Geologic units in Page county, Virginia

Additional scientific data in this geographic area

Martinsburg and Oranda Formations (Ordovician) at surface, covers 18 % of this area

Martinsburg Formation (Geiger and Keith, 1891). Three lithologic packages are recognized in the Martinsburg (in ascending order): black shale and limestone; sandstone and shale; and sandstone (Rader and Biggs, 1976). The lower unit consists of 200 to 250 feet of black calcareous shale, black aphanic, argillaceous limestone; and thin, light-brown K-bentonites. The bulk of the formation, more than 2800 feet, is composed of olive-green to gray, fine- to medium-grained, lithic sandstone and greenish-gray shale and siltstone. These lithologies occur in base-truncated Bouma cycles. Graded bedding, flute casts, and load casts are common. The upper unit, about 170 feet thick, is brownish-gray, medium- to coarse-grained, quartz sandstone. The lower two-thirds of this sandstone contains near-shore, marine fossils. Oranda Formation (Cooper and Cooper, 1946). Limestone and siltstone. Limestone, medium- to dark-gray, fine-grained, very argillaceous. Siltstone, black to dark-gray; both lithologies silicified where in contact with a K-bentonite. Five K-bentonites identified in the type section (Rader and Read, 1989).

Beekmantown Group (Ordovician) at surface, covers 15 % of this area

Includes the Pinesburg Station Dolomite, the Rockdale Run Formation, and the Stonehenge Limestone (northern Virginia only) or the Beekmantown Formation and Stonehenge Limestone (central and southwestern Virginia). Pinesburg Station Dolomite (Sando, 1956). Dolostone, dark- to light-gray, fine- to medium-grained, medium- to thick bedded with minor nodular white chert. It ranges from 0 to 400 feet in thickness and is equivalent to beds in the upper Beekmantown Formation. Present only in Clarke and Frederick counties and is conformable with the underlying Rockdale Run Formation and unconformable with the overlying New Market or Lincolnshire Limestones. Rockdale Run Formation (Sando, 1958). Dominantly limestone and dolomitic limestone, lesser dolostone with minor chert throughout. Limestone, light- to medium-gray, fine-grained generally, but coarse, bioclastic limestone locally, medium- to thick-bedded. Dolostone, light-gray, fine- to medium- grained, thick-bedded with "butcher block" weathering and minor nodular or bedded chert in both limestone and dolostone. Unconformably overlain by the New Market Limestone where the Pinesburg Station Dolomite is absent. It is laterally equivalent to the Beekmantown Formation and conformably overlies the Stonehenge Limestone. The formation is about 2700 feet thick. Beekmantown Formation (Clarke and Schuchert, 1899). Dominantly dolostone and chert-bearing dolostone with lesser limestone. Dolostone, light- to very-dark-gray, fine- to coarse grained, mottled light- and dark-gray, with crystalline beds locally contains nodular, dark-brown or black chert and thick, hill forming, lenticular chert beds in lower part. Limestone, very-light- to medium-gray, fine-grained, medium- to thick bedded, locally dolomitic and locally fossiliferous. The formation is present from Page and Shenandoah counties southwestward in the easternmost exposures of the Lower Ordovician rocks. It and the underlying Stonehenge Limestone, are equivalent to the Mas cot and Kingsport Dolomites of the upper part of the Knox Group. It is unconformably overlain by Middle Ordovician limestones and conformably overlies the Stonehenge Limestone. Erosion, related to the unconformity at the top of the Beekmantown Group and Knox Group, has produced erosional breccias, local topographic relief, and paleokarst topography as well as significant regional thinning of the rock units. The Beekmantown Group thins from about 3000 feet in Page County to less than 700 feet in Washington County, largely because of post-Beekmantown erosion. Stonehenge Limestone (Sando, 1956). Limestone with interbedded dolostone in north western Virginia. Limestone, dark-gray, fine-grained, laminated to massive, with black nodular chert. Dolostone, light-gray, fine-to very-coarse-grained, as thin- to medium-interbeds or as coarse- grained, massive, reefoidal bodies. Reefoidal bodies are restricted to the middle portion of the formation. The formation conformably overlies the Conococheague Formation and thins northwestward from 400 or 500 feet in the southeasternmost exposures (Page County) to a few tens of feet in the north western exposures (western Rockingham County) and is not recognizable or included in the lower Beekmantown or upper Conococheague in much of southwestern or western Virginia. It is equivalent to the lower part of the Kingsport Dolomite.

