Beekmantown Formation (Ob)

The Beekmantown Formation is composed of interbedded dolomite and dolomitic limestone. This unit is generally massive and provides good exposure in outcrop. The dolomite tends to weather to a beige-gray color and often exhibits elephant-skin type weathering patterns on exposed surfaces where fractures are weathered to form linear troughs on the rock surfaces. Chert is abundant in the middle part of the unit and may form along bedding plane partings or form in irregular, spongy masses that weather out and become scattered on the surface as part of the regolith overlying the rock. In Giles County, the equivalent units have been mapped as Mascot Dolomite and Kingsport Dolomite, undivided (Schultz and others, 1986). Further to the south, this unit is mapped as part of the Knox Group (Miller and Meissner, 1977). The Beekmantown is at least 1,312 feet (ft; 400 m) thick; however, the complete thickness of the formation is not exposed in Monroe County because of truncation of the unit along the St. Clair thrust fault. The Beekmantown Formation is a karst-forming unit containing caves and abundant sinkholes.

Lincolnshire and New Market Limestones, Undivided (Oln)

The New Market Limestone is primarily a fine-grained, microcrystalline, high-purity dove-gray limestone throughout most of its thickness. Previous work by Kay (1956) identified the same interval of rocks as the Lurich formation; however, this name was not formalized, and recent mapping for this study indicates the equivalency to the New Market. This limestone unit is generally massive and forms good outcrop (fig. 6).

Thin lags of conglomerate composed of carbonate clasts are sporadically found within the layers of finer-grained limestone (fig. 7). Good exposures of the New Market Limestone can be seen along Limestone Hill Road between Gap Mills and Zenith where the unit is approximately 243 ft (74 m) thick. The base of the formation has distinctive pinkish-orange beds that contain lithic fragments and conglomeratic clasts of chert. At the contact with the underlying Beekmantown Formation, the clasts in the conglomerate are mostly angular, can be as much as 10 centimeters across, and are composed of angular clasts of chert, dolostone, and some lithic fragments. This lower conglomeratic interval is as much as 115 ft (35 m) thick and may be equivalent to the Tumbez Formation (Cooper, 1945); however, the lower conglomeratic interval is discontinuous in Monroe County and, thus, is included in the New Market Limestone. The New Market is a significant karst-forming unit, containing the largest caves in the Ordovician limestone units and abundant sinkholes.

The Lincolnshire Limestone lies immediately above the New Market and is a very dark gray to black limestone containing abundant black chert, is generally thin-bedded, and contains wispy argillaceous layers. The Lincolnshire Limestone is very thin and discontinuous in Monroe County. Good outcrops of the Lincolnshire exist in the Paint Bank and Gap Mills 7.5-minute quadrangles near Gap Mills where it has been measured to be approximately 33 ft (10 m) thick. Due to its discontinuous outcrop in Monroe County, the Lincolnshire has been mapped along with the New Market Limestone as a single unit.

Big Valley Formation (Obv)

The Big Valley Formation is used in place of the previously defined Benbolt, McGlone, and McGraw Limestones that are either not recognized or too thin to be mapped as formations in Monroe County. The type section of the Big Valley Formation is in Big Valley, Virginia (Bick, 1962) and is dominated by limestones that may be thin- or thick-bedded, containing wispy layers of siliceous sediment or dolomitic mottling. The Big Valley also contains dark-gray, fine-grained, cobbly limestone, and dark-gray argillaceous limestone. The Big Valley Formation is approximately 184 ft (56 m) thick as measured near Zenith along a section parallel to Limestone Hill Road.

Moccasin Formation (Omoc)

The Moccasin Formation is a heterolithic unit composed of interbedded argillaceous limestone and calcareous shale. Shale in the Moccasin Formation is commonly pinkish-grey, as well as tan and greenish-beige in color (fig. 8). Mud cracks are commonly observed in the pinkish shale. Limestone beds in the formation are generally less than 1 ft (30 centimeter [cm]) thick. The top of the Moccasin Formation is marked by the thin 6.5 ft (less than [<] 2 m) thick Walker Mountain Sandstone Member (Butts and Edmundson, 1943; Haynes and Goggin, 1993). The total thickness of the Moccasin Formation is estimated to be 73 ft (24 m) based on sections measured along Limestone Hill Road near Zenith. Karst features are not commonly observed in the Moccasin Formation, although solutional voids have been observed along fractures and bedding partings.

Eggleston Formation (Oeg)

The Eggleston Formation consists of interbedded thin limestone and brownish-gray calcareous shale. Limestone beds in the Eggleston may be as much as 1.5 ft (45 cm) thick. Two prominent ridge-forming zones in the lower part of the Eggleston can be followed throughout most of the length of southeast Monroe County along the base of Peters Mountain. These ridge-forming zones are held up by competent beds of fossiliferous limestone. The Eggleston Formation is approximately 486 ft (148 m) thick as determined in the Paint Bank 7.5-minute quadrangle along a farm road extending off Mountain View Lane. Determination of an accurate thickness for the formation is hindered by lack of outcrop and frequent internal deformation of the strata.

Reedsville Shale (Or)

The Reedsville Shale consists of interbedded grayish-brown siltstone and fossiliferous shale, with more abundant shale in the lower third of the formation. The lower third of the Reedsville Shale weathers to a distinctive yellowish orange color and may be equivalent to a similar facies of the Martinsburg Formation that is recognized elsewhere in West Virginia. This lower part of the unit is less competent than the upper part, contains sparse brachiopod and graptolite fossils, and does not form good outcrop. Excellent exposure of the contact between the lower and upper part of the Reedsville can be observed in road cuts along Route 311, between Sweet Springs and the crest of Peters Mountain heading southeast toward Paint Bank. The upper part of the Reedsville Shale is a mixture of interbedded siltstone and calcareous shale with some zones of the shale being very fossiliferous. The upper part of the Reedsville Shale is generally chocolate brown to brownish-gray in color. The brachiopod Orthorhynchula linneyi is abundant within a conspicuous zone in the upper part. Total thickness of the Reedsville is estimated to be approximately 705 ft (215 m); however, in most areas in Monroe County the map pattern shows a much greater thickness because of the repetition of strata resulting from local tight folding.

Juniata Formation and Oswego Sandstone, Undivided (Ojo)

The Oswego Sandstone is a thin, heterolithic unit and varies from arkosic sandstone to fine-grained mudstone. The Oswego Sandstone is best exposed along Route 311 near the state line between Virginia and West Virginia. At that locality, the Oswego is approximately 33 ft (10 m) thick. The Juniata Formation is a series of interbedded sandstone and mudstone that has a characteristic brownish-red color. The Juniata is best exposed along Route 3 at Gap Mills. At this locality, the beds are overturned and approximately 295 ft (90 m) thick. These units have been described in the study area in greater detail by Diecchio (1985).

