive, salt and pepper sandstone. Contains three sandstone intervals interbedded with dark-gray, fissile shale. Maximum mapped thickness about 550 ft.' 1699|'This unit is equivalent to Mowry Shale and Thermopolis Shale undivided as mapped by Roberts (1964c). Top of unit (Mowry) is interbedded, siliceous, very fine- to fine-grained sandstone, siltstone, and shale. Contains several prominent bentonite beds. Sandstones and siltstones mostly light gray to medium gray, with a silvery sheen. Fish scales on bedding planes of sandstones and siltstones are characteristic of this formation. Thermopolis Shale is predominantly dark-gray fissile shale, bentonitic shale, and several beds of bentonite. Has hematitic concretionary zone near base. Fall River Sandstone is brownish-gray, thin-bedded, argillaceous, fine-grained, quartz sandstone. Generally poorly exposed in map area, mostly covered by glacial deposits. Total thickness is approximately 1300 ft.' 1700|'Mostly reddish-brown, olive-gray, and dusky-purple mudstone with interbedded, lenticular, fine- to coarse-grained sandstones. Locally thick, lenticular, fluvial, fine-grained sandstone, the Greybull Sandstone, is present at the top. The basal Pryor Conglomerate Member is brown conglomerate and pebbly coarse-grained sandstone and is 20 to 60 ft thick. The total thickness of the Kootenai Formation is about 500 ft.' 1701|'Total thickness about 900 ft. Morrison Formation: Mainly variegated greenish-gray and pale-reddish-brown mudstone. Very fine- to fine-grained, quartzose, calcareous, cross-bedded sandstones are commonly present at about mid-section, 5 to 10 ft thick but locally can be as much as 30 ft thick. Fossil dinosaur remains common. Upper contact placed at the base of the Pryor Conglomerate. The basal contact is placed at the top of fossiliferous limy sandstone and coquina of the underlying Swift Formation. Thickness about 300 ft. Ellis Group: Includes the Swift, Rierdon, and Piper Formations. The Swift is interbedded medium-gray shale, limestone and limy sandstone, fossiliferous. Brownish-gray, fossiliferous, very sandy limestone occurs at the top of the formation, which generally includes brownish-gray coquina at the top. The Rierdon Formation is mostly pale greenish-gray very fossiliferous shale with minor interbedded brownish gray limestone. Typically poorly exposed, forming smooth slopes littered with fossils, including oysters (Gryphaea and Ostrea), belemnites (Pachyteuthis), and crinoid fragments (Pentacrinus). The Piper is interbedded medium-gray, and pale reddish-gray, thin-bedded limestone and medium-gray shale. Includes thin interbedded gypsum. Forms ledge below smooth slopes of the Rierdon shale. Thickness of the Ellis Group is about 600 ft.' 1702|'Mainly variegated greenish-gray and pale-reddish-brown mudstone. Very fine- to fine-grained, quartzose, calcareous, cross-bedded sandstones are commonly present at about mid-section, 5 to 10 ft thick but locally can be as much as 30 ft thick. Fossil dinosaur remains common. Upper contact placed at the base of the Pryor Conglomerate. The basal contact is placed at the top of fossiliferous limy sandstone and coquina of the underlying Swift Formation. Thickness about 300 ft.' 1703|'Includes the Swift, Rierdon, and Piper Formations. The Swift is interbedded medium-gray shale, limestone and limy sandstone, fossiliferous. Brownish-gray, fossiliferous, very sandy limestone occurs at the top of the formation, which generally includes brownish-gray coquina at the top. The Rierdon Formation is mostly pale greenish-gray very fossiliferous shale with minor interbedded brownish gray limestone. Typically poorly exposed, forming smooth slopes littered with fossils, including oysters (Gryphaea and Ostrea), belemnites (Pachyteuthis), and crinoid fragments (Pentacrinus). The Piper is interbedded medium-gray, and pale reddish-gray, thin-bedded limestone and medium-gray shale. Includes thin interbedded gypsum. Forms ledge below smooth slopes of the Rierdon shale. Thickness of the Ellis Group is about 600 ft.' 1704|'Formations not mapped separately because of narrow outcrop width. Phosphoria is poorly exposed and includes, yellowish-gray shale, light gray limestone, sandstone and quartzite, commonly grayish pink, cherty; thickness is 50 to 75 ft. The Phosphoria interval is present in the southeast part of the map area. It thins westward and was not recognized by Richards (1957) or Roberts (1964). The Tensleep Sandstone is light-brown to very-pale-orange sandstone, fine grained, well sorted, well rounded, cross-bedded. Locally contains thin limestone beds, locally cherty near the top, and is locally silicified to form quartzite; about 250 ft thick. The Amsden Formation is interbedded grayish-pink to light-red mudstone, limestone, and siltstone. Limestones are commonly cherty. Unconformably overlies karst surface developed on limestone of the Madison Group. Characteristically produces pink stain on underlying cliffs of Madison Group; thickness about 200 ft but locally tectonically thinned to only a few feet along the mountain front. Total thickness about 500 to 600 ft.' 1705|'Used in areas where Phosphoria is absent and Quadrant and Amsden outcrop bands are too narrow to map separately. Quadrant Formation: Quartzite, well sorted quartzose sandstone, and dolomite (Roberts, 1964f). Amsden Formation: Dolomite, calcareous sandstone, and siltstone in upper part; argillaceous limestone in middle part; massive sandstone and red calcareous siltstone and limestone in lower part; mostly concealed (Roberts, 1964b).' 1706|'Quartzite, well sorted quartzose sandstone, and dolomite (Roberts, 1964f).' 1707|'Dolomite, calcareous sandstone, and siltstone in upper part; argillaceous limestone in middle part; massive sandstone and red calcareous siltstone and limestone in lower part; mostly concealed (Roberts, 1964b).' 1708|'Gray and gray-brown limestone and dolostone; red, pink and yellow-orange sandstone; red and pink siltstone; red to purple shale; and red-stained breccia (Skipp and others, 1999).' 1709|'Limestone and dolomitic limestone, light gray to light brownish gray. Thick bedded to massive in the upper part (Mission Canyon Limestone) and thin-bedded to thick bedded in the lower part (Lodgepole Limestone). Also contains thin interbedded gray shales. Fossiliferous and cherty beds are present throughout. Thickness of the Madison Group is about 1000 ft in the southeastern part of the map area.' 1710|'Pale yellow-brown, massive, poorly bedded limestone and dolostone that weather light gray and form cliffs and castellated ridges. Unit has a gradational lower contact with Lodgepole Limestone. Thickness is about 720 to 950 ft (Skipp and others, 1999).' 1711|'Gray and brown limestone and minor dark-brown to black silty shale. Formation unconformably overlies Sappington Member of the Three Forks Formation. Total thickness is 750 to 800 ft (Skipp and others, 1999).' 1712|'Light- and dark-brown dolostone; gray and gray-green, nodular limestone; yellowish-brown sandstone; pink and gray siltstone; black and green shale; and minor dolostone breccia. Unit disconformably overlies upper Cambrian beds. This entire interval is 740 to 940 ft thick. (Skipp and others, 1999).' 1713|'The Three Forks is mainly yellowish-weathering argillaceous limestone and medium-gray shale, very poorly exposed. The Jefferson Formation is dolomitic limestone, light brownish gray, fetid, poorly exposed, typically occurs as float. The Big Horn Dolomite is cliff-forming dolomite and dolomitic limestone, very light gray to very pale orange, lower part massive, thin to thick bedded in upper part. Has characteristic pock-marked surface because of differential weathering. Interval thickness about 600 ft.' 1714|'Dark- and light-gray mottled and grayish-brown limestone, limestone-pebble conglomerate, and grayish-green shale. Entire interval about 530 to 700 ft thick (Skipp and others, 1999).' 1715|'Light red sandstone and quartzite, greenish-gray shale and sandy shale, gray thin-bedded limestone and greenish-gray flat-pebble limestone conglomerate. Includes the Flathead, Wolsey, Meagher, Park, and Pilgrim formations. Thickness is 600 to 800 ft.' 1716|'Grove Creek Formation is limestone, intraformational conglomerate, and shale; Snowy Range Formation is green shale and gray limestone that is partly a flat-pebble conglomerate; bed of columnar fractured limestone locally present in lower part of Snowy Range Formation; mostly concealed. (Roberts, 1964b). Note: Skipp and others, 1999 assigned a Lower Ordovician age to the Snowy Range Formation in the Sedan 15'' quadrangle.' 1717|'Massive, cliff-forming oolitic, mottled, magnesian and dolomitic limestone with edgewise intraformational limestone-pebble conglomerates, interbedded shale and siltstone, and glauconitic limestone. Thickness is 360-430 ft (Skipp and others, 1999 and Roberts, 1964b).' 1718|'Park Shale: Multicolored shale with thin beds of argillaceous limestone, siltstone, and sandstone; mostly concealed. (Roberts, 1964b). Meagher Limestone: Thin-bedded, silty, medium-gray limestone and yellowish-gray dolomite; locally oolitic (Roberts, 1964b). Wolsey Shale: Greenish gray calcareous shale and mudstone with thin beds of limestone; mostly concealed (Roberts, 1964b). Flathead Quartzite: Gray to red quartzose sandstone and quartzite; conglomeratic in lower part locally. (Roberts, 1964b).' 