Chilhowee Group (Cambrian) at surface, covers 14 % of this area

Chilhowee Group (Keith, 1903). The Chilhowee Group includes the Antietam, Harpers, and Weverton Formations in the northeastern portion of the Blue Ridge Province and the Erwin, Hampton, and Unicoi Formations in the southwestern portion of the Blue Ridge Province. Antietam Formation (Williams and Clark, 1893). Quartzite, medium-gray to pale-yellowish-white, fine- to medium grained, locally with very minor quartz-pebble conglomerate, cross-laminated, medium- to very-thick-bedded, very resistant, forms prominent cliffs and ledges, contains a few thin interbeds of light-gray phyllite, has calcareous quartz sandstone at the top that is transitional with the overlying Tomstown Dolomite, and many beds contain Skolithos linearras. It is laterally equivalent to the Erwin Formation to the southwest. The formation interfingers with the underlying Harpers Formation and ranges in thickness from less than 500 feet in Clarke County to nearly 1000 feet in Rockingham County (Gathright and Nystrom, 1974; Gathright, 1976). Harpers Formation (Keith, 1894). Metasandstone, metasiltstone, and phyllite. Metasandstone, dark-greenish gray to brownish-gray, fine-grained, sericitic, thin- to medium-planar bedded, locally bioturbated, Skolithos-bearing litharenite; dark-gray, fine-grained, cross-laminated, thickbedded, laterally extensive bodies of quartzite; and very-dark gray, medium- to coarse-grained, thick-bedded, ferruginous, very resistant, quartzitic sandstone. These beds were extensively mined for iron ore north of Roanoke (Henika, 1981). Metasiltstone, dark-greenish-gray, thin, even bedded, sericitic, and locally bioturbated. Phyllite, medium- to light-greenish gray, bronze weathering, laminated, sericitic. The Harpers is laterally equivalent to the Hampton Formation to the southwest and they are so similar that the names have been used interchaneably in the northern Blue Ridge (Gathright, 1976; Brown and Spencer, 1981). The Harpers conformably overlies the Weverton or Unicoi Formations, thickens northeastward from about 1500 feet north of Roanoke to about 2500 feet in Clarke County. The thicker sections are dominated by phyllite and metasiltstone and the thinner sections by metasandstone and quartzite. Weverton Formation (Williams and Clark, 1893). Quartzite, metasandstone, and phyllite. Quartzite, medium- to very dark-gray, weathers light-gray, fine- to coarse-grained, well rounded quartz-pebble conglomerate beds locally, medium- to thick-bedded, cross-bedded, very resistant, with interbedded metasandstone, dark-greenish- gray, feldspathic, thick-bedded, with ferruginous cement in some beds. Phyllite, light- to dark-greenish-gray or dark-reddish-gray, laminated, sericitic, with coarse sand grains and quartz-pebble conglomerate in a few thin beds, generally in lower part. Formation ranges in thickness from more than 600 feet in Clarke County to less than 200 feet in Augusta County (Gathright and Nystrom, 1974; Gathright and others, 1977). The Weverton is lithologically very similar to strata in the upper portion of the Unicoi Formation to the south to which it may be equivalent. The Weverton appears to unconformably overlie the Catoctin and Swift Run Formations and the Blue Ridge basement complex and is present northeast of Augusta County.

Catoctin Formation - Metabasalt (Proterozoic Z-Cambrian) at surface, covers 14 % of this area

Grayish-green to dark-yellowish-green, fine-grained, schistose chlorite- and actinolite-bearing metabasalt, commonly associated with epidosite segregations. Mineralogy: chlorite + actinolite + albite + epidote + titanite ± quartz + magnetite. Relict clinopyroxene is common; biotite porphyroblasts occur locally in southeastern outcrop belts. Geophysical signature: The Catoctin as a whole has a strong positive magnetic signature. However, between Warrenton and Culpeper the lowest part of the Catoctin, which consists of low-titanium metabasalt and low-titanium metabasalt breccia, is non-magnetic, and displays a strong negative anomaly. Metabasalt (CZc) is by far the most widespread unit comprising 3000 feet or more of section (Gathright and others, 1977). Primary volcanic features are well preserved in many places. In the north west ern outcrop belt, these include vesicles and amygdules, sedimentary dikes, flow-top breccia, and columnar joints (Reed, 1955; Gathright, 1976; Bartholomew, 1977); relict pillow structures have been reported in Catoctin greenstones east of Buena Vista (Spencer and others, 1989). In the southeastern outcrop belt, amygdaloidal metabasalts are common, as are volcanoclastic rocks interbedded with basaltic fl ows (Rossman, 1991). Fragmental zones occur locally between individual lava fl ows; map-scale hyaloclastite pillow breccias occur at three strati raphic levels within the southeastern outcrop belt (CZcb, CZhb, CZlb; Espenshade, 1986; Kline and others, 1990).