Tuscarora Sandstone (St)

The Tuscarora Sandstone is a hard, well-cemented, very light gray to white quartz sandstone that forms conspicuous ridges throughout its extent in Monroe County. The Tuscarora Sandstone holds up the ridges along Peters Mountain and Gap Mountain. A thickness of 39 ft (12 m) is well exposed along Route 3 at Gap Mills where the beds are overturned along the southeast limb of the Glen Lyn syncline and probably thinned to some extent. In the far southeast part of Monroe County, the Tuscarora is composed of two distinct zones of quartz sandstone, each 20 ft (6 m) thick, separated by approximately 33 ft (10 m) of light-tan shale. This package of rock is well exposed along Route 311 between Paint Bank and the state fish hatchery and has a total thickness of approximately 73 ft (22 m).

Keefer Sandstone and Rose Hill Formation, Undivided (Skr)

The character and thickness of the Rose Hill Formation and Keefer Sandstone across Monroe County vary and, therefore, these two formations have been lumped into a single map unit at the county scale. The Rose Hill Formation is predominantly composed of maroon-to-red hematitic sandstone, maroon-to-red mudstone, and thin intervals of fossiliferous shale. The Rose Hill Formation is approximately 220 ft (67 m) thick. Above the Rose Hill Formation lies the Keefer Sandstone, which, in the lower part, is a friable rusty yellow weathering quartz sandstone approximately 20 ft (6 m) thick, followed by an interval of approximately 20 ft (6 m) of shale, and overlain by a hard well-cemented quartz sandstone at least 30 ft (10 m) thick that forms prominent ridges in the topography. The upper part of the Keefer Sandstone weathers to a red color and often contains iron oxides and manganese oxides along fractures; it is brecciated in places, such as along Route 3 near Gap Mills.

Devonian and Silurian Limestone and Sandstone, Undifferentiated (DSls)

This map unit represents a heterolithic package of limestones and sandstones with intervals of shale that is approximately from 230 to 295 ft (70–90 m) thick. The lowermost part of this unit contains limestones of the Tonoloway Limestone and Helderberg Group. Incomplete sections of the Tonoloway and Helderberg Group limestones have been observed in the southeastern part of Monroe County near Paint Bank. Overlying the Helderberg Group is the Oriskany Sandstone, which is a friable and rusty weathered quartz sandstone that forms low topographic ridges but does not generally provide good outcrop. The Oriskany Sandstone is vuggy and is often iron cemented with occasional manganese oxide cement. The contact zone between the Oriskany Sandstone and the underlying limestones has historically been a target for iron and manganese mining. Although the limestones and sandstones described here are locally recognized in Monroe County, these units are believed to thin out to the southwest and become amalgamated into the Rocky Gap Sandstone as described by Dorobek and Read (1986).

Millboro Shale (Dm)

The Millboro Shale is a dark-gray to black, fissile, platy-weathering shale that is interspersed with siltstone. This shale often contains iron sulfide minerals and weathers to a dusky yellow color. The Millboro Shale is approximately 738 ft (225 m) thick, although true thickness is difficult to determine because of poor outcrop and internal deformation.

Brallier Formation (Db)

The Brallier Formation is a sequence of intercalated siltstone, sandstone, and shale. The rocks of this formation are generally dark gray in color (fig. 9). Beds of fine-grained graywacke can be as much as 3.3 ft (1 m) thick and form local ridges in the topography. The lower part of the Brallier generally has thinner beds than the upper part of the unit. The rocks tend to weather a pinkish orange and tan color. The Brallier Formation in Monroe County is approximately 2,640 ft (800 m) thick; however, the true thickness is difficult to estimate since most outcrop belts contain intraformational folding. The Brallier Formation is mostly exposed in the eastern part of the county and outcrops can be seen in the southeast side of the valley along Moncove Lake Road and along Hollywood-Glace Road in the core of the Glace anticline. A belt of outcrop of the Brallier Formation also occurs in the footwall of the St. Clair thrust fault that extends from the northeast to the far southwest of the county below Peters Mountain. A third zone of outcrop occurs in the southeastern extension of Monroe County in the vicinity of the town of Waiteville.

Foreknobs Formation (Df)

The Foreknobs Formation is a series of intercalated siltstones, sandstones, and shales that are generally fossiliferous. The thickness of the Foreknobs Formation is approximately 1,205 ft (365 m) as measured along the southeast limb of the Pedro syncline in the Glace 7.5-minute quadrangle. The lower half of the Foreknobs Formation is similar in character to the Brallier Formation except for the greater abundance of sandstone and greater abundance of fossils, especially brachiopods and crinoids. The base of the Foreknobs Formation is marked by distinctive quartz-rich sandstone and conglomerate; this basal sandstone can be as much as 33 ft (10 m) thick and traced throughout Monroe County. This unit may be equivalent to the Briery Gap Sandstone of Dennison (1970) and forms an excellent marker bed for mapping purposes as it is readily visible in lidar-derived topographic terrain imagery. The upper half of the Foreknobs Formation is finer-grained than the lower half and consists primarily of thin bedded siltstone and shale that is brownish-gray to tan and reddish-gray in color. The uppermost beds of the Foreknobs Formation are not well-exposed in the county; however, red mudstone, shale and residual red soils are common at this stratigraphic position. The top of the Foreknobs Formation is an erosional surface marked by a series of quartz pebble conglomerate beds within the overlying Price Formation.

Price Formation (Mp)

The Price Formation is a siliciclastic unit composed of sandstone, siltstone, and shale that is approximately 1,337 ft (405 m) thick as measured along the southeast limb of the Pedro syncline in the Glace 7.5-minute quadrangle. The rocks of the Price Formation vary in color from very light gray to very dark gray, with intervals of reddish and tan sandstones and shales. The Price Formation is fossiliferous in places and is marked by the presence of spheroidal siderite nodules at the basal surfaces of certain sandstone and siltstone beds in the middle part of the unit. The base of the Price Formation is marked by a conspicuous quartz pebble conglomerate with clasts ranging in size from millimeters to several centimeters in diameter (fig. 10). This basal conglomerate may be equivalent to the Cloyd conglomerate of Butts (1940) and Carter and Kammer (1990).

Much like the uppermost Foreknobs Formation, the lower part of the Price Formation is characterized by dark gray to reddish-brown marine and terrestrial strata that grades upward into terrestrial mudstones and sandstones. The mudstones in the upper part tend to be red and tan in color and the sandstones tend to be light gray. The top of the Price Formation is marked by a massive cross-bedded sandstone that is as much as 33 ft (10 m) thick (fig. 11).