1719|'Multicolored shale with thin beds of argillaceous limestone, siltstone, and sandstone; mostly concealed. (Roberts, 1964b).' 1720|'Thin-bedded, silty, medium-gray limestone and yellowish-gray dolomite; locally oolitic (Roberts, 1964b).' 1721|'Greenish gray calcareous shale and mudstone with thin beds of limestone; mostly concealed (Roberts, 1964b).' 1722|'Gray to red quartzose sandstone and quartzite; conglomeratic in lower part locally. (Roberts, 1964b).' 1723|'Dark-grayish green, reddish-weathering, coarse-grained, massive, poorly bedded arkose and conglomeratic arkose. Thickness ranges from 7,000 to 10,000 ft (Skipp and others, 1999).' 1725|'Upper anorthosite zone - Plagioclase cumulate with 0-50 percent pyroxene, much of which is post-cumulus. A convoluted relationship between coarse-grained and fine-grained rock characterizes the uppermost part of the zone on the ridge southwest of Burnt Gulch (northwest of Picket Pin Mountain) and on a knoll at the west side of the Boulder River. Plagioclase cumulate with post-cumulus magnetite (including magnetite oikocrysts) constitute some of the coarse-grained phase. Euhedral (cumulus) plagioclase laths in a groundmass of anhedral pyroxene and plagioclase grains (probably post-cumulus) constitute the fine-grained phase: in places the rock is porphyritic, with plagioclase crystals as much as 1-2 cm long in a fine-grained groundmass. Plagioclase-magnetite cumulate also occurs in two thin layers near the top of the section west of Wright Gulch (north side of Contact Mountain). Plagioclase cumulate in the rest of the zone contains increasing amounts of clinopyroxene (10 to 50 percent) down-section, and near the bottom of the zone some of the pyroxene is orthopyroxene. Megascopically and in thin sections, practically all pyroxene in the unit appears to be anhedral, and hence interstitial. However, McCallum and others (1980), who made a detailed petrographic study of a section across the east end of Contact Mountain, found much cumulus clinopyroxene and inverted pigeonite in the zone. Their "Gabbronorite III unit" contains the Upper anorthosite and Upper gabbro zones of this map. The Upper anorthosite zone has a maximum exposed thickness of 800 m, but east of Picket Mountain and west of the Boulder River the unit is truncated completely by the unconformity at the base of the Paleozoic sedimentary rocks. Upper gabbro zone - Principally plagioclase-two pyroxene cumulate. Ratios of orthopyroxene to clinopyroxene and of plagioclase to pyroxene vary, but plagioclase is generally dominant and the rock tends to be leucocratic. Plagioclase-clinopyroxene cumulate is rare or absent throughout the zone. The upper contact is the highest limit where abundant cumulus clinopyroxene is seen in hand specimen. A lens of pegmatoid orthopyroxene cumulate 1 m thick is exposed at the top of the zone north of Picket Pin Mountain. Turbulent flow structure is exhibited near the middle of the zone on a low ridge south of a cirque northeast of Picket Pin Mountain. There, wispy xenoliths, chiefly of plagioclase cumulate, are enclosed in the two pyroxene rock. A prominent lens of plagioclase extends from about 100 m to 200 m above the base of the zone; it is well exposed in cuts along the cirque road east of Picket Pin Mountain. This lens thins westward and disappears near the east end of Contact Mountain. Plagioclase-orthopyroxene cumulate and plagioclase cumulate form several minor layers along and near the base. The zone attains a maximum thickness of 600 m on the north side of Contact Mountain, but is truncated out by the unconformity at the base of the Paleozoic sequence east of Picket Pin Mountain and west of the Boulder River. Upper mixed zone - Plagioclase-olivine cumulate dominant, but there are interlayers and lenses of plagioclase cumulate and, rarely, of olivine cumulate. Immediately down-section from the upper contact, nonparallel lenses of plagioclase-olivine cumulate are enclosed in plagioclase cumulate. Lens endings are generally pointed, but some are blunt and squared off. These features possibly are evidence of tilting of the magma chamber during deposition of compositional layering. A lens of olivine cumulate 3-5 m thick crops out near the bottom of the map unit on the southeast side of Picket Pin Mountain, but pinches out completely in 100-200 m between there and the summit. Plagioclase-orthopyroxene cumulate and orthopyroxene-plagioclase cumulate form a layer 2-3 m thick at the base of the zone 1 3/4 km ESE of Picket Pin Mountain. In the Upper mixed zone and in underlying units of the Stillwater Complex the exposed grains of olivine tend to weather out, so that typical exposures of plagioclase-olivine cumulate have pitted surfaces. Magnetite is a characteristic alteration product of the olivine. Despite chemical decomposition of the olivine, the Upper mixed zone is locally erosion-resistant, and holds up Picket Pin Mountain and the ridge between there and the East Boulder River. Because the lithology is so distinct from that of the overlying units and next underlying unit, the zone is a reliable stratigraphic marker. Thickness is 60 to 110 m, except where this unit is truncated by the unconformity on Mount Rae, to the west, and in the drainage of Picket Pin Creek, to the east.' 1726|'Almost 100 percent plagioclase cumulate: an exceptionally monolithologic rock unit. A zone of sulfides, generally less than a few centimeters thick, is exposed near the top of the zone a little south of the summit of Picket Pin Mountain. In 1979 this zone was claimed, drilled, and extensively sampled by the Anaconda Mining Co. on both sides of Picket Pin Mountain and on the north side of Contact Mountain to the west, but the project was subsequently dropped for lack of exploitable concentrations of nickel, copper, and platinum-group metals. Pods of gossan as much as 30 cm thick are also exposed near the top of the zone at and near a pinnacle on top of the northeast edge of the faceted west end of Contact Mountain. Centers of the pods are replete with pyrrhotite. Oikocrysts of clinopyroxene appear locally throughout the unit. Thickness is 0-690 m; the zone is thickest on Contact Mountain, less than half as thick near Picket Pin Mountain, and it is truncated both east and west by the unconformity at the base of the Paleozoic sequence.' 1727|'Middle mixed zone - Contains three cycles. Sequence from top to bottom in each cycle is plagioclase-olivine and (or) plagioclase-orthopyroxene cumulate, and plagioclase cumulate. Though it is dominant throughout, the cumulus plagioclase component decreases and the cumulus olivine component increases from one cycle to the next lower one. Only the lowermost cycle seems to be present west of the Boulder River. The zone is cut out by unconformity on Mount Rae, to the west, and by the Horseman Thrust, on the ridge between Picket Pin Creek and West Fork Stillwater River to the east. However, part of the unit reappears from under the unconformity near the east end of the complex. About 90 m of plagioclase cumulate with a little postcumulus clinopyroxene is exposed along the road to Benbow Mine. Plagioclase-olivine cumulate occurs on a ridge northwest of the road. Thickness of the Middle mixed zone is 0-400 m. Middle gabbro zone - Chiefly plagioclase-orthopyroxene-clinopyroxene cumulate and plagioclase-clinopyroxene cumulate on Contact Mountain, where the zone was first described and defined by Segerstrom and Carlson (1977). West of the Boulder River the lower third of the zone is plagioclase cumulate. A sharp change of lithofacies eastward across the East Boulder River brings in more plagioclase cumulate and cumulus olivine. A distinctive feature of the zone near Picket Pin Mountain is augen-like segregations of plagioclase parallel to regional foliation. Four-phase rock, plagioclase-orthopyroxene-clinopyroxene-olivine cumulate is identified in some outcrops, especially on the eastern part of the ridge between Picket Pin Creek and Iron Creek. To the west the zone is truncated by the un-conformity on Mount Rae. Near West Fork Stillwater River the unit is cut out along the Horseman Thrust, but farther east it reappears south of Horseman Flats. It is cut out again in the Stillwater River valley and farther east until it finally reappears near the road to Benbow Mine. There, the zone is chiefly plagioclase cumulate with post-cumulus orthopyroxene. Some plagioclase-clinopyroxene cumulate present. Thickness 0-280 m. Lower mixed zone - Principally plagioclase-olivine cumulate with 5-40 percent cumulus olivine and smaller amounts of post-cumulus clinopyroxene. Plagioclase cumulate and plagioclase-clinopyroxene cumulate with scattered grains of cumulus olivine (<5 percent) make up some beds. The middle third of the zone is almost all plagioclase cumulate on the west side of the Boulder River. Alternating bands of plagioclase cumulate and plagioclase-olivine cumulate characterize outcrops at the bottom of the zone on the first spur ridge east of the Stillwater River. The exposure is discontinuous, as it is cut out by faulting and unconformity at five different places. The westernmost exposure of the unit is just south of Mount Rae. Thickness of the zone is 0-300 m.' 1728|'Mostly plagioclase cumulate with oikocrysts of clinopyroxene. The upper contact is defined at the top of a relatively thick plagioclase cumulate layer: sulfides were noted in this rock near the east end of Contact Mountain and on the first spur ridge east of the Stillwater River. Along the road to Benbow Mine cumulus olivine occurs in varying amounts throughout the zone, mostly <5 percent; some of the olivine is poikilitic in oikocrysts of clinopyroxene. Poikilitic olivine in plagioclase cumulate is also noted at the bottom of the zone on the first spur ridge east of the Stillwater River. The zone is exposed continuously along the entire outcrop of the Stillwater Complex, except where it is concealed by Quaternary deposits and except for one short stretch at West Fork Stillwater River and another a little west of the main Stillwater River that are cut out along the Horseman Thrust. Thickness 0-480 m.' 1729|'Lower gabbro zone - Largely plagioclase-two pyroxene cumulate. Layers of plagioclase cumulate, plagioclase-clinopyroxene cumulate, and plagioclase-orthopyroxene cumulate are interspersed throughout the sequence. "Pillow troctolite" (Hess, 1960, p. 82-83), a pegmatoid plagioclase-olivine cumulate, occurs as a crosscutting injection or replacement in fine-grained plagioclase cumulate and plagioclase-clinopyroxene cumulate at the top of the zone about 750 m west of the 10,056-ft summit of Contact Mountain. Lenses of plagioclase-olivine cumulate and olivine cumulate occur at the top of the zone southeast of Contact Mountain and southwest of Picket Pin Mountain. "Inch-scale" layered rock is exposed a few tens of meters below the top of the zone at various localities along the strike of layering. The classic exposure of this unusual rock is in a landslide along the road to the Mountain View mine on west side of valley of Stillwater River, where disc-shaped oikocrysts of clinopyroxene are alined edgewise to form vertical dark layers a few millimeters to 2 or 3 cm thick. The repeating dark layers are generally separated by 2-3 cm of light-colored plagioclase cumulate. Alternation of layers is fairly uniform for 10-20 m across strike, but deviations occur in which the layers are paired or even more complexly grouped. Other exposures of inch-scale layering, at the same stratigraphic position, are: (1) on both sides of the gorge of lower Brownlee Creek, about 1 km west of the East Boulder River, (2) on the spur ridge about 1.8 km east of the Stillwater River (a little olivine is present in the upper part of the layered zone here), and (3) on the spur ridge immediately west of Prairie Creek. A thick sequence of plagioclase cumulate is sandwiched in plagioclase-two pyroxene cumulate of the Lower gabbro zone between Brownlee Creek and Iron Creek. The middle of the Lower gabbro zone is interrupted by a few meters of plagioclase-olivine cumulate and olivine cumulate overlain by plagioclase cumulate well exposed as vertical beds in a cut on the road to Mountain View Mine in SW corner sec. 16, T. 5 S., R. 15 E. Plagioclase-olivine cumulate is interbedded with plagioclase cumulate through much of the sequence on the spur ridge about 1.8 km east of the Stillwater River, so that the Lower gabbro zone there bears little resemblance to a typical section on the south side of the 10,065-ft summit of Contact Mountain. Olivine gradually diminishes toward the eastern end of the complex, and it is not seen in exposures of the Lower gabbro zone along the road to Benbow Mine. Repeating cycles of plagioclase cumulate, plagioclase-orthopyroxene cumulate, and plagioclase-two pyroxene cumulate can be recognized in the middle of the Lower gabbro zone for a short distance westward from Brownlee Creek. There are at least two showings of inch-scale layering in plagioclase cumulate a little over 100m above the bottom of the zone. One of these is on the spur ridge east of the Stillwater River. The other is about 75 m north of down-faulted limestone on the spur ridge west of Prairie Creek. Contact of the zone with the underlying Norite zone tends to be transitional, particularly from the East Boulder River westward to the divide between the East Boulder and main Boulder River. The transition is between plagioclase-two pyroxene cumulate above and plagioclase-orthopyroxene cumulate below. All gradations between these two rocks are exhibited within a stratigraphic range of about 200 m, where interstitial clinopyroxene and fine-grained cumulus clinopyroxene occur in varying proportions with coarser-grained cumulus orthopyroxene. Except where the unit is covered with fill deposits in the main river valleys, the Lower gabbro zone is continuously exposed from southwest of Mt. Rae to the east side of Black Butte, a distance of 40 km. Thickness 0-590 m. Near where it is cut off at either end by unconformity, the zone is only 100-200 m thick. Norite zone - The upper contact is where rock containing cumulus orthopyroxene readily seen in hand specimen underlies rock devoid of easily recognizable cumulus orthopyroxene. The upper part of the zone is mainly plagioclase-orthopyroxene cumulate. Oikocrysts of clinopyroxene are common in this rock and also in interlayers of plagioclase cumulate. A group of mixed layers provides a major break in continuity about halfway down the sequence. There, through a stratigraphic interval of about 100 m, 28 distinguishable layers a meter or so thick have been counted in one section. This section includes eight olivine-bearing layers interspersed between layers of plagioclase cumulate and plagioclase-orthopyroxene cumulate. Among these strata lies a zone, centered approximately at the divide between Brownlee Creek and Graham Creek that for a distance of 5.5 km and through a thickness of 2.1 m, has an average grade of 0.65 troy oz of Pt and Pd per short ton. This is one of the richest known deposits of platinum-group metals (PGM) in the world. The PGM zone there occurs where a layer of olivine cumulate and a layer of plagioclase cumulate together first became dominant upsection from the base of the Norite zone (Todd and others, 1979, p. 461-462, 464). The Pt and Pd are contained in sulfides, native alloys, arsenides, antimonides, bismuthinites (Cabri and La Flamme, 1974). The principal PGM-bearing minerals are pyrrhotite and pentlandite. Geochemical sampling and the driving of two adits by Johns-Manville in 1976-1980 have shown that the package of mixed layers containing one or more zones of PGM anomalies extends through the entire 40 km of strike length of the Norite zone. Drilling and the driving of an adit by Anaconda in 1979-1980 confirmed the existence of ore-grade PGM near the main Stillwater River. Facies changes within the zone result locally in fewer olivine-rich layers, lensing of pegmatite, and changes in stratigraphic position and grade of PGM anomalies. Two ore zones with a stratigraphic separation of 6-8 m have been locally encountered by Johns-Manville (E. L. Mann, oral commun., August 1980). Ore-grade PGM is encountered through a stratigraphic interval of 20-26 m by Anaconda in the Minneapolis adit (R. N. Miller, oral commun., August 1980). Below the group of mixed layers there is an interval of plagioclase-two pyroxene cumulate, then a much wider interval of principally plagioclase-orthopyroxene cumulate to the bottom of the section. Off-lap and slump structures indicating floor tilt, as well as graded bedding are seen in the lower interval of plagioclase-orthopyroxene cumulate on the west side of the Stillwater River. Scattered injections of pegmatoid orthopyroxene cumulate (bronzite), irregular in shape, crosscutting the layering, and locally rich in sulfides occur at the base of the Norite zone in two areas: (1) for about 1 km northwest of the East Boulder River, and (2) between Iron Mountain and Iron Creek. These sulfides also contain anomalous concentrations of platinum-group metals and were intensively drilled and trenched by Johns-Manville before the more persistent zone of interest was discovered higher in the Norite zone. At the maximum thickness of the zone, across the east end of Contact Mountain, the generalized sequence from top to bottom is: plagioclase-orthopyroxene cumulate, 250 m; mixed group (distinctively plagioclase cumulate and plagioclase-olivine cumulate), 100 m; plagioclase-two pyroxene cumulate, 75 m; and plagioclase-ortho-pyroxene cumulate 275 m making a total thickness of 700 m. The plagioclase-two pyroxene interval thins considerably west of the Brownlee Creek-Graham Creek divide and east of the West Fork of the Stillwater. To the west, the entire Norite zone is faulted out by the Lost Creek fault, near the Boulder River, and to the east the outcropping zone disappears in the moraine of West Fishtail Creek.' 1731|'Includes trondhjemitic gneiss-amphibolite, paragneiss, and heterogeneous gneiss sequences of trondhjemitic gneiss, tonalitic gneiss and amphibolite. Also includes minor schist, quartzite, and iron formation (Van Gosen and others, 2000).' 1732|'Predominantly granitic gneiss and migmatite; commonly consists of alternating bands of more felsic and more mafic gneiss; contains inclusions of metasedimentary rocks (granitic gneiss of Van Gosen and others, 2000).' 1733|'Interlayered quartzite and amphibolite. Amphibolite typically contains 60 percent hornblende, 30 percent feldspar, 5 percent garnet, and 5 percent quartz. The abundance of garnet distinguishes this amphibolite from the hornblende-rich gneiss in the quartzofeldspathic gneiss. Quartzite is coarse-grained and locally contains minor garnet, feldspar, anthophyllite, biotite, fuchsite, and sillimanite. A few layers of chlorite schist occur in this unit.' 1734|'Gneiss that consists mainly of feldspars and quartz with variable amounts of hornblende and biotite. Ranges from gray to pink gneiss because of abundant orthoclase. Layers and small concordant bodies of amphibolite occur scattered throughout this gneiss.' 1735|'Biotite schist that includes minor quartzite, iron formation and amphibolite (Van Gosen and others, 1993).' 1736|'Gneiss composed of sillimanite, garnet, quartz, feldspar, and biotite. Contains distinctive twinned, white plagioclase porphyroblasts. Coarse-grained marble that contains both calcite and dolomite with minor biotite and magnetite(?) forms layers up to 100 ft thick.' 1737|'Includes Barney Creek amphibolite, George Lake marble, and Jewel quartzite of Van Gosen and others (2000). Includes amphibolite, dolomitic marble and marble, quartzite, minor schist, and iron formation (Van Gosen, and others, 1993). Exposed south of Livingston in the northwestern part of the Beartooth Mountains.' 1738|'Sills are usually associated with hornfels.' 1740| 1741| 1750|'Includes modern stream alluvium, terrace gravel, talus and related slope material, landslide debris, and unconsolidated glacial moraines and outwash gravel.' 1751|'Floodplain, terrace, and alluvial-fan deposits that include stream-deposited gravel, sand, and silt. Gravel and peat are in filled ponds and lakes.' 1752|'Shown separately only in area near Challis and Antelope Flat (44°18’ N., 114°04’ W.), 30 km north of southeast corner of quadrangle.' 1753| 1754|'Unsorted boulders, cobbles, pebbles, sand, silt, and clay in moraines and in glaciofluvial outwash. Belt of coalescent moraines along eastern front of Sawtooth Range has been attributed to two major late Pleistocene glaciations, the Bull Lake and the Pinedale (Williams, 1961).' 1755|'Yellowish-white to cream-colored, irregularly thin- to medium-bedded, locally finely banded deposits of travertine showing varying degrees of irregular cellular and concentric texture with local cavities and encrustations. Layering in most of the deposit is nearly horizontal. Many exposures at eastern margin have steeply inclined layering; some of these clearly are of slumped blocks; others could be of undisturbed travertine deposited on steep slopes. Thought to once have covered about 2.5 km2 on southwest side of Bradbury Flat (44°26’ N., 114°10’ W.). Probably related to presently extinct hot springs that issued from faults in underlying Paleozoic carbonate strata. Estimated thickness 100 m.' 1756|'Dark-gray to black, fine-grained ophitic basalt; consists chiefly of plagioclase laths embedded in augite, and scattered crystals of magnetite. Exposures include small cap on ridge immediately west of Grimes Pass (44°01’ N., 115°49.8’ W.) and small outcrops west of Paddy Flat (44°45’ N., 115°59’ W.), near northwest corner of quadrangle.' 1757|'Stratified, tan to gray, loosely consolidated arkosic sandstone and siltstone and interstratified conglomerate and thin-bedded, dark-gray to black shale in which are abundant impressions of upper middle to lower upper Miocene leaves (W. C. Rember, written commun., 1983). Formation also contains seams of low-rank coal as much as 30 cm thick, bedded intervals several meters thick of light-gray to white diatomaceous earth, and beds of gray volcanic ash as much as 30 cm thick. Formation crops out along west side of Middle Fork of Payette River northwest of Crouch (44°07’ N., 115°58’ W.). It was deposited in an intermontane basin and is now part of a west-tilted, down-faulted block. Minimum thickness 1,680 m; base not exposed.' 1758|'Thunder Mountain cauldron complex and environs: These small rhyolite intrusions have potassium-argon ages of 39.8±0.9, 41.1±-0.9, 42.4±1.5, and 45.2±1.5 m.y. (see table 1). Additional description see below. Western and south-central parts of quadrangle: Rocks are light colored and generally porphyritic, containing rounded and embayed quartz phenocrysts. Potassium feldspar phenocrysts are more plentiful than plagioclase. Matrices are pinkish gray and aphanitic. Modal compositions include rhyolite, quartz-latite, and rhyodacite. The rocks are altered: sericite is developed from feldspars and iron oxides from pyrite. Dikes range from a few meters to more than 30 m wide and are as much as several hundred meters long. Rhyolite dikes are more numerous near bodies of granite (Tg), to which they probably are related, and, like that rock, are more radioactive than other plutonic rocks or dikes. Granophyric matrices common in dikes cutting Idaho batholith rocks north of Stanley. Some dikes in that area also are composite, having marginal zones more mafic than rhyolite.' 1760|'Thunder Mountain cauldron complex and environs: Sanidine from a sample taken from Monumental Creek (44°56’ N., 115°12’ W.) was dated by potassium-argon methods at 44.6±1.5. For additional description, see below. Northern part of Van Horn Peak cauldron complex and Panther Creek graben: Dikes and plugs of mostly light-gray, dense, phenocryst-poor rhyolite; locally contains conspicuous phenocrysts of sanidine as long as 8 mm and sparse phenocrysts of quartz, as at Red Rock Peak, along northern border of Challis quadrangle at long 114°25’30" W. Rock at Red Rock Peak grades into, and forms composite bodies with, quartz porphyry intrusions (Tqp). Sanidine from Red Rock Peak yielded potassium-argon age of 44.6±1.5 m.y. (see table 1). Custer graben area: Gray, white, and pink dikes, plugs, and domes that contain phenocrysts of alkali feldspar and quartz, and minor amounts of biotite and plagioclase.' 1764|'Thunder Mountain cauldron complex and environs: Gray dacite porphyry and minor diorite porphyry. Consists of both extrusive and intrusive rock. These rocks mostly have a dense or cryptocrystalline groundmass but, locally, have a holocrystalline, fine-grained groundmass (diorite porphyry). Phenocrysts of plagioclase generally are conspicuous and, in Indian Creek area (44°48’ N., 115°13’ W.), commonly are 2-3 mm long. Quartz is sparse. Mafic minerals consist of varying proportions of biotite, hornblende, and pyroxene. Phenocryst content varies from about 20 to 50 percent. In places, dacitic rocks clearly intrude lapilli tuff (Tdq) and possibly younger tuff units as well. In other places, rocks appear to be stratiform, having flow layering that defines gentle dips and containing interlayered zones of flow breccia characteristic of contact zones between lava flows. As mapped, therefore, this unit in some places includes dacite lava that is correlative with the lower latite lava (Tll) and in other places includes dikes and irregularly shaped intrusives that probably are younger than lapilli tuff (Tdq). Also correlates in part with dacite and diorite porphyry (Tdc) mapped in western part of quadrangle (see p. 20). Northern part of Van Horn Peak cauldron complex and Panther Creek graben: A mixed sequence of gray and green-gray extrusive and intrusive rocks of intermediate composition; most are dacite porphyry that locally displays flow layering, flow brecciation, and dense to glassy groundmass indicating they are lavas. In places, porphyry is massive, has crystalline groundmass, and locally exhibits intrusive contacts with country rock as young as basal part of tuffs of Camas Creek-Black Mountain (Tc, see p. 7). Therefore, this unit, as mapped, probably includes intermediate lava as old as 51 m.y. and younger dikes and irregularly shaped intrusive masses 48 m.y. old or younger. Phenocrysts (30-45): q, 0 (in most rocks) to 5; af, 0; pf, 60-75; b, 1-12; hb, trace-25; px (mostly altered but in places includes both fresh opx and cpx), trace-20. Contacts are gradational over short distances from hornblende- and biotite-rich varieties to pyroxene-rich varieties. Thickness 0 to more than 500 m. Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: Biotite from sample collected near Casto (44°34’ N., 114°50’ W.) gave potassium-argon age of 48.6±1.4 m.y. (see table 1). Sample collected from probable extrusive rock 8 km north of Casto yielded hornblende age of 46.9±2.8 m.y. and biotite age of 49.5±1.4 m.y. (see table 1).' 1778|'Thunder Mountain cauldron complex and environ: Thickness 0 to more than 50 m. Western and south-central parts of quadrangle: Includes three small ridge caps of rhyolite and discordant rhyolitic tuff near headwaters of North Fork of Boise River (44°05’ N., 115°11’ W.) and, in northern part of area, exposures on headwaters of Rapid River (44Ί40’ N., 115Ί02’ W.) and on headwaters of Riordan Creek (44°49’ N., 115°24’ W.).' 1779|'Dikes and plugs of pink porphyry that varies greatly in size of phenocrysts and texture of groundmass; typically contains smoky quartz phenocrysts as much as 3-4 mm long, tabular alkali feldspar and sparse plagioclase phenocrysts as much as 6-8 mm long, and sparse flakes and books of biotite as much as 3 mm long, in dense micrographic groundmass of quartz and alkali feldspar; commonly, however, has microgranular or cryptocrystalline groundmass. Locally contains extremely large phenocrysts of resorbed quartz in bipyramids as much as 1 cm long and alkali feldspar and plagioclase phenocrysts as much as 3 cm. long in graphic groundmass. Single large sanidine crystal 3 cm long from dike east of Yellowjacket Creek (44°59’ N., 114°24’ W.) yielded a potassium-argon age of 44.4±1.0 m.y. (see table 1). The quartz porphyry is described as pink granophyre by Ross (1934). Some dikes contain rhyolite without quartz phenocrysts; in places these quartz-poor dikes were included with lapilli tuff (Tdq), in other places with rhyolite intrusions Tr, see below). Phenocrysts (19-35): q, 15-58; af, 37-83; pf, 0-12; b, trace-4.' 1781|'Dikes and plugs of basaltic composition similar to or same as basaltic lava (Tb, see below). Includes mafic to intermediate rocks, near Van Horn Peak and elsewhere, that are conspicuously porphyritic, containing phenocrysts of pyroxene (hypersthene, pigeonite, or both) and plagioclase as long as 5 mm; locally contains sparse biotite and hornblende.' 1783|'Thunder Mountain cauldron complex and environs, Northern part of Van Horn Peak cauldron complex and Panther Creek graben: Corresponds to "Casto pluton" of Ross (1934) and of Cater and others (1973). Mostly equigranular pink granite: q, 22-34 (0.2-5 mm long); af (perthitic cloudy orthoclase and microcline), 31-57 (0.5-10.0 mm. long); pf, 18-33; b, 20-37; hb, 0-3.5; trace amounts of sphene and allanite. These rocks plot in granite field of both Johannsen (1948) and Streckeisen (1973). Also includes light-gray rocks that are distinctly richer in plagioclase and fall in quartz monzonite field of Johannsen (1948). Armstrong (1975) reports potassium-argon age on biotite of 43.9±1.3; Leonard and Marvin (1982) report potassium-argon age from the same locality, also on biotite, of 47.8±1.9 m.y. (see table 1). On Loon Creek (44°43’ N., 114°48’ W.), biotite from the granite has a potassium-argon age of 45.1±1.6 m.y. (see table 1). Biotite from fine-grained, light-gray rock exposed near Grouse Lake (44°50’ N., 114°41’ W.) has a potassium-argon age of 46.6±1.6 m.y. Western and south-central part of quadrangle: Pink to gray, medium- to coarse-grained granite characterized by pink perthitic feldspar. Relative proportion of salic minerals (calculated from the CIPW norm): q, 39; af, 34; pf, 27. Rock contains miarolitic cavities, locally lined with smoky quartz crystals, and is more radioactive than granodiorite of Idaho batholith. Hornblende from one sample was dated by potassium-argon methods at 44.3±1.3 m.y. (see table 1). Granite includes part of Sawtooth batholith (Reid, 1963).' 1788|'Varicolored, nonwelded to partly welded ash-flow and ash-fall rhyolite tuff. Sequence overlies and underlies a single genetically related, densely welded cooling unit of tuff of Castle Rock (Tck) having identical phenocryst mineralogy. Lower part of monolith-forming tuff locally includes unrelated soft, bedded tuff containing abundant biotite; includes several zones rich in small, variegated volcanic lithic fragments. Rock above and below tuff of Castle Rock (Tck) weathers to tepee-shaped hoodoos and other variously shaped monoliths. Unit typically contains vitric, silky pumice lapilli and fresh black glass shards, but zeolitized and otherwise altered in many places. Phenocrysts, (15-25): q, 33-50; af, 50-60; pf, 0-3. b, 0-1; hb, 0-trace; px (mostly pseudomorphs), 0-3; commonly contains smoky quartz and chatoyant alkali feldspar. Thickness 0-300 m.' 1789|'Within cauldron includes five cooling units of reddish ash-flow tuff and thin interbeds of ash-fall tuff. Upper cooling unit is densely welded and about 228 m thick. Beneath upper unit are, successively, two thin, partly welded, lithic-rich cooling units, each about 30 m thick; a cliff-forming, moderately welded cooling unit about 150 m thick; and a lowermost slope-forming, partly welded cooling unit rich in lithic fragments, about 190 m thick. Densely welded upper unit is inferred to correlate with densely welded part of outflow unit (Tmt). If this is the case, soft tuff preserved above densely welded unit in Panther Creek graben has been stripped away at Castle Rock (44°49’ N., 114°25’ W.). Phenocrysts, upper cliff-former (28): q, 47; af, 50; pf, 0.7; b, 0.3-0.7; altered cpx, 0.7; (base) to 2.9 (top). Phenocrysts in lower cliff former show considerable variation from base to top: q, 15 (base) to 45 (top); af, 78 (base) to 49 (top); pf, 3.8 (base) to 4.1 (top); b, trace (base) to 1.0 (top); hb, trace; cpx, 3.1 (base) to 1.4 (top). Same minerals are present in thin cooling units between cliff-formers and in thick, moderately welded lower unit. Quartz is slightly smoky throughout sequence, and all units contain zones of chatoyant alkali feldspar. Total thickness 620 m.' 1791|'Black and brownish-gray lava that probably ranges in composition from potassium-rich basalt to trachyandesite or latite; vesicular to dense; contains scattered small (0.5-2 mm long) phenocrysts of pyroxene and plagioclase in pilotaxitic groundmass composed of plagioclase laths, pyroxene, and glass; locally contains small altered olivine phenocrysts and commonly contains xenocrysts of quartz that have reaction rims composed of tiny pyroxene prisms and glass. Unit is intercalated with rocks as young as basal tuff of Castle Rock (Tck) and with rocks as old as tuff of Ellis Creek (Te, see p. 8). Thickness 0-50 m.' 1792|'Light-green-gray and reddish-gray, locally flow-layered, densely welded ash-flow tuff erupted during collapse of Castle Rock cauldron segment (see table 4). Consists of at least two cooling units. Locally, unit in Panther Creek graben is flow laminated and layered from base to top; these characteristics indicate that hot ash flows there coalesced to liquids before coming to rest. Elsewhere, especially on slopes south of West Fork of Camas Creek (44°47’ N., 114°35’ W.), rock is only locally flow layered and displays zones containing abundant brown-green pumice lapilli that are darker than light-gray or light-greenish-gray enclosing matrix. Abundance and relative proportions of phenocrysts vary greatly. Phenocrysts (20-35): q, 19-50; af, 25-45; pf, 15-40; b, 2-20; hb, trace-5; px, trace. Some quartz phenocrysts in this unit show a peculiar tendency to be biaxial, even where they are not obviously strained. They tend to be smoky and nearly everywhere are incipiently to strongly resorbed. Allanite is a common accessory mineral. Thickness 0-300 m.' 1793|'Light-greenish gray and reddish-gray tuff, commonly altered to yellow and various pastel shades; flow-layered and laminated, densely welded, locally containing recognizable well-flattened pumice lapilli. Ash flows in this unit coalesced to liquids before they stopped flowing. Named for sparse, but conspicuous, euhedral phenocrysts of water-clear, "plate glass" tabular alkali feldspar crystals as long as 5-6 mm and altered and inconspicuous euhedral plagioclase crystals as long as 5-6 mm. Spherulitic in several localities; unit has an altered perlitic basal vitrophyre. Phenocrysts (6-10): af, 30-40 (locally as much as 50); pf, 45-66; altered mafic mineral (probably pyroxene), 2-8. Unit is intruded by dense rhyolite at Singheiser (44°51’ N., 114°24’ W.) and Rabbit Foot (44°53’ N., 114°20’ W.) mines and is weakly mineralized. Thickness 0-300 m.' 1795|'A cauldron-filling sequence of mostly very densely welded ash-flow tuff consisting of 10 or more separate cooling units that all are characterized by small phenocrysts (about 2 mm long) that in upper part of sequence consist of plagioclase and fairly abundant mafic minerals, in middle part consist of plagioclase and sparse to moderate amounts of alkali feldspar, and in lower part consist of plagioclase and sparse alkali feldspar and quartz. Entire sequence contains abundant mafic minerals consisting mostly of altered pyroxene, and includes variable amounts of biotite and sparse hornblende. Lower part of sequence has more biotite and hornblende than pyroxene. Assemblage is similar to tuff of Eightmile Creek (Tem, see below) although phenocrysts make up a greater proportion of tuff of Eightmile Creek. Phenocrysts in upper cooling unit (11-40): q, 0-trace; af, 0-trace; pf, 67-75; b, trace-13; altered hb, 0-trace; altered pyroxene, 10-20. Phenocrysts in middle cooling unit (5-25): q, 0-trace; af, trace-20; pf, 65-77; altered b, trace-9; altered hb, trace-3; altered px, 3-10. Phenocrysts in lower cooling unit (3-15): q, 2-12; af, 2-8; pf, 60-75; altered b, 2-10; altered hb, trace-3; altered px, trace-10. Sequence appears to have been almost entirely confined to cauldron complex formed principally as result of eruption of tuff of Ellis Creek (Te, see below). Only thin, partly welded ash-flow tuff and minor ash-fall tuff appear outside the complex. Outflow units are mapped with tuff of Pennal Gulch (Tp, see below). Thickness 0-3,000 m.' 1796|'Northern part of Van Horn Peak cauldron complex and Panther Creek graben: Outflow equivalent of tuff of Van Horn Peak. See below for additional description. Corral Creek cauldron segment and area north and east of Challis: Pale-grayish-red to yellowish-brown, densely welded rhyolite or quartz latite ash-flow tuff, probably a simple cooling unit. Phenocrysts (10-13): pf, 82-90; b, 7-13; altered px, 2-5. Black basal vitrophyre a few meters thick contains abundant small volcanic lithic fragments a few millimeters to a few centimeters in diameter. Biotite from vitrophyre has a potassium-argon age of 47.8±1.7 m.y. (see table 1). Unit has reversed magnetic polarity. Source for this unit is a vent at Van Horn Peak (44°46.5’ N., 114°19.5’ W.)-(Ekren, 1981). Thickness 0 to more than 100 m.' 1797|'Thin ash-flow tuff cooling unit of Camas Creek-Black Mountain lithology interbedded with thin-bedded ash-fall tuff of similar lithology and lacustrine tuffaceous siltstone, sandstone, and conglomerate. Thickness 0-100 m.' 1800|'Pink and orange-gray, flow-laminated, hydrated rhyolite glass that has retained its perlitic texture despite deep weathering and alteration; mapped in all localities to include subjacent and superjacent horizontal beds of tuff and tuffaceous sandstone. Quartz is amethystine to smoky and as much as 4 mm long. Phenocrysts (21-33): q, 27-35; af, 9-31; pf, 19-47; b, 6-12; hb, 4-6.5; altered px, 2. Thickness 0-35 m.' 1801|'Northern part of Van Horn Peak cauldron complex and Panther Creek graben: Two or more cooling units of green-gray quartz latitic and rhyodacitic ash-flow tuff that together are more than 300 m thick near Sleeping Deer Mountain (44°46’ N., 114°41’ W.) within cauldron complex, but rarely exceed a few tens of meters each outside cauldron complex or Custer graben. Unit overlies tuff of Ellis Creek (Te), underlies the tuffs of Camas Creek-Black Mountain (Tc), and locally shares some characteristics of each unit. Included in tuff of Pennal Gulch (Tp) south of Table Mountain. Where densely welded and rich in phenocrysts, it is not easily distinguished from tuff of Ellis Creek; it differs only in containing significant amounts of alkali feldspar and fewer mafic minerals. Phenocrysts (14 (weakly welded) to 46 (densely welded within the cauldron complex)): q, 10-22; af, 6-16; pf, 44-63; b, 6-16; hb, 1-5; px, trace-2. Unit contains abundant apatite, modest amounts of zircon, and sparse allanite as accessories. Most quartz phenocrysts are slightly embayed. Large grains that are extensively embayed are common. Biotite from exposures of this unit near West Fork of Morgan Creek (44°43’ N., 114°15’ W.) has potassium-argon age of 48.4±1.7 m.y. (see table 1), which agrees with stratigraphic position of this tuff, below 47.8-m.y.-old tuff of Table Mountain (Ttm). Thickness 0 to more than 300 m. Corral Creek cauldron segment and area north and east of Challis: Light-gray and green-gray, massive-weathering, partly welded and densely welded quartz latite or rhyodacite ash-flow tuff; appears to be simple cooling unit in Corral Creek area (44°45’ N., 114°14’ W.), where it is poorly exposed and, in most areas, was not mapped separately at base of tuff of Pennal Gulch (Tp). Phenocrysts (25-35): q, 13-19; af, 12-18; pf, 47-57; b, 9-14; hb, trace-2; cpx, trace. Thickness 0-50 m. Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: Erosional remnant of this tuff on lower Mill Creek (44°32’ N., 114°17’ W.) overlies thin-bedded tuffaceous sandstone, siltstone, and reworked pumice beds that in turn overlie aphanitic intermediate lava (TI, see p. 12). Tuff is light greenish gray to brownish gray, nonwelded, and only slightly devitrified; it contains pale-green to white pumice lapilli. Phenocrysts (approximately 26): af, 15; q, 22; pf (strong oscillatory zoning), 33; b, 23; hb (green), 7. On southwest side of Corkscrew Mountain (44°32’ N., 114°23’ W.), erosional remnants of tuff of Eightmile Creek are light gray to greenish gray, nonwelded, and pumiceous, containing medium-brownish-gray pumice lapilli that are darker than matrix. Crystals, as much as 2 mm long, commonly are broken. Phenocrysts (42): af, 15; q, 8; pf (strong oscillatory zoning), 64; b, 12; hb, 1. At Red Butte (44°39’ N., 114°20.5’ W.) about 60 m of this tuff underlies rhyolite of Red Butte (Trb). Tuff is nonwelded to partly welded, very pale gray, and rich in pumice. Phenocrysts (as much as 2.5 mm long) (27-33): af, 11-12; q, 17-22; pf (strong oscillatory zoning), 52-55; b, 13-15; hb, 0-2. From upper Spider Creek to upper Parker Creek (44° 38’ N., 114° 33’ W.), unit is greenish-gray, densely welded, and pumiceous; contains whitish-green pumice lapilli that generally are lighter than matrix. The tuff is altered to calcite and chlorite; sericite, clay minerals, and epidote locally are present. Biotite from sample from West Fork of Morgan Creek (44° 42’ N., 114° 18’ W.) has potassium-argon age of 48.4±1.7 (see table 1). Phenocrysts (25-35): af, 12-17; q, 9-27; pf, 43-66; b, 7-18; hb, 2-4. Custer graben area: Gray, greenish-gray, pale-brown, and grayish-pink, bluff-forming, pumice-rich, crystal-rich, densely welded, quartz latitic ash-flow tuff characterized by phenocrysts, as much as 4 mm long, of plagioclase, quartz, sanidine, biotite, amphibole, clinopyroxene, orthopyroxene, zircon, apatite, and allanite. Biotite from vitrophyre exposed west of mouth of Tenmile Creek (44°28’ N., 114°35’ W.) has potassium-argon age of 47.5±1.3 m.y. (see table 1); this age supersedes previously published age of 46.9±1.6 m.y. (McIntyre and others, 1982). Unit has reversed magnetic polarity. On west side of Eightmile Creek about 4 km above its mouth (44Ί27’ N., 114Ί39’ W.), four cooling units having total thickness of more than 215 m are exposed.' 1803|'Northern part of Van Horn Peak cauldron complex and Panther Creek graben: Rhyodacitic, crystal-rich ash-flow tuff characterized by phenocrysts of plagioclase, strongly resorbed quartz, and abundant biotite and hornblende, all of which are present in subequal amounts. Rocks exposed within cauldron complex commonly are partly altered to chlorite, epidote, and associated minerals. Locally, as along West Fork of Camas Creek (44°49’ N., 114°18’ W.), abundant very fine-grained magnetite darkens rock. Sample from northeast of Duck Peak (44°55’ N., 114°29’ W.), in Panther Creek graben, has potassium-argon age on biotite of 48.4±1.6 m.y. (see table 1). Thickness 0 to more than 2,000 m. Corral Creek cauldron segment and area north and east of Challis: Light-green-gray, massive-weathering, densely welded rhyodacite ash-flow tuff that is outflow from Van Horn Peak cauldron complex. A multiple-flow compound cooling unit or, possibly, two cooling units; everywhere contains conspicuous pumice; contains zones within which pumice lapilli are lighter than matrix and zones within which lapilli are darker than matrix. Phenocrysts (36-50): q, 4-15 (commonly "worm-eaten" and as long as 4 mm); pf, 60-75 (as long as 6 mm); b, 12-20; hb, 8-16; cpx, trace. Thickness 0-300 m. Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: Along lower Eddy Creek this unit consists of two outflow cooling units, each about 60 m thick, separated by 1-2 m of thin-bedded tuff and reworked, cross-bedded, fluvial tuffaceous sandstone containing abundant biotite. Ash-flow tuff in both cooling units is gray green and moderately welded; contains pale-green pumice lapilli as much as 3 cm long. Phenocrysts (28-34): q, 3-6; pf, 66-67; b, 22-24; hb, 5-6.' 1804|'Northern part of Van Horn Peak cauldron complex and Panther Creek graben: Principally cliff forming, dark-gray, conspicuously porphyritic lava of dacitic composition; blocky weathering; inconspicuous flow layering; unit locally includes phenocryst-poor, dark purplish-gray latite or andesite. Phenocrysts, dacitic lava (25-40): q, 0-trace; pf, 60-70 (as long as 6 mm); b, 0-4; hb, 15-23; cpx, trace-15; opx, trace. In some areas mafic minerals are oxidized to black opaque iron oxides or replaced by chlorite and iron oxide. Latite or andesite lava contains 5-15 percent phenocrysts (0.5-2 mm long) of plagioclase and clinopyroxene, and traces of orthopyroxene and oxidized hornblende; locally, pyroxene and hornblende phenocrysts are as long as 1 cm. Biotite from sample collected near Red Rock Peak (45°00’ N., 114°25’ W.), in Panther Creek graben, gave potassium-argon age of 48.6±1.7 m.y. (see table 1). Sample from Little West Fork of Morgan Creek (44°42’ N., 114°19’ W.) has potassium-argon age on biotite of 51.1±1.8 m.y. (see table 1). Thickness 0 to more than 1,000 m. Corral Creek cauldron segment and area north and east of Challis: Thickness 0-900 m. Twin Peaks caldera and southern part of Van Horn Peak cauldron complex:: Rhyodacite lava and agglomerate exposed southeast of Challis Creek near Corkscrew Mountain (44°32’ N., 114°23’ W.) and north of Challis Creek (44°36’ N., 114°20’ W.) between Pats Creek and Eddy Creek. Dark-brown-weathering, brown to gray, porphyritic, locally vitrophyric, massive rhyodacite lava and coarse agglomeratic breccias containing 0.2-3 m blocks of monolithologic rhyodacite porphyry in matrix of glassy to devitrified rhyodacite. Phenocrysts (28-44): pf (as much as 5 mm long), 60-75; b, 1-14; px (both opx and cpx), 7-35; hb, 0-13. Contacts between massive and agglomeratic phases of unit are irregular and commonly steep. Cavities filled with chalcedonic silica locally abundant. Biotite from sample collected from southwest side of Corkscrew Mountain (44°32’ N., 114°23’ W.) has potassium-argon age of 50.4±1.8 m.y. (see table 1). Custer graben area: In Custer graben consists chiefly of crystal-poor, potassium-rich andesite that contains phenocrysts, generally smaller than 2 mm long, of plagioclase and clinopyroxene. Characterized by propylitic alteration in Sunbeam mine-Estes Mountain area (44Ί27’ N., 114Ί44’ W.) and in area surrounding Custer (44Ί23.5’ N., 114Ί41’ W.). Southeastern part of quadrangle: Contains heterogeneous group of rocks erupted from numerous vents scattered over area from Challis to south edge of quadrangle. Individual, local units generally can be consistently distinguished during mapping at large scales (see, for example, Hobbs and others, 1975). Compositions mainly dacite and rhyodacite; contain as phenocrysts various combinations of the minerals plagioclase, biotite, amphibole, clinopyroxene, orthopyroxene, and, uncommonly, olivine or quartz. Includes minor amounts of potassium-rich andesite characterized by phenocrysts of pyroxene and(or) olivine. Has normal and reversed magnetic polarity. Ranges in age from about 49 m.y. to about 51 m.y. Potassium-argon ages on biotite are 49.2±1.2 m.y. and 51.1±1.7 m.y.; plagioclase is dated at 49.3±1.4 m.y. (see table 1).' 1809|'Corral Creek cauldron segment and area north and east of Challis: Similar to intermediate and mafic lavas (TI), listed on p. 17. Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: Includes dark-gray to bluish-black, dark-brown-weathering, aphanitic, porphyritic, intermediate intrusive rocks containing 3-5 percent euhedral plagioclase phenocrysts as much as 6 mm long and 1-2 percent altered euhedral pyroxene phenocrysts. Exposed on Challis Creek at mouth of Eddy Creek (44°35’ N. 114°19’ W.) and on Moose Creek (45°48.5’ N., 114°10’ W.).' 1814|'Northern part of Van Horn Peak cauldron complex and Panther Creek graben, southeastern part of quadrangle, and Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: Predominantly aphyric, reddish-brown-weathering, gray, purple, and greenish-gray, blocky to platy lava, locally containing interbedded oxidized breccia. Olivine and pyroxene occur as phenocrysts in some samples. Plagioclase not commonly found as phenocrysts. Microphenocrysts include olivine, clinopyroxene, orthopyroxene, and sieve-textured plagioclase. Quartz xenocrysts commonly present. Plagioclase-rich groundmass commonly is trachytic or pilotaxitic and may contain apatite and reddish-brown, strongly pleochroic mica. In region surrounding Challis, rock compositions range from potassium-rich andesite to potassium-rich basalt, andesite probably predominating. Farther south, in area between East Fork of Salmon River and Jerry Peak (44°03.5’ N., 114°06.5’ W.), compositions range from potassium-rich andesite to latite. Latite is more common toward top of section. Magnetic polarity indeterminate in area near Challis; lava toward south has reversed magnetic polarity. Near Challis, the lava overlies tuff of Ellis Creek (Te, 48.4±1.6 m.y.) and is overlain by tuff of Eightmile Creek (Tem, 48.4±1.6 m.y. and 47.5±1.3 m.y.) and tuff of Table Mountain (Ttm, 47.8±1.7 m.y.). Toward south, lava overlies tuff dated at 49.0±2.9 m.y. (Tvs) and is overlain by tuff of Herd Lake (Th, 48.1±1.7 m.y.). Corral Creek cauldron segment and area north and east of Challis: In Corral Creek area (44°45’ N., 114°14’ W.), thickness is 0-700 m.' 1821|'Light-gray, tan, and pinkish-red, generally crystal-poor or only moderately crystal-rich, porphyritic intrusive rocks and minor flows. Rock typically contains phenocrysts of quartz and sanidine in variable amounts; plagioclase locally present in minor amounts. Locally contains minor biotite and, rarely, trace amounts of pyroxene. These rocks display fabrics ranging from massive to flow laminated to autobrecciated. In thin section, textures vary from glassy to devitrified to fine grained, allotriomorphic, and granular. In ridges between East Fork of Mayfield Creek and Yankee Fork (44°30’ N., 114°40’ W.) at least two generations of rhyolite are present. Older rhyolite is glassy to devitrified and is intruded by dikes and pluglike masses of more massive rhyolite. Narrow rhyolite dikes and irregularly shaped intrusive masses pervade west and north margins of Twin Peaks caldera and crosscut lower part of tuff of Challis Creek (Tcrl, see p. 12) and the caldera-wall slump debris (Tsd, see p. 11). Potassium-argon age of sanidine is 46.5±1.7 (see table 1).' 1822|'Light-gray to brown-gray, light-brown-weathering dikes of porphyritic trachyte and(or) rhyolite that contain phenocrysts of sanidine, as long as 5 mm, and locally contain minor amounts of pyroxene and biotite. Plagioclase crystals, where present, generally are small, resorbed, and mantled by alkali feldspar. These dike rocks occur principally within Twin Peaks caldera, where they intrude tuff of Challis Creek (Tcrl, see p. 12) and caldera-wall slump debris (Tsd, see p. 11).' 1824|'Light-gray to pinkish-red or lavender, red- to lavender-weathering, fissile to flaggy, flow-laminated, and locally autobrecciated intrusive rock along north and east margins of Twin Peaks caldera. Phenocrysts (9-21): af, 0-25; pf (strong oscillatory zoning), 53-69; b, 17-34; hb, 0-5. Locally, near contacts, rock is black and vitrophyric or perlitic, devitrified, and gray-green weathering. Rock has steep to vertical flow foliation and locally has isoclinal flow folds with steep to vertical axial planes of variable strike. Locally, this rock contains cavities lined with chalcedonic quartz. In hand specimen, rock resembles rhyolite of Red Butte (Trb, see p. 13), except that, unlike rhyolite of Red Butte, it generally contains scattered sanidine phenocrysts. Unit is in intrusive contact with tuff of Challis Creek (Tcr) in upper Pats Creek area (44°36’ N., 114°21’ W.) and in upper Eddy Creek above Eddy Basin (44°40’ N., 114°27.5’ W.). A pluglike mass (not shown on map) containing plagioclase, biotite, sanidine, and hornblende phenocrysts is on Spider Creek (44°40.5’ N., 114°26.5’ W.). This rock is light greenish-gray, medium-brown weathering, and steeply flow foliated; it contains abundant gas cavities lined with rosettes of gypsum and minor calcite.' 1827|'Dark-brown to purple-brown, dark-brown-weathering, porphyritic dikes or flows containing abundant phenocrysts as much as 3 mm in length of plagioclase and pyroxene, less abundant hornblende, and minor biotite. On lower Bear Creek (44°34.5’ N., 114°24’ W.) a probable dike of this unit, which appears to occupy ring-fracture fault of Twin Peaks caldera, contains phenocrysts of sanidine in addition to above minerals. Contacts of this unit near Mosquito Flat Reservoir (44° 31’ N., 114°26’ W.) are not exposed, and relations with tuff of Challis Creek (Tcr) are uncertain.' 1828|'Northern part of Van Horn Peak cauldron complex and Panther Creek graben: Not shown separately in this area. Massive to faintly flow-laminated and locally vitrophyric at intrusive margins. Present in area of East Fork of Mayfield Creek (44°29’ N., 114°37’ W.) and in Custer graben. In upper Tenmile Creek area (44°28’ N., 114°37’ W.), reddish-brown to lavender, porphyritic rhyodacite intrudes tuff of Eightmile Creek (Tem) and is intruded by porphyritic rhyolite (Trf). Phenocrysts (22-25): pf, 54-58; b, 10-15; hb, 23-26; op, 5-7. Potassium?argon age of intrusive mass on upper Tenmile Creek is 46.0±1.7 m.y. (biotite); 48.9±2.9 m.y. (hornblende). Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: Massive to faintly flow-laminated and locally vitrophyric at intrusive margins. Present in area of East Fork of Mayfield Creek (44°29’ N., 114°37’ W.) and in Custer graben. In upper Tenmile Creek area (44°28’ N., 114°37’ W.), reddish-brown to lavender, porphyritic rhyodacite intrudes tuff of Eightmile Creek (Tem) and is intruded by porphyritic rhyolite (Trf). Phenocrysts (22-25): pf, 54-58; b, 10-15; hb, 23-26; op, 5-7. Potassium-argon age of intrusive mass on upper Tenmile Creek is 46.0±1.7 m.y. (biotite); 48.9±2.9 m.y. (hornblende) (see table 1). Custer graben area: Brown- and reddish-brown-weathering, crystal-poor rocks that contain phenocrysts less than 2 mm in length of plagioclase, amphibole or pyroxene, and minor biotite.' 1829|'Medium- to dark-gray, brownish-gray, gray- to brown-weathering, porphyritic intrusive rock containing abundant phenocrysts of plagioclase as long as 7 mm and less abundant hornblende, pyroxene, and minor biotite in pilotaxitic microcrystalline to devitrified, blotchy matrix. Where fine-grained, rock has steep flow foliation of variable strike and in places flow folds have steep axial planes. This rock intrudes tuffs of Camas Creek-Black Mountain (Tc) at head of Parker Creek (44°37’ N., 114°32’ W.). Unit is intruded by rhyolite (Trf).' 1831|'Heterogeneous deposit of coarse talus and megabreccia (following usage of Lipman, 1976) interfingering and admixed with ash-flow tuff. Deposits of this material are localized along ring-fracture system on southeast margin of Twin Peaks caldera (44°34’ N., 114°23’ W.) and in northern part of caldera, where they extend into caldera from its north and west margins. In lower part of unit are bedded, indurated, coarse conglomeratic sandstone and angular talus deposits, resembling concrete on fresh fracture; these deposits also form thin, lens-like layers and pods throughout the map unit. Megabreccia consists of tuffaceous matrix-supported to block-supported deposits of angular, unsorted pebble- to boulder-size clasts and house-size blocks of ash-flow tuff, intermediate and siliceous lava, epiclastic rock, and abundant fragments of tuff of Challis Creek. Ash-flow tuff matrix of megabreccia is nonwelded and crystal poor; it resembles tuff of Challis Creek except for higher plagioclase content. Phenocrysts (4-20): af, 40-47; q, 37-57; pf, 3-17; b, trace. Thickness more than 400 m.' 1832|'Multiple flow, compound cooling unit of crystal-rich rhyolite ash-flow tuff. Phenocrysts (22-26 in basal nonwelded tuff, 36-44 in remainder of unit): af, 54-69; q, 25-45; pf, 0-3; b, trace; hb, rare; px, 0-4.5. Accessory minerals are zircon, allanite, and trace amounts of apatite. Sanidine phenocrysts, as long as 5 mm, are chatoyant. Quartz phenocrysts commonly are slightly resorbed, bipyramidal, and smoky. Phenocrysts are somewhat smaller and less abundant in basal part of unit than in densely welded tuff. This unit, like middle and lower cooling units (Tcrm and Tcrl), has little or no plagioclase or ferromagnesian minerals. Plagioclase does not generally exceed 0.5 percent of total phenocrysts, biotite is uncommon, and minor pyroxene grains are preserved only in most densely welded tuff. Except locally near base, this unit is medium-gray to brownish-gray, brown-weathering, densely welded, devitrified ash-flow tuff containing pumice lapilli as much as 12 cm long and 3 cm thick (uncommonly as much as 26 cm long and 2 cm thick) and scattered pebble- to cobble-size fragments of gray to lavender, aphyric intermediate to rhyolitic lava. Pumice lapilli are in places medium to rusty brown and darker than matrix or light gray and lighter than matrix. Sanidine has been dated by 40Ar-39Ar methods at 45.5±0.3 m.y. (see table 3). At base, this tuff is light gray to light brown, nonwelded to moderately welded, and glassy and contains more abundant lithic fragments than do rocks exposed above; lithic fragments less than about 8 cm in diameter constitute 4-6 percent of rock and include porphyritic intermediate lava, light-gray, flow-banded rhyolite (in places as much as 1 m long at base of unit), and fragments of ash-flow tuff that has mineralogy of tuff of Challis Creek. The most pumiceous nonwelded to moderately welded basal part of tuff is as much as 60 m thick and is overlain by densely welded tuff at the base of which is a laterally discontinuous vitrophyre as much as 6 m thick. Original top of unit is not exposed. Thickness is more than 700 m.' 1833|'Two, or possibly three, cooling units of light-gray to greenish-gray or brownish-gray, partly to densely welded, pumiceous rhyolite ash-flow tuff that weathers gray, light brown, or lavender. Phenocrysts (as much as 2 mm long) (15-26): af, 43-59; q, 38-57; pf, 0-trace; b, 0-1.5; px, 0-2. Pumice lapilli in upper cooling unit are greenish brown, darker than matrix, and as much as 6 cm, but generally less than 1 cm, long. In lower cooling unit, pumice lapilli are pale greenish gray and lighter than matrix. Sanidine from zeolitized middle unit gives an 40Ar-39Ar age of 45.9±0.2 m.y. (see table 3). Middle unit (Tcrm) typically is in sharp contact with overlying upper unit (Tcru). There are concentrations of lithic fragments near contact. Middle unit locally overlies thin tuffaceous sediments at contact with underlying lower unit (Tcrl). Thickness 22-48 m.' 1834|'Light- to dark-brownish-gray, brownish-gray-weathering, moderately to densely welded, crystal-rich, pumiceous rhyolite ash-flow tuff. Phenocrysts (2-4 mm long) (15-48): af, 50-71; q, 26-47; pf, 0-3 (generally less than 0.5); b, rare; px (preserved only in vitrophyre), 0-2. Variable amounts of pumice and vertical variation in compaction (welding zonation) within this unit, especially in upper part, suggest compound cooling. Unit shows similar alternating variations in crystal and lithic fragment content. Lithic fragments generally are small and scattered but locally are as much as 0.3 m in diameter and constitute as much as 10 percent of rock. They consist principally of flow-banded, aphyric, felsic lava and moderately crystal-rich, quartz- and sanidine-bearing ash-flow tuff that resembles tuff of Challis Creek. Pumice is lighter or darker than matrix depending on degree of welding, and lapilli commonly range from 1 to 6 cm in diameter but can be as much as 15 cm. Vitrophyric zones of this unit locally are as much as 22 m thick. Tuff forms distinct buff to pale-green, treeless or sparsely tree-covered outcrops where altered to zeolites north and west of Twin Peaks (44°36’ N., 114°28’ W.). Sanidine crystals from this unit were dated at 45.8±0.2 m.y. and 46.5±0.1 m.y. (see table 3). Thickness more than 610 m (base not exposed).' 1835|'Corral Creek cauldron segment and area north and east of Challis: See both descriptions below. Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: Reddish- to gray-brown, partly to densely welded rhyolite ash-flow tuff containing 13-45 percent phenocrysts (as much as 4 mm long): af, 38-70; q, 27-60; pf, 0-3; b, trace; px, 0-3. Comprises rocks outside Twin Peaks caldera that cannot be correlated with certainty with units within caldera. Outflow tuff of Challis Creek, where it caps high mountain peaks north and west of Twin Peaks caldera, consists of single cooling unit at least 200 m thick. Locally, as exposed on uppermost Tenmile Creek (44°29’ N., 114°39’ W.), outflow tuff of Challis Creek grades upward from crystal-rich, moderately welded tuff to very fine grained, crystal-poor ash-fall tuff. Tuff there and in nearby exposures on upper East Fork of Mayfield Creek (44°30’ N., 114°39’ W.) is interpreted to be associated with an early collapse segment of Twin Peaks caldera that is west of west margin of Twin Peaks caldera proper. This early collapse segment is obscured by a mass of intrusive rhyolite (Trf). Sanidine from this unit was dated by 40Ar-39Ar methods at 46.3±0.2 m.y, Southeastern part of quadrangle: Red, reddish-purple, yellowish-brown, or gray, densely welded, devitrified ash-flow tuff containing 5-20 percent crystals as long as 3 mm of chatoyant alkali feldspar and smoky quartz in matrix of fine shards. Locally contains abundant pumice. Also contains sparse zircon, allanite, biotite, and a few crystals of plagioclase. Near Challis, single cooling unit overlies an irregular erosion surface carved on underlying tuff of Pennal Gulch (Tp). Two, or possibly three, densely welded cooling units present in Spar Canyon-Sand Hollow area. One of these units may be younger than tuff of Red Ridge (Trr). Unit has normal magnetic polarity. Potassium-argon age of sanidine is 45.0±1.3 m.y.' 1838|'Heterogeneous deposit of cobble- to house-size blocks of volcanic rock. Blocks include highly fractured pyroclastic rocks that resemble tuffs of Camas Creek-Black Mountain (Tc), rocks that may be tuff of Ellis Creek (Te) and tuff of Eightmile Creek (Tem), intermediate porphyritic lava, and blocks of coarse-grained volcaniclastic rock. Indurated matrix consists of smaller fragments of same materials that make up the blocks. Exposed in slope west of lower Ibex Creek (44°34.5’ N., 114°43.5’ W.). Megabreccia is intruded on all sides by rhyolite (Trf) and gray porphyry (Tgp). Megabreccia is interpreted to represent caldera-wall slump debris shed from west margin of Van Horn Peak cauldron complex after eruption of tuffs of Camas Creek-Black Mountain (Tc). Thickness more than 150 m.' 1839|'Northern part of Van Horn Peak cauldron complex and Panther Creek graben, Corral Creek cauldron segment and area north and east of Challis - both descriptions below apply to these two areas: Twin Peaks caldera and southern part of Van Horn Peak cauldron complex: In Darling Creek area (44°38’ N., 114°17’ W.), unit consists of gray-brown to buff to pale-green or red bedded tuff, tuffaceous sediment, nonwelded ash-flow tuff, and pumice flows. Typical pumice-rich ash-flow tuff or pumice flows contain phenocrysts (2 mm in length) (9-14): af, 0-15; q, 0-7; pf, 62-83; b, 12-26; hb, trace. Ash-flow tuff ranges in thickness from 1 to 52 m and is rich in pumice. Noncollapsed pumice fragments in these tuff beds generally are 1-2 cm. in maximum diameter and constitute 30-70 percent of rock, so many of these units are pumice flows. Lithic fragments in these tuff beds typically are pebble-size clasts of aphyric felsic lava and intermediate porphyritic lava containing feldspar and biotite phenocrysts. Interbedded with pumice-flow tuff beds are buff to green and lavender, thin-bedded, fine- to medium-grained fluvial tuffaceous sandstone, siltstone, reworked pumice beds, and coarse conglomerate beds that contain debris-flow lenses and siltstone and range from 1 to 35 m thick. Map unit is at least 355 m thick and is unconformably overlain by tuff of Challis Creek (Tcr). Southeastern part of quadrangle: Gray, pale pink, or pale green, silicic, crystal-poor pyroclastic flows and air-fall, both subaqueous and subaerial. Chiefly massive to crudely bedded, pumice-rich, coarse tuff and pumice-lapilli tuff, but also includes beds of sorted, thin-bedded volcanic sandstone and mudstone. Commonly contains crystals as much as 2 mm long of plagioclase, sanidine, and biotite, and subordinate or very minor quartz, amphibole, allanite, zircon, and apatite. Sparse carbonized or silicified plant fragments are present in some pyroclastic flows. Locally in