Qs
Surficial deposits, undivided
Quaternary
Unconsolidated beach, eolian, delta, lagoon, landslide, glacial, and alluvial deposits composed of silt, sand, gravel, and boulder-sized material.  Primary and reworked pyroclastic material is a common component.  In many locations, this deposit includes
Wilson, F.H., Mohadjer, Solmaz, and Grey, D.M., in press, Reconnaissance geologic map of the Western Aleutian Islands, Alaska - Integrated geologic map databases of the United States:  U.S. Geological Survey Scientific Investigations Map SIM-XXXX, scales
Qa
Alluvium
Sand, gravel, and silt including reworked cinders and ash.  Includes old beach deposits in lower parts of some valleys on Attu and northern Adak Islands (Gates and others, 1971; Waythomas, 1995) as well as alluvial terrace deposits in low benches and unp
Qf
Alluvial fan deposits
Sand, gravel, and cobbles on Attu Island (Gates and others, 1971).  On northern Adak Island, may contain variable amounts of debris-flow deposits and talus, usually mantled by tephra (Waythomas, 1995)
Qc
Colluvial deposits
Boulders and boulder-sized debris in slope wash, talus, landslide, and mudflow deposits on Little Sitkin Island (Snyder, 1959).  Locally, clasts consist of kaolinized, silicified, and pyritized rock.  Includes accumulations of poorly sorted, angular rock
Qls
Landslide and mudflow deposits
Poorly sorted accumulations of gravel, sand and silt on Great Sitkin Island (Simons and Mathewson, 1955; Waythomas and others, 2003) and Kanaga Volcano (Miller and others, 2003).  Also common on the lower slopes of Mount Moffett and Mount Adagdak and fou
Qb
Beach deposits
Sand, pebbles, gravel, and boulders and some narrow stretches of bare rock shore platform.  These deposits are distinguished on Adak (Waythomas, 1995), Great Sitkin (Waythomas and others, 2003), Kanaga (Miller and others, 2003), Ogliuga and Skagul (Frase
Qd
Eolian deposits
Sand dunes, typically consisting of sand derived from beaches mapped on northern Adak, Kiska, Semisopochnoi, Little Sitkin, Ogliuga, Skagul, and Amchitka Islands (Coats, 1956a; Coats, 1959b; Coats and others, 1961; Snyder, 1959; Fraser and Barnett, 1959;
Qg
Glacial deposits, undivided
Ground, terminal, or lateral moraine deposits as well as local outwash.  Consists of subangular and rounded boulders, many faceted, in a semi-indurated tough and clayey matrix on northern Adak Island (Coats, 1956a).  On southern Adak and Kagalaska Island
Qhdd
Anthropogenic deposits
Holocene
Unconsolidated deposits concentrated around the former Adak Naval Air Station derived from filling low-lying marsh areas and reworking other deposits to construct runways and other facilities.  Equivalent to "disturbed land" map unit of Waythomas (1995)
QTs
Older silt, sand, and gravel
Quaternary or Tertiary
Boulder-rich conglomeratic deposit on the east flank of Mount Adagdak, mapped as Tertiary by Coats (1956a), is probably Quaternary in age, certainly Pliocene or younger.  The cobbles and boulders range in size from a few inches (~5 cm) to 10 feet (3m) an
QTf
Faneto Formation
early Pleistocene or late Tertiary
Pebble, cobble, and boulder conglomerate, coarse sandstone, coarse graywacke, and thin argillite lenses 1,500 feet thick on Attu Island (Gates and others, 1971).  Beds are predominantly red (from abundance of pebbles of red argillite or lava) and greenis
QTkh
Kiska Harbor Formation
Quaternary(?) and late Tertiary
Subaerial lava flows, autoclastic breccias, pyroclastic rocks, water-laid pumiceous sand and conglomerate (Coats and others, 1961) on Kiska and Little Kiska Islands.  Flows are predominant in the formation to the north and sedimentary (water-laid) beds t
QTkb
Kiska Harbor Formation, breccia member
Volcanic breccia consisting of coarse angular to sub-rounded basalt fragments in a dark-brownish-gray, fine-grained volcanic debris matrix (Coats and others, 1961).  The proportion of basalt fragments varies; locally, fragments are uncommon and the brecc
QTkf
Kiska Harbor Formation, flow member
Vitrophyric basalt lava flow, lacking pillows and therefore presumably of subaerial origin.  Flows in the Kiska Harbor Formation range in thickness from "* * * a few to several hundred feet." (1 to 100 m?) (Coats and others, 1961)
Tcp
Chitka Point Formation
Tertiary, Miocene
Subaerial hornblende- and pyroxene-andesite lava flows, breccia, tuff, and marine conglomerate on Amchitka Island (Powers and others, 1960).  Thickness exceeds 1000 feet on Amchitka Island as reported by Powers and others (1960).  Carr and others (1970)
Tch
Chuniksak Formation and similar rocks
Fine-grained, well-bedded, laminated siliceous, argillic, and limy marine sedimentary rocks on Attu Island (Gates and others, 1971).  According to Gates and others (1971), the formation consists principally of siliceous shale, argillite, calcareous argil
Tc
Chirikof Formation
Tertiary, Miocene?
Boulder and pebble conglomerate, coarse sandstone, carbonaceous shale and sandstone, and possibly a single lava flow on Attu Island southwest of Chirikof Point (Gates and others, 1971).  The boulder conglomerate contains rounded boulders, whereas fragmen
Tn
Nevidiskov Formation
Tertiary, early Miocene? or late Oligocene?
Coarse graywacke and conglomerate in a generally fining upward sequence on Attu Island (Gates and others, 1971).  Conglomerate clasts are as much as 2 feet (0.7 m) in diameter, consisting of basement rocks of Attu Island (map unit TMzu), which include po
Tgc
Gunners Cove Formation
Tertiary
Tuffaceous conglomerate and sandstone, crystal-vitric basaltic tuff, thin basalt flows, and basaltic dikes (Lewis and others, 1960) forming two-thirds of Rat Island.  Sandstone and conglomerate contain moderately to well-rounded mafic volcanic clasts, pr
Tbp
Banjo Point Formation
Tertiary, Oligocene or late Eocene
Bedded marine sandstone, conglomerate, tuffaceous shale, and lapilli tuff of basaltic composition on Amchitka Island (Powers and others, 1960).  Carr and others (1970) slightly revised the unit definition and the defined outcrop area and also indicated t
Tal
Andrew Lake Formation and similar rocks
Tertiary, late Eocene
Tuffaceous sandstone and siltstone and siliceous and cherty shale interbedded with  mafic flows or penecontemporaneous sills on Adak and Amatignak Islands.  "Graded sandstone beds as much as 0.3-m-thick are common and alternating beds of sandstone and sh
Tk
Krugoli Formation
early Tertiary or late Mesozoic?
Bedded argillite, siltstone, chert, basaltic and spilitic lava flows, tuff, tuff-agglomerate, and conglomerate about 7,400-feet-thick on Agattu Island (Gates and others, 1971).  Lithologically similar to the basement rocks of Agattu and Attu Islands (map
Tkc
Krugoli Formation, Coarse-grained and conglomeratic sedimentary rocks
Coarse graywacke and conglomerate, interbedded with pillow lava flows, pyroclastic rocks, and fine siliceous rocks.  Clasts consist of sandstone, fine-grained siliceous sedimentary rocks, basalt and tuff; no schistose or granitic clasts were observed (Ga
Tkv
Krugoli Formation, Tuff and lava; chiefly basaltic and spilitic
A single sequence of volcanic rocks about 2,400 feet above base of unit consists of 200 feet of massive green tuff and tuffaceous sandstone overlain by a 50-foot-thick porphyritic basalt flow or by porphyritic basalt pillow lava, in turn overlain by 70 f
TMzu
Basement rocks, undifferentiated
Basement rocks - undifferentiated.  On Agattu Island, consist of marine argillite, sandstone, graywacke, and conglomerate, submarine pyroclastic rocks, and columnar-jointed and pillow lava flows (mostly basaltic).  Sedimentary rocks are subdivided on Att
TMzf
Basement rocks, Fine-grained sedimentary rocks
Fine-grained, thin-bedded, sedimentary rocks including chert, siliceous siltstone, argillite, limestone, and fine tuffaceous graywacke in "* * * thin varicolored black, green, red, yellow, and gray beds" (Gates and others, 1971, p. 715), distinguished on
TMzc
Basement rocks, Coarse-grained and conglomeratic sedimentary rocks
Coarse graywacke and conglomerate, interbedded with pillow lavas, pyroclastic rocks, and fine siliceous rocks on Attu Island.  Generally drab dark-gray, grayish-purple, or grayish-green in thin beds of graywacke and much thicker conglomerate horizons.  T
Tbv
Basement rocks, Volcanic rocks
Tertiary?
Largely tuff and tuff-agglomerate of generally basaltic composition and interbedded pillow lava flows on Attu and Agattu Islands.  Pyroclastic rocks range from coarse, bomb-filled lapilli tuff to fine-grained, silt-sized tuff, agglomerate, and breccia.
Qvu
Volcanic rocks and deposits, undivided
Consists of  basalt and andesite lava flows, minor intrusive rocks, and pyroclastic rocks and deposits, including scoria,  tuff breccia, and vent agglomerate throughout the western Aleutian Islands.  These include the rocks and deposits of the older stag
Hv
Younger volcanic rocks
Largely consists of basalt flows and interbedded pyroclastic rocks associated with Moffett Volcano and Mt. Adagdak on Adak Island (Coats, 1956a), on Kiska Volcano on Kiska Island (Coats and others, 1961), and Mount Cerberus, Sugarloaf Peak, and Lakeshore
Hp
Pyroclastic rocks
Holocene or late Pleistocene?
Includes pyroclastic rocks and deposits of dacitic and andesitic composition and minor lava flows associated with Great Sitkin Volcano on Great Sitkin Island (Qat, Simons and Mathewson, 1955; Qat, Qhts, Qpf, Waythomas and others, 2003), the older composi
Ha
Ash and tephra deposits
Constructional surfaces on Segula and possibly Davidof and Khvostof (Nelson, 1959), and on northern Adak Island (Waythomas, 1995) are covered by a pyroclastic mantle.  On Segula Island, it consists of clasts ranging in size from fine sand to large blocks
Hd
Little Sitkin Dacite and Double Point Dacite
Holocene and late Pleistocene?
White to black low-silica dacite and rhyodacite flows of the Little Sitkin Dacite locally subdivided into the West Cove and Pratt Point members (Snyder, 1959) and a sequence of light- to dark-gray high-silica dacite flows containing many miarolitic inclu
Hvd
Basalt and andesite domes
In part consists of dark-gray to black, glassy, porphyritic basalt dome extruded in crater of Great Sitkin Volcano in 1945, largely removed by 1974 eruption (Unit Qld45, Waythomas and others, 2003).  Also includes a dark-gray to black, basaltic andesite
Ql
Lahar and debris avalanche deposits
Poorly sorted gravel, sand, silt, and boulders in fan-shaped deposits on Great Sitkin and Kanaga Volcanoes (Waythomas and others, 2003; Miller and others, 2003).  These deposits are present in most major valleys ranging from 1 to 5 meters in thickness.
Qvv
Vent volcanic rocks and deposits
Holocene and late Pleistocene
Dark-gray, glassy or fine-grained, compact or diktytaxitic, porphyritic feldspathic basalt associated with the central and rift vents of Segula Volcano (Nelson, 1959) as well as the tuff-breccia cone of Mount Moffett Volcano on Adak Island (Coats, 1956a)
Qep
East Point Formation
Andesite and basalt flows more than 500-feet-thick exposed at East Point on Little Sitkin Island (Snyder, 1959).  Flows tend to be massive with clinkery, vesicular flow tops.  These flows were erupted onto a deeply dissected topography and this relations
Qsp
Sitkin Point Formation and similar rocks
Water laid pyroclastic material including tuffaceous graywacke, dacite-boulder tuff-breccia, andesitic tuff, and pumiceous tuff on Little Sitkin Island (Snyder, 1959).  Locally includes areas of kaolinized, silicified, and pyritized rock.  Also includes
QTv
Older volcanic rocks, undivided
Quaternary and Tertiary
Includes the Pochnoi Volcanics of Semisopochnoi Island (Coats, 1959b) consisting of tuff-breccia, lava flows, and agglomerate, as well as largely basaltic and olivine and hypersthene basalt and andesite flows of ancestral Mt. Kanaton volcano on Kanaga Is
QTb
Basalt plugs and associated breccia
Includes basalt plugs of the Great Sitkin Volcanics (Simons and Mathewson, 1955), basalt flows and tuff-breccia from a vent older than caldera of Mt. Kanaton near west coast of Kanaga Island (Coats, 1956b).  Also includes porphyritic fine-grained, column
QTob
Olivine basalt flows
Olivine basalt flows from vent older than caldera of Mt. Kanaton near Round Head on northern Kanaga Island (Coats, 1956b; Miller and others, 2003).  A K-Ar whole-rock age determination yielded an zero age for basaltic andesite (Bingham and Stone, 1972),
QTqk
Quartz keratophyre dikes
Early Quaternary or Late Tertiary
Quartz keratophyre dikes (Gates and others, 1971) on widely separated parts of Attu Island.  Dikes consist of chalky white euhedral feldspar (albite) crystals and glassy quartz in a bluish-gray to light-gray groundmass of fine-grained albite, quartz and
QTha
Hornblende andesite and dacite
Porphyritic dikes and stocks of hornblende andesite and dacite on Attu Island (Gates and others, 1971).  A hornblende dacite porphyry from this unit that cuts the Chirikof Formation, map unit Tc, yielded a 6.1 ñ 0.4 Ma whole-rock K-Ar age (DeLong and McD
Tlp
Lavas and pyroclastic deposits, undifferentiated
Miocene or older
Basaltic or andesitic lava and pyroclastic deposits on Kavalga, Ogliuga, Ulak, and Skagul Islands (Fraser and Barnett, 1959).  Locally subdivided into predominantly lava and flow breccia, and tuff and breccia units, in some places fractured and silicifie
Tvb
Vega Bay Formation
middle Tertiary
Marine deposits of volcanic breccia, tuff, and a few pillow lava flows, all of basaltic composition, with a few interbeds of conglomerate and sandstone of similar material on Kiska Island (Coats and others, 1961).  Samples from flows, a dike, and a basal
Tvo
Finger Bay Volcanics and similar rocks
Tertiary, Oligocene or older
Herein consists of the Finger Bay Volcanics of Adak and Kagalaska Islands and most of the Andreanof Islands east of Adak Island (Coats, 1956a; 1956c; Fraser and Snyder, 1959), the Amchitka Formation of Amchitka Island (Powers and others, 1960; Carr and o
Tid
Dikes and hypabyssal intrusive rocks
Dikes and small intrusive bodies largely of basalt and andesite, but includes altered basalt in small dikes and sills on southern Adak and Kagalaska Islands (Coats, 1956a; Fraser and Snyder, 1959).  According to Fraser and Snyder (1959), dikes in the Fin
Td
Diabase and gabbro
Diabase and gabbro dikes, sills and small plutons are widespread and form two-thirds of the intrusive rocks on Attu Island as well as being very common on Agattu Island (Gates and others, 1971).  These dikes and sills intrude all Attu Island units as old
Tqgd
Quartz diorite and granodiorite
Younger granodiorite, quartz diorite, and gabbro in stocks, sills, and dikes on Adak and Kagalaska Islands (Fraser and Snyder, 1959) and quartz diorite on Amchitka Island (Powers and others, 1960).  On Amchitka Island, medium-grained quartz diorite intru
Tagr
Albite granite
Albite granite is found on Attu Island in the Steller Cove and Mikhail Point areas (Gates and others, 1971).  It occurs in irregular masses at Steller Cove and as a small stock east of Mikhail Point.  The granite is gray, pale-pink, or purplish, has wide
Tgd
Granodiorite, quartz diorite, and gabbro
Older granodiorite, quartz diorite, and gabbro in stocks, sills, and dikes on Adak Island; originally mapped as the same unit as our Tqgd by Fraser and Snyder (1959, their unit gd), K-Ar dating has shown that these plutons are of 2 separate ages (Marlow