Charnockite (Proterozoic Y) at surface, covers 11 % of this area

Includes dusky-green, mesocratic, coarse- to very-coarse-grained, equigranular to porphyritic, massive to vaguely foliated pyroxene-bearing granite to granodiorite; contains clinopyroxene and orthopyroxene, intermediate-composition plagioclase, potassium feldspar, and blue quartz. Reddish-brown biotite, hornblende, and poikilitic garnet are present locally; accessory minerals include apatite, magnetite-ilmenite, rutile, and zircon. Geophysical signature: charnockite pods in the southeastern Blue Ridge produce a moderate positive magnetic anomaly relative to adjacent biotite gneisses, resulting in spotty magnetic highs. This unit includes a host of plutons that are grouped on the basis of lithology, but are not necessarily consanguineous. These include Pedlar charnockite, dated at 1075 Ma (U-Pb zircon, Sinha and Bartholomew, 1984) and Roses Mill charnockite (Herz and Force, 1987), dated at 1027±101 Ma (Sm-Nd, Pettingill and others, 1984).

Conococheague Formation (Cambrian-Ordovician) at surface, covers 7 % of this area

Conococheague Formation (Stose, 1908). Dominantly limestone with significant dolostone and sandstone beds in lower part and locally in upper part. Limestone, medium- to very-dark-gray, fine-grained, thin-bedded with wavy siliceous partings that weather out in relief. Vertically repetitious primary sedimentary features such as sharpstone conglomerate, laminated bedding, and algal structures indicate cyclic sedimentation. Dolostone, medium-gray, fine- to medium-grained, laminated to massive-bedded with primary features similar to those in the limestones. Sandstone, medium-gray, brown weathering, cross-laminated, medium to thin-bedded, forms linear ridges, largely associated with dolostone beds but quartz sand common in most lithologies. Formation is present throughout the Valley of Virginia southeast of the Pulaski and North Mountain faults. It ranges in thickness from about 2200 feet in northern Virginia to 1,700 feet near Abingdon. The Conococheague is approximately equivalent to the Copper Ridge and Chepultepec Formations and conformably overlies the Elbrook Formation.

Elbrook Formation (Cambrian) at surface, covers 5 % of this area

Elbrook Formation (Stose, 1906). Dolostone and limestone with lesser shale and siltstone. Dolostone, medium-to dark-gray, fine- to medium-grained, laminated to thick-bedded. Limestone, dark-gray, fine-grained, thin- to medium-bedded, with algal structures and sharpstone conglomerate. Shale and siltstone, light- to dark-gray, dolomitic, platy weathering, with minor grayish-red or olive-green shales. Interbedded limestone and dolostone dominate the upper part of the formation; dolomitic siltstone and shale and thin- bedded argillaceous limestone dominate the lower part. The formation ranges be tween 1500 and 2900 feet in thickness in the southeasternmost exposures but is incomplete elsewhere due to faulting. The Elbrook of northern Virginia is transitional with the Nolichucky and Honaker Formations (locally the limestone facies of the Nolichucky has been differentiated from the Elbrook by Bartlett and Biggs (1980). It is also approximately equivalent to the rock sequence comprised of the Nolichucky and Maryville Formations, the Rogersville Shale, and the Rutledge Formation. Farther southwest the Conasauga Shale is the Elbrook equivalent. The Elbrook appears to be conformable and gradational with the underlying Waynesboro or Rome Formations. From Washington County to Augusta County much of the Elbrook Formation adjacent to the Pulaski and Staunton faults is a breccia of the "Max Meadows tecontic breccia type" (Cooper and Haff, 1940). These breccias are composed of crushed rock clasts that range from sand size to blocks many feet long, derived almost entirely from the lower part of the Elbrook Formation. The breccia commonly forms low lands characterized by karst features.

Massanutten Sandstone (Silurian) at surface, covers 4 % of this area

Massanutten Sandstone (Geiger and Keith, 1891).Quartzite and sandstone, very-light-gray, fine-grained, well-sorted, locally conglomeratic, thick-bedded and crossbedded, very resistant with minor shale partings in upper part. Thickness ranges from 600 to nearly 1200 feet. Present only in the Massanutten Mountains, the sandstone unconformably overlies the Martinsburg Formation and is a lateral equivalent to the Tuscarora, and Rose Hill Formations, and Keefer Sandstone (Perry, 1977).