Maccrady Shale (Mm)

The Maccrady Shale is a distinctive red mudstone and shale with thin horizons of reduced iron content that exhibit a pale green color; root casts indicate paleosols are common at these greenish horizons (fig. 12A). The Maccrady Shale has calcareous beds throughout and contains vugs of calcite and gypsum (fig. 12B). Good exposures of the Maccrady Shale were observed along County Road 11 (Hillsdale Road) between Pickaway and Hillsdale. In southern Greenbrier County off of Route 219, the top of the Maccrady Shale is marked by a very thin, fine-grained calcareous unit that may be equivalent to the Little Valley Limestone; however, this same interval has not been observed in Monroe County because of the lack of exposure. The total thickness of the Maccrady Shale is approximately 363 ft (110 m).

Hillsdale Limestone of the Greenbrier Group (Mgh)

The Hillsdale Limestone is primarily a cherty, argillaceous carbonate unit with a total thickness of approximately 330 ft (100 m). The lowermost part of the Hillsdale Limestone contains abundant black chert that often forms curled nodules. The upper two thirds of the formation do not appear to resist weathering in most of Monroe County and, instead, are found as light tan to orangish-yellow, clay-rich regolith with abundant chert nodules that are generally light in color, ranging from white to tan. Therefore, the topographic expression of the Hillsdale Limestone does not often exhibit distinct karst features even though this limestone contains some of the largest cave passages within the Greenbrier Group. A common marker fossil within the Hillsdale Limestone is the coral Lithostrotionella (which, when silicified, is the West Virginia state gem), although this fossil has not been observed by the authors in Monroe County.

Taggard and Denmar Formations of the Greenbrier Group, Undivided (Mgtd)

The Denmar Formation (Wells, 1950) is a unit composed of interbedded, light gray, oolitic limestones and thinner yellowish-tan shales. The lower part of the Denmar Formation is characterized by being predominantly massive limestone with abundant black chert nodules; the lower part was previously referred to as the Sinks Grove Limestone by Reger and Price (1926), but this unit was not formalized. The upper part of the Denmar Formation is composed of thick-bedded, fossiliferous, oolitic limestone and beige-to-tan calcareous shale; the upper part was previously referred to as the Patton Limestone by Reger and Price (1926), but the unit was not formalized. The upper part of the Denmar has at least three shale horizons that can be as much as 4 to 5 ft (1.2 to 1.5 m) thick and are generally light tan in color. Reger and Price (1926) described an interval of a “gray and highly calcareous but somewhat lenticular deposit, varying in thickness from 10 to 20 ft, when present, and separating the Patton Limestone from the Sinks Grove Limestone” (p. 483). Reger named this the Patton Shale, but noted, “It is named from association with the Patton Limestone but was not observed at the type locality of the latter” (Reger and Price, 1926, p. 483). Wells (1950) later redefined the Sinks Grove and Patton Limestones as the Denmar Formation, under protest by Reger (1950). During geologic mapping for this project, the authors could not distinguish a singular, mappable Patton shale, thus no discernible contact to define the Sinks Grove Limestone separate from the Patton Limestone was found. In fact, the authors of this study observed multiple discontinuous shales within this interval of strata, as did Reger and Price (1926, p. 482). We have, therefore, held to mapping the unit defined as Denmar Formation by Wells (1950).

The shale horizons within the Denmar are laterally discontinuous. One of them is well exposed in the back of Haynes Cave, as well as on the surface along the farm road leading to the cave entrance. This shale was mistaken for the shale in the Taggard Formation by Hirko (2012); instead, surface mapping demonstrates that this shale is within the lower part of the Denmar Formation. Between the shales are limestones that tend to be oolitic and highly fossiliferous. Good exposures of the Denmar Formation can be seen along Charles Booth Road in the eastern part of Monroe County, and along Hillsdale Road. The Denmar Formation is approximately 429 ft (130 m) thick in Monroe County.

The Taggard Formation is primarily composed of red calcareous shale and oolitic limestone, with calcareous sandstone at its base (fig. 13A). This formation had previously been described by Reger and Price (1926) as having an upper and lower shale separated by a limestone; however, detailed work in Monroe County has revealed those divisions to only be expressed locally. The thickness of this unit varies greatly and can be anywhere from 6.6 to 33 ft (2 to 10 m) thick where observed. A complete section of the Taggard Formation is exposed in a roadcut along County Route 43/1 on the west side of the Greenbrier River between Fort Spring and Frazier. Here the entire formation is approximately 30 ft (10 m) thick.

The unit is discontinuous, but where recognized the shale usually has a distinctive red color, but the shale can also be tan, or pale pinkish-green in color. Locally the Taggard Formation is expressed as a rusty red weathered shale regolith having abundant spheroidal chert nodules that are generally light in color from white to light tan, much like the upper part of the Hillsdale Limestone (fig. 13B).

At its type locality along Taggard Branch, the Taggard Formation has an interval of very light gray to white oolitic limestone that is approximately 5 ft (1.5 m) thick and is sandwiched between red and tan shales above and below. Exposure at the type locality is poor, but a complete section is visible along Route 3 approximately 3.2 mi (2 km) to the north. The base of the Taggard Formation is marked by a quartz-rich, reddish-brown calcareous sandstone that is traceable through much of the county and is at least 5 ft (1.5 m) thick but may be thicker in places (fig. 14).

Pickaway Limestone of the Greenbrier Group (Mgp)

The Pickaway Limestone is a sequence of massive- to thin-bedded limestones and thin shales approximately 445 ft (135 m) thick. A variety of textures occurs within the Pickaway Limestone; however, the most distinctive texture is of vertical wavy fractures that are oriented vertically with respect to bedding and are often filled with secondary calcite and siliceous minerals (fig. 15).

These vertical fractures appear to have formed in response to compressive forces and represent anastomosing cleavage in the rock. The zones of spaced, disjunctive cleavages are most often seen in the finer grained and laminated limestone beds that tend to cap cycles of fining-upward sequences. Four of these fining-upward sequences have been observed in the Fraser Quarry near the Greenbrier and Monroe County line where thin discontinuous shales separate each of the cycles. The shales are typically light beige or tan or pale green in color. The base of each of these cycles is characterized by coarse-grained, often fossiliferous limestones and packstones. The lower parts of these cycles tend to yield good outcrop in the formation and can easily be confused with fossiliferous grainstones of the overlying Union Limestone. The Pickaway exhibits abundant karst features at the land surface and contains caves. Solution channels are commonly visible in outcrops of the Pickaway Limestone, indicating that the formation is likely a water-bearing zone where saturated (fig. 16).

Alderson Limestone, Greenville Shale and Union Limestone of the Greenbrier Group, undivided (Mgagu)

The Union Limestone is a medium-gray, massive, fossiliferous limestone throughout much of its thickness. The Union Limestone generally forms good outcrops expressed as whalebacks in the land surface (fig. 17).

Solutional karst features and sinkholes are common within it, as well as large cave passages. The Union Limestone is approximately 347 ft (105 m) thick in Monroe County as measured along Knobs Road heading north and west out of the Town of Union. The top of the Union Limestone occasionally contains high-energy crossbeds with rounded eolian quartz grains that were reworked into the marine environment (fig. 18). Horn corals (fig. 19A) and crinoid stem fragments (fig. 19B) are commonly found within the Union Limestone; excellent exposures containing these fossils can be seen at the West Virginia University farm property north of Willow Bend.

The Greenville Shale is a distinctive black shale that is from approximately 15 to 20 ft (4.5 to 6 m) at its maximum thickness and is discontinuous in Monroe County. The Greenville Shale was apparently misidentified at its type section near Greenville; the exposure at that locality has been determined through new mapping to be the Lillydale Shale Member of the Bluefield Formation. The Greenville Shale is lumped within the map unit Mgagu because it is too thin to map separately.

The Alderson Limestone is a medium gray, argillaceous, generally thin-bedded limestone that weathers to a yellowish-beige color limestone and generally does not provide good outcrop. Thickness of the Alderson Limestone is approximately 396 ft (120 m) as measured along Knobs Road (fig. 20). In central and western Monroe County, the Alderson Limestone may contain coarser-grained intervals that are cross-bedded and more massive (fig. 21).

The Alderson limestone does not contain significant caves; however, solutional features are observed along bedding plane partings and fractures. At least one interval of shale exists within the formation, but this shale unit has not been mapped with consistency through the county because of poor outcrop; however, good exposure can be seen along Knobs Road near the crest of the Knobs.

Bluefield Formation, Lower Part (Mbfl)

The Bluefield Formation is a heterogeneous unit composed of both siliciclastic and carbonate rocks. In this report, it has been subdivided informally into a lower and upper unit because the lower part contains the most carbonate. The lower part of the Bluefield Formation includes shale and siltstone of the Lillydale Shale Member at its base, the Glenray Limestone Member atop the Lillydale Shale Member, and extends upward through the top of the Reynolds Limestone Member. The thickness of the lower part of the Bluefield Formation is variable, thickening east to west from approximately 363 ft (110 m) along Knobs Road to 726 ft (220 m) on Wolf Creek Mountain.

The Lillydale Shale Member is a thick package of very dark gray to greenish-gray shale interspersed with zones of more resistant siltstones, calcareous and micaceous shale, and thin limestone beds. In places, the Lillydale Shale Member is a dark reddish-brown color and, in those intervals, contains a greater proportion of siltstone. The Lillydale Shale Member is utilized as a resource for shale fill and road spoil with numerous small quarries found scattered throughout the county. Good exposures can be found along Knobs Road. Nodules of iron carbonate (siderite) have been observed within the Lillydale Shale Member. Thickness varies from 80 to 130 ft (24 to 40 m).

The Glenray Limestone Member of the Bluefield Formation has about 20 ft (6 m) of coarse-grained, fossiliferous, dark-gray limestone that forms a conspicuous marker bed throughout the county (fig. 22).

The Reynolds Limestone Member of the Bluefield Formation is a sequence of interbedded calcareous shale and more massive fossiliferous limestone beds. Much of the Reynolds Limestone Member is argillaceous and does not yield good outcrop; however, it is expressed as a topographic bench throughout most of the county and can be discerned using the elevation models derived from lidar (fig. 23) The Reynolds Limestone Member has solutional channels formed along vertical fractures in the thicker limestone beds that may contribute to weakening of steeper slopes, making them prone to slump failure, and potentially a water-bearing unit where saturated.

Between the Glenray Limestone Member and Reynolds Limestone Member is a sequence of generally brown to maroon-red siltstone, shale, and sporadic beds of thicker sandstone. The thickness of the lower Bluefield Formation is approximately 800 ft (244 m) in Monroe County.

Bluefield Formation, Upper Part (Mbfu)

The upper part of the Bluefield Formation is primarily fine-grained siltstone and shale grading upward into more massive sandstone beds. The upper part of the Bluefield thickens east to west from approximately 479 ft (145 m) along Knobs Road to 1,320 ft (400 m) on Wolf Creek Mountain. The first stratum overlying the shaly Reynolds Limestone Member is a fairly thick interval of red and greenish-grey shale, with minor interbedded siltstone and sandstone. Above this interval are two marker zones of sandstone that form benches in the topography.

The lowest sandstone above the red or tan shale is only from 3 to 4 ft (0.9 to 1.2 m) thick, with a maximum thickness of around 6 ft (1.8 m). It is very well sorted, with grain size that is upper very fine (0.095 to 0.125 millimeter [mm]), subrounded to rounded grains, with zones of Fe-oxide weathered out, and low angle cross-bedding that weather out to give the rock a ropy texture.

Above this thin sandstone is a sequence of olive-grey shale and siltstone, at least 30 ft thick. This is capped by a thicker sandstone (as much as 20 ft [6 m]), with similar character as the thinner one below it. Both are very fine, well-sorted sandstones, low angle crossbeds, with iron staining and concretions. The upper one of these sandstone packages may be equivalent to the Droop Sandstone Member of Reger and Price (1926); the lower sandstone package is as yet unnamed. Above this is some beige and red shale, until the sequence is capped by the coarser sandstone of the Stony Gap Sandstone Member of the Hinton Formation.

Stony Gap Sandstone Member of the Hinton Formation (Mh)

The highest stratigraphic unit exposed in Monroe County is the Hinton Formation, and what is exposed of this formation in the county is limited to exposures of the Stony Gap Sandstone Member, which is the lowest member of the formation (fig. 24). The Stony Gap Sandstone Member is a hard, light gray, very coarse to conglomeratic quartz sandstone. It varies in thickness but has been observed to be as much as 20 ft (6 m) thick. The Stony Gap is coarser and less well-sorted than the sandstones below it, possesses high-angle crossbedding, and weathers light gray. It forms distinct topographic mesas and ridges because it is more resistant to weathering.

Carbonate Rock Units

Two packages of carbonate strata of different age provide the principal bedrock aquifers within Monroe County. The first is a package of Ordovician carbonate rocks within an elongated valley at the northwest base of Peters Mountain situated structurally in the hanging wall of the St. Clair thrust fault and a second smaller exposure in the far southeast of Monroe County in the valley to the northwest of Potts Mountain (fig. 4). The units in this package and their abbreviations on the geologic map are the Beekmantown Formation (Ob), the Lincolnshire and New Market Limestones (Oln), the Big Valley Formation (Obv), the Moccasin Formation (Omoc), and the Eggleston Formation (Oeg). The overlying Reedsville Shale (Or) can be calcareous in certain zones and, thus, is mentioned here as some small springs on the northwest flank of Peters Mountain appear to have their source within the Reedsville Shale.

The second package of carbonates include the units of the Greenbrier Group. The Greenbrier Group limestones are collectively more than 1,000 ft (300 m) thick and provide productive bedrock aquifers for Monroe County. Carbonate units within the Greenbrier Group include the Hillsdale (Mgh), Denmar (Mgtd), Pickaway (Mgp), and Union and Alderson Limestones (Mgagu). Each of these units provides large water-bearing zones where solutionally enlarged fractures and bedding planes occur. The degree of solutional karst development is locally controlled by geologic structure and the prevailing hydraulic gradient. Local development of subsurface karst features is extremely difficult to predict given that karst aquifer development took place over hundreds of thousands or perhaps millions of years, so hydraulic gradients of the past do not necessarily reflect those of the present. Numerous caves in the county, their position, and their passage development provide insight into the regional karst aquifer and are summarized by White (2018a) and Dasher (2019). For example, caves often occur at the contact between the Hillsdale Limestone, which sits at the base of the Greenbrier Group, and the underlying Maccrady Shale; the contact between the formations is often targeted by well drillers seeking high yields of water for agricultural wells (fig. 5).

In addition to the principal carbonate aquifers within the belt of Ordovician carbonates in the southeast of the county and the Greenbrier Group, two thin zones of limestone within the Bluefield Formation are important carbonate aquifers. The Reynolds Limestone Member is a significant water-bearing zone and marks the top of the lower part of the Bluefield Formation. A dye-tracer test (Jones, 1997) documented water sinks at Green’s Cave that resurges at Indian Draft Cave about 0.6 miles south of Wayside and flows through the Reynolds and Glenray Limestone Members and emerges within the Alderson Limestone. The Glenray Limestone Member in the lower part of the Bluefield Formation is also another significant water-bearing zone that is between overlying and underlying low permeability shale layers.

Siliciclastic Rock Units

Three distinct packages of siliciclastic rocks are within Monroe County. The first is a package of younger Mississippian rocks in the western part of the county comprised of rocks within the lower and upper parts of the Bluefield Formation (Mbfu and Mbfl) and the Hinton Formation (Mh; fig. 4). The second is a package of older Ordovician, Silurian, and Devonian siliciclastic rocks in the eastern part of the county, east of the St. Clair thrust fault, which places older Ordovician, Silurian, and Devonian rocks over younger Mississippian rocks. This second package of siliciclastic rocks comprises Ordovician rocks of the Moccasin Formation (Omoc), Eggleston Formation (Oeg), Reedsville Formation (Or), Juniata Formation and Oswego Sandstone (Ojo), Silurian rocks of the Tuscarora Sandstone (St), Keefer Sandstone and Rose Hill Formation (Skr), Wills Creek Formation (DSls), Tonoloway Limestone (DSls), Oriskany Sandstone (DSls), and Huntersville Chert (DSls), and Devonian strata of the Millboro Shale (Dm), and lower part of the Brallier Formation (Db). The third is a package of Devonian and Mississippian rocks comprised of the upper part of the Brallier Formation (Db), Foreknobs Formation (Df), and the Price Formation (Mp; fig. 4).

These three siliciclastic rock packages are composed of various percentages of sandstone, shale, siltstone, and thin limestone. Data collected for this study was not sufficient to identify particular units within these three packages that contribute substantially to groundwater flow. However, secondary permeability resulting from bedding plane contacts, fractures associated with fold axes, joints, faults, and other secondary permeability features within the units provide most of the groundwater storage because these units are well cemented and primary porosity within the strata comprising the packages is negligible.

Ogden (1976) and Heller (1980) surmised that some wells that appear to be completed in limestone units may actually be producing water from shaley units within the Greenbrier Group or the underlying Maccrady Shale. Short circuiting of natural groundwater flow may occur where wells are drilled because open boreholes allow water from overlying aquifers to comingle with water in deeper aquifers, sometimes manifesting as flow into the ceiling of cave passages. Given that shale units are often highly fractured, it is entirely possible for shale layers to act as significantly productive aquifers. Even though a few wells were logged for this project, the lack of complex well logs, which include flow logs, stratigraphic logs, and mapping of specific bedding planes and fractures that produce water, makes mapping perched zones and their lithologic composition impossible at anything other than a local scale in the few places where well logs were collected for this study.

Alluvial Aquifers

Deposits of surficial sediments (fig. 4) are situated throughout Monroe County as the result of streams reworking detritus from weathered bedrock, slump deposits, landslides, and other erosional processes that have aggregated at the base of steeper slopes. Alluvial deposits along streams and alluvial fan deposits may be important aquifers in certain areas. For example, part of the water supply for the Red Sulphur PSD comes from a small spring box buried in colluvium high up on the slope of Peters Mountain at an elevation of 2,340 ft, which is fed by perforated infiltration lines buried within the colluvium (Baker, 2005). However, flow rates from this secondary, smaller source have not been reported, so the relative significance in terms of water quantity has not been assessed. The surficial colluvial deposits at this site cover fractured bedrock of the Reedsville Shale, and flow from the bedrock may serve to supply part of the spring flow at this and other small spring sites along the northwestern slope of Peters Mountain (see “Springs” section). In the lower valleys, alluvial terrace deposits in floodplains may also serve as shallow water sources. Such floodplain deposits are thickest in the floodplain of Potts Creek in the far southeast extension of the county near Waiteville along parts of the floodplain of Indian Creek, upstream from the confluence between Hans Creek and Indian Creek, and at the downstream end of Wolf Creek near the county boundary. No known wells tap these deposits.

Well Depths and Casing Length

Median, maximum, and minimum well depths for the 58 siliciclastic wells were 203, 700, and 60 ft, respectively, and median, maximum, and minimum well depths for the 220 carbonate wells was 340, 880, and 20 ft, respectively. The deeper median well depths for the carbonate wells is a result of well drillers often drilling for the geologic contact between the Hillsdale Limestone of the Greenbrier Group and the underlying Maccrady Shale for agricultural wells, which require a larger well yield than is necessary for a common residential homeowner well.

Wells drilled in the study area are typically completed as open holes, and screened intervals are rarely needed. Median, maximum, and minimum casing lengths for the 58 siliciclastic wells was 40, 108, and 20 ft, respectively, and median, maximum, and minimum casing lengths for the 220 carbonate wells was 40, 241, and 4 ft, respectively. The similarity in median casing lengths likely reflects a similar depth of weathered rock and regolith in the siliciclastic and carbonate bedrock within the county.

Well Yield and Transmissivity

Well yield, expressed in gal/min, and specific capacity, expressed in gal/min/ft of drawdown, are two common measures of a well’s ability to transmit water from an aquifer. Median, maximum, and minimum well yield for the 58 siliciclastic wells was 8.0, 80.0, and 1.5 gal/min, respectively, and median, maximum, and minimum well yield for the 220 carbonate wells was 10.0, 100.0, and 0.5 gal/min respectively. Median well yields of 8 to 10 gal/min are sufficient for a residential supply but are marginal for agriculture or public supply.

Transmissivity is a measure of an aquifer’s ability to transmit water and is expressed in cubic feet per foot of cross-sectional area of the aquifer per day (ft²/day). Transmissivity was computed based on borehole geophysical flow-meter data collected at 10 of the wells, 6 completed in siliciclastic rocks and 4 completed in carbonate rocks. The method used to analyze the well log data and compute transmissivity was the USGS Flow-Log Analysis of Single Holes (FLASH) software (Day-Lewis and others, 2011). FLASH uses fracture data identified in the boreholes using either an optical televiewer or acoustic televiewer and measurements of borehole flow made with an EM flowmeter to analyze data collected to analytically determine transmissivity. Results of the analysis indicate that most of the transmissivity in the wells analyzed is derived predominantly from shallow fractures in the upper 20 to 100 feet of the borehole, even though there can be multiple fractures within the borehole (table 5). Fracture data from the acoustic and optical televiewer logs are summarized in tables 6 and 7 and show most fractures within a borehole are not significant water-bearing fractures, and most water-bearing fractures are at shallower depths within the boreholes.

Limitations of the Potentiometric Surface Map

The approach to create the potentiometric surface map has one major assumption. This assumption is that the entire Monroe County aquifer system can be modeled as an unconfined aquifer, such that surface-water features are visible representations of that aquifer. This is a particularly significant assumption for the Greenbrier aquifer, where the presence of separate aquifers above and below the Taggard Formation had been postulated (Ogden, 1976). The major problems, both historically and in this work, with delineating separate aquifers is twofold. First, there is a lack of detailed stratigraphy for the separate Greenbrier units, with very few detailed geologic logs or borehole geophysical well logs for wells completed in Monroe County. Second, it is unclear which unit any given well is drilled into in the Greenbrier because of either missing information on well depth or an open borehole well where multiple units contribute water to a given well. These two issues do not preclude the use of well data in reconstructing a potentiometric-surface (water-table) map of the Greenbrier karst of the Appalachian Plateau nor of the Ordovician karst of the Valley and Ridge; it does, however, open the possibility that any anomalous points of water-level elevation might be due to this major assumption being broken.

The potentiometric-surface map (fig. 25) therefore represents a combined heads map representing water levels from multiple water-bearing zones of varying potentiometric head within the aquifer. The potentiometric-surface map essentially represents a subdued replica of topography. However, the Greenbrier aquifer is much more complex because there is a drainage divide between the northern Greenbrier Valley and Indian Creek. An additional confounding factor is that there is a possibility that some mapped springs and shallow wells are representative of shallow perched or phreatic subsurface streams or springs representing discharge from epikarst (Williams, 2008). Epikarst is defined as the near-surface karstification that has the tendency to produce perched aquifers above major water-bearing formations. For example, Mnr–0183 and Mnr–0184 are less than 400 feet apart with over 100-foot difference in hydraulic head. The former well, Mnr–0183, has no documented well depth, and, if significantly different in depth, could explain why the water levels are so different.

Well completion reports (app. 1) obtained from the Monroe County Health Department in Town of Union, W. Va., indicate that many wells are drilled as uncased or partially cased wells, often penetrating multiple water-bearing fractures, bedding planes, or subsurface water-filled cave or solution conduits. Upon completion of these type of wells, the water level will eventually equilibrate. The combined head often represents the overall head in the aquifer associated with the predominant water-bearing zone. As such, the potentiometric-surface map can be used to infer directions of groundwater flow, to map subsurface groundwater divides, and to assess the depth to groundwater within specific areas of the county. The issue related to multiple water-bearing zones is common in the fractured-rock aquifers typical in West Virginia but can be more problematic for karst aquifers, such as that within the Greenbrier. For this study, mapping of individual water-bearing zones or perched aquifers associated with specific geologic units within the Greenbrier Group was not possible, even given the large number of wells measured for this investigation.

Karst Basin Delineation

Based on previous dye-tracer tests, groundwater basins were estimated within Monroe County (Jones, 1997). This study revised those estimates (fig. 26) based on two additional dye-tracer tests and the countywide potentiometric-surface map developed for the study (fig. 25). The countywide potentiometric-surface map was especially useful for revising the previously identified karst basins, as it provided additional constraints on the groundwater divides among the respective groundwater basins.

The karst basins assessed are shown in figure 26 and match both the tracer tests conducted (for this and previous studies) and the groundwater divides as identified on the countywide potentiometric-surface map. These identified groundwater basins and the countywide water-table map provide valuable data that may be used to assess potential directions of groundwater flow in the event of an unanticipated contamination event and for managing water availability and use in the basins.

Relation of Lithology to Groundwater Flow

Borehole geophysical surveys documented 492 fractures within 4,646 feet of borehole in 14 of 15 wells logged for the study (one log did not map borehole fractures), resulting in a distribution of approximately 1 fracture per each 9.4 feet of borehole. The number of water-bearing fractures (tables 6 and 7), however, was much lower with only 28 of the 492 fractures, or 5.7 percent of all mapped fractures determined to be water bearing. This very low overall distribution of water-bearing fractures emphasizes the importance of understanding controls on fracture occurrence and distribution within the aquifer.

For water-bearing fractures, 7 were mapped in limestone units, 3 were mapped in argillaceous sandstone units, 3 were mapped in shale units, 3 were mapped in sandstone units, 1 was mapped in a siltstone unit, 6 were mapped in a sandy or silty shale unit, and 5 were mapped in argillaceous limestone units. Thus 12 of 28 (42.9 percent) water-bearing fractures were associated with limestone or argillaceous limestone units, 6 of 28 (21.4 percent) were associated with sandstone or argillaceous sandstone units, and 10 of 28 (35.7 percent) were associated with shale, sandy or silty shale, or siltstone units. The high proportion of water-bearing fractures associated with shale dominant units was unexpected as most of the water-bearing fractures were anticipated to be derived from limestone dominant units. This is likely because of the distribution of wells logged, as most wells logged (10 of 15) were in the Bluefield Formation (5 in the upper and 5 in the lower Bluefield Formations), which is dominated by sequences of sandstone and shale and only contains a few thin limestone units. However, shale units are easily fractured by tectonic stress and by stress relief and other compressive and tensional stress and, as such, are not necessarily aquitards but can yield sufficient water for residential supply, making them productive aquifers.

If we assess the fracture distribution for the karst limestone forming units, four of which are in the Mississippian Greenbrier Group, the (1) Hillsdale Limestone, (2) Taggard and Denmar Formations, (3) Pickaway Limestone, and (4) Alderson Limestone-Greenville Shale- Union Limestone, and with one in the (1) the Ordovician Beekmantown Formation, 201 total fractures were mapped, with 12 of 201 (6 percent) being identified as water-bearing fractures. This is similar to the distribution of water-bearing fractures in the more sandstone and shale dominated Bluefield Formation with 16 water-bearing fractures (5.5 percent) out of a total of 291 mapped fractures. For the karst water-bearing fractures, 5 were in argillaceous limestone units, 4 were in limestone units, and 3 were in argillaceous sandstone units. Thus, 9 of the 12 water-bearing fractures in karst limestone lithologies were in calcareous, potentially karst-forming, units.

Relation of Geologic Structure to Groundwater Flow

Mapping of the strike and dip of fractures can provide insight into potential structural controls on groundwater flow. Relatively flat lying to slightly dipping fractures (0 to 25° dip) are more commonly associated with bedding plane partings, while higher angle fractures are commonly associated with joints or faults. Figure 33A and B shows the poles to planes stereographic projection of the dip and azimuth of fractures mapped using borehole geophysical methods in Monroe County, W. Va., and figure 34 is rose diagrams showing the azimuth of the mapped fractures. Data collected for the borehole geophysical surveys are summarized in table 7 with accompanying lithologic interpretations. Original logs are available at the USGS GeoLog Locator (https://webapps.usgs.gov/GeoLogLocator/#!/).

Although the orientation (dip and dip azimuth) of fractures varies locally because of folds (anticlines and synclines) and variation in bedrock dip across the county, assessment of the fracture data can provide significant insight into the nature of fractures within the bedrock of various geologic formations within the study area. Figure 33A and B shows the fractures mapped in 15 wells logged in various geologic formations and areas within Monroe County. Most of the water-bearing fractures (identified as blue dots on fig. 33A and B) are primarily shallow features, less than 375 feet below land surface with the majority less than 200 feet in depth. The dip of the fractures varies greatly, with fractures less than 15 degrees being associated primarily with bedding planes. Higher angle fractures are likely a result of joints within the bedrock except for the fractures mapped in well Mnr–0435. Mnr–0435 is completed in the Beekmantown Formation along the St. Clair thrust fault and the fractures mapped in this well likely reflect the general dip of the thrust fault in the Ordovician limestone belt in the eastern part of the county.

Fracture data gathered from the borehole analysis and grouped by geologic formation revealed similarity in patterns of fractures (fig. 33A and B). These fracture data not only indicate the predominance of bedding plane partings on fracture distribution within the lower and upper parts of the Bluefield Formation (dip less than 20°) but also document that the Greenbrier Group has a high concentration of high angle fractures (60° to 90°) and that the dip of the St. Clair thrust fault influences the data for the well logged in the Beekmantown Formation. Rose diagrams show the dip azimuth direction (fig. 34A) and overall count and dip magnitude (fig. 34B) for the fracture data aggregated by geologic formation. There is a strong dip azimuth orthogonal to bedrock strike (fig. 34A) to the northwest and southeast for fractures mapped in the Greenbrier Group and the upper and lower parts of the Bluefield Formation, which is expected. Fracture azimuth data for the well logged along the St. Clair thrust fault in the Beekmantown Formation, however, indicate a dip direction predominantly to the south. The number of fractures and magnitude of the dip (fig. 34B) show similar trends to that of the tadpole plots, with mean dip typically less than 8° for the lower and upper parts of the Bluefield Formation and the Greenbrier Group, and corresponding higher distribution of steeper mean dip (23.8°) for the fractures within the Beekmantown Formation.

Relation Between Groundwater and Surface Water

Because the synoptic streamflow and spring survey was conducted in the midst of a moderate seasonal drought, all streamflow and spring discharge was derived from groundwater base-flow contribution to the springs or tributary streams. Measured streamflow, for those sites with measurable flow, was highly variable and ranged from 0.001 to 11.7 ft³/s (app. 2). Measured unit streamflow yield for those sites with measurable flow was also highly variable, ranging from a minimum of 0.0001 (ft³/s)/mi² to a maximum of 4.21 (ft³/s)/mi². For the watersheds containing more than 50 percent karst bedrock in their recharge areas, median streamflow yield was 0.028 (ft³/s)/mi², and for watersheds containing less than 50 percent karst bedrock, median streamflow yield also was 0.028 (ft³/s)/mi². Therefore, type of bedrock does not seem to be the controlling factor affecting variability in unit streamflow yield. Higher base-flow yields appear distributed in the streams at the base of Peters Mountain emerging from the Ordovician age limestone units (figs. 5 and 35) and lower base-flow yields appear for streams emerging from areas dominated by the Upper and Lower Bluefield Formations. A base-flow synoptic-survey conducted during more normal or high streamflows and groundwater level conditions would result in higher (ft³/s)/mi² streamflow yields. Regardless, the uniform 0.028 (ft³/s)/mi² median streamflow yield provides a lower statistic for assessing water availability for similar drought conditions.

Disappearing streams in Monroe County also complicate the understanding of streamflow distribution. Near Greenville, Hans Creek completely sinks into the Alderson Limestone and emerges approximately 600 meters downstream. Also, in the Greenville area, Laurel Creek completely sinks twice and reemerges in Indian Creek. Such disappearing streams are not common in Monroe County, so overall streamflow yield can be estimated effectively. Since the watersheds and basins for the groundwater basin map (fig. 26) and the base-flow measurement map (fig. 35) do not coincide, interbasin transfer of groundwater is occurring within the karst, as would be expected; therefore, the groundwater basin map is more reliable for assessing groundwater flow than the base-flow delineated surface-water subwatersheds.

Peters Mountain Alluvial Aquifers

Alluvial aquifers below the crest of Peters Mountain are derived from prior colluvial deposits, typically large angular blocks of sandstone, alluvial fan, and other deposits from the resistant Oswego and Tuscarora Sandstones, and, to a lesser extent, the Keefer Sandstone that lies above the Moccasin and Eggleston Formations and Reedsville Shale, which acts as confining units below these deposits. These alluvial deposits are not everywhere along Peters Mountain but are mapped on the county surficial geologic map prepared for this study (fig. 4). The bedrock in this hard resistant sandstone dips to the east-southeast and is partially confined by Devonian shale to the east. Recharge from precipitation falling on Peters Mountain primarily discharges as springs in the alluvial aquifers on the west side of Peters Mountain. These springs are a principal source of water for public supply in Monroe County, with the Red Sulphur PSD, the Town of Union, and the Gap Mills PSD all deriving water for public supply from these prolific alluvial aquifers (Richards, 2006; U.S. Geological Survey, 2019; George Dasher, West Virginia Speleological Survey, written commun., 2018; Nick Schaer, West Virginia Department of Environmental Protection, written commun., 2018). Data for springs (fig. 5) clearly show the alluvial deposits correlate with the presence of many of the alluvial features along Peters Mountain.

Greenbrier Aquifers

The principal aquifer within the Greenbrier Group is controlled by the geologic contact between the base of the Greenbrier Group, at the geologic contact between the Hillsdale Limestone and the underlying Maccrady Shale. This same geologic contact is also a major control on cave development within the Greenbrier aquifer, as large cave systems, such as Scott Hollow Cave, Destitute Cave, and Hurricane Ridge Cave, form at this interval. This geologic contact is also targeted by local well drillers especially for agricultural wells which require a sufficient yield to meet needs for cattle and dairy operations. Well completion reports for Monroe County commonly reference this geologic contact as the final depth of these often deep agricultural wells, which can be more than 800 feet.

In addition to this deep aquifer, evidence from water-level data collected for this study also support the presence of shallow perched aquifers in intervals of limestone in the Denmar Formation and above the Taggard Formation within the Pickaway Limestone. Ogden’s doctoral dissertation (1976) pointed to the presence of perched aquifers within what he called the Patton-Taggard aquifer and the Union-Greenville aquifer. While the geologic maps used for Ogden’s dissertation and the one produced for this study differ significantly, both allude to a perched aquifer above the Taggard Formation. Ogden also points towards a perched aquifer, in what he termed the Union-Greenville aquifer, which is mapped as the Union Limestone, Greenville Shale and Alderson Limestone in this report. Groundwater-level data collected for this study indicate a possible perched aquifer in the Pickaway Limestone. Regardless, neither data available for the Ogden study nor collected for this study conclusively identify the specific extent of perched aquifers or the confining or semi-confining units controlling the perched aquifers. The primary reason for this is that wells are only drilled to sufficient depth to provide the required well yield, and often penetrate multiple geologic formations or aquifers. The possibility of comingling water from shallow perched aquifers with deeper confined aquifers is a common process, as wells are often completed as open boreholes across multiple aquifers, which makes identification of the existence and extent of perched aquifers difficult. As a result, the potentiometric-surface map (fig. 25) prepared for this study is based on composite heads across multiple geologic formations and potentially multiple aquifers.

Ordovician Age Karst Valley Aquifers

In the long linear valley that lies at the base of Peters Mountain and extends from Peterstown in the south to Sweet Springs in the north, numerous large springs emanate from the Ordovician Beekmantown Formation and the New Market and Lincolnshire Limestones. These limestones are likely fed by water from two sources, (1) recharge from precipitation falling directly on these karst aquifers, and (2) recharge from sinking streams originating from the Peters Mountain alluvial aquifers which is then captured by sinkholes predominantly within the New Market and Lincolnshire Limestones and Big Valley Formation, and, to a lesser extent, in the Moccasin Formation. Most of the groundwater then discharges as springs and base-flow near the center of the valley in the Ordovician Beekmantown Formation and the New Market and Lincolnshire Limestones.

Mississippian Age Siliciclastic Fractured-Rock Aquifers

In the western part of Monroe County, the aquifers are predominantly siliciclastic fractured-rock aquifers of Mississippian age, not only within the Bluefield Formation but also within the Hinton Formation. While the Bluefield Formation is predominantly sandstone, siltstone, and shale, thin limestone or calcareous shale strata are also within the formation. The Bluefield Formation has been mapped stratigraphically as an upper part and a lower part, with the Reynolds Limestone Member forming the base of the upper part of the Bluefield Formation. The Reynolds Limestone Member has been identified as karst forming with a few small sinkholes mapped along its outcrop. This unit has also been associated with small-scale landslides in underlying strata triggered by groundwater discharge from solution conduits within the Reynolds that causes underlying soils and rock strata to become saturated with water, locally elevating pore pressures in overlying sediments, thus providing conditions that can cause localized landslides. By and large however, the Bluefield and Hinton Formations form siliciclastic fractured-rock aquifers that are controlled by stress-relief processes (Wyrick and Borchers, 1981). Stress-relief fractures form in valley settings because of the isostatic rebound of overburden strata, which has been eroded resulting in upward arching of bedrock in valley bottoms and near vertical stress-relief fractures in hillsides. This associated stress-relief fracturing results in typically increased permeability in valley settings.

Ordovician, Silurian, Devonian, and Mississippian Age Siliciclastic Fractured-Rock Aquifers

Another predominantly siliciclastic fractured-rock aquifer is in the eastern and north-central parts of Monroe County and comprises predominantly siliciclastic rocks of Ordovician, Silurian, Devonian, and Mississippian age. The eastern part of this aquifer is east of Peters Mountain and consists predominantly of sandstones and shales of the Ordovician age Reedsville Shale, Oswego Sandstone, and Juniata Formation, Silurian Tuscarora Sandstone, Rose Hill Formation, and Keefer Sandstone, the Devonian Millboro Shale, Brallier Formation, and undifferentiated sandstone and thin limestones. Ordovician limestones of the Big Valley, Moccasin, and Eggleston Formations also crop out in this area; however, this aquifer has relatively less karst development because few sinkholes were mapped in this part of Monroe County.

The north-central part of the non-karstic aquifer consists predominantly of shale and sandstone of the Devonian Millboro Shale, Brallier Formation, and Foreknobs Formation, and the Mississippian Price Formation, and Maccrady Shale. Both the north-central and eastern Ordovician, Silurian, and Devonian aquifers are therefore similar to the Mississippian siliciclastic rock aquifers and are dominated by stress-relief fractures along hillsides and in valley bottoms.