Waynesboro Formation (Cambrian) at surface, covers 3 % of this area

Waynesboro Formation (Stose, 1906). Largely dolostone and limestone with distinctive upper and lower sequences of interbedded red mudrock, red sandstone, and dolostone. Dolostone, light- and dark-gray, mottled, fine- to coarse-grained, thick-bedded, calcareous. Limestone, medium-gray, fine-grained, thick-bedded, locally with black chert nodules. Mudrock, grayish-red, locally fissile, interbedded with dolostone and sandstone. Sandstone, dark- grayish red, fine- to medium-grained, medium- to thin-bedded, forms low ridges and hills. The Waynesboro Formation is laterally equivalent to the Rome Formation and is only present northeast of Roanoke. It is well exposed in Botetourt County (Haynes, 1991) and in Clarke County (Gathright and Nystrom, 1974) where lower shale beds of the Elbrook Formation were incorrectly included in the Waynesboro as an upper member. It is conformable with the underlying Tomstown Dolomite and is between 1100 and 1200 feet thick.

Ridgeley Sandstone, Helderberg and Cayugan Groups (Silurian-Devonian) at surface, covers 3 % of this area

Ridgeley Sandstone and Helderberg and Cayuga Groups. Ridgeley Sand stone (Swartz, 1913). Sandstone, gray, fine-to coarse-grained, locally conglomeratic, weathers yellowish- to dark-yellowish-brown, friable, calcareous, and fossiliferous. Thickness ranges up to 150 feet but is highly variable locally; occurs in western Virginia north of Craig County. Same as the Oriskany Sandstone of Butts (1933), and is continuous with the Rocky Gap Sandstone to the southwest. It grades downward into the Licking Creek Limestone and has been extensively mined for iron ore (Lesure, 1957). Helderberg Group: Licking Creek Limestone (Swartz, 1929). Upper member is light-gray, coarse-grained, arenaceous limestone; lower member is medium-to dark-gray, fine-grained, chert bearing limestone. Thickness ranges from 0 to150 feet and is present northeastward from Craig County; same as the Becraft (upper member) and New Scotland (lower member) of Butts (1940). It conformably over lies the Heal ng Springs Sandstone where the sandstone is present. It was extensively mined for iron with the Ridgeley Sandstone. Healing Springs Sandstone (Swartz, 1929). Sandstone, light-gray, medium- to coarse-grained, cross-laminated, and calcareous with local lenses of chert. Present in Alleghany, Bath, and Augusta Counties where it is generally less than 20 feet thick and conformably overlies the New Creek Lime stone. It appears to be a northeast extending tongue of Rocky Gap Sandstone. New Creek Limestone (Bowen, 1967; Coeymans Limestone of earlier reports). Limestone, light- to-medium gray with pink calcite crystals, very-coarse-grained, crinoidal, with lenses of quartz sandstone locally in the lower part. Occurs as local reefoidal buildups northeast of Alleghany County. Keyser Formation (Swartz, 1913). Limestone, sandstone, and shale. Limestone (upper), medium- to dark-gray, fine- to medium-grained, nodular, scattered, small chert nodules, biohermal, fossiliferous. Limestone (lower), medium- to dark-gray, fine- to coarse-grained, medium- to thick-bedded, very nodular, shaly, with thin (1- to 3-inch thick) crinoidal layers. Sandstone, medium-light-gray, medium-grained, calcareous, cross-bedded. Shale, medium-gray, calcareous. Upper and lower boundaries are conformable north of Clifton Forge. Thickness ranges from 250 feet in Highland County to 50 feet in Augusta County. In Highland and Bath counties the upper and lower limestones are separated by a calcareous shale unit (Big Mountain Shale Member). To the south and southeast the shale is replaced by sandstone (Clifton Forge Sandstone Member). From Craig County southwestward, the Keyser becomes all sandstone and is equivalent to the lower portion of the Rocky Gap Sandstone. Southwest of Newcastle the lower contact is disconformable. For mapping purposes the Keyser is considered to be part of the Helderberg Group. Cayuga Group: Tonoloway Limestone (Ulrich, 1911). Limestone, very-dark-gray, fine-grained, thin-bedded to laminated, with some arenaceous beds; celestite locally occurs in vugs and as veins. Thickness ranges from a few feet in southwestern Virginia to more than 500 feet in Highland County. It is conformable with the underlying Wills Creek Formation and equivalent to the Hancock Formation of Southwest Virginia. Wills Creek Formation (Uhler, 1905). Limestone, medium-to dark-gray, fine-grained, arenaceous, thin-bedded, with calcareous shale and mudstone, and thin, quartzose sandstone beds. Occurs only in western Virginia where the thickness ranges from 0 to more than 400 feet. It conformably over lies the Bloomsburg Formation and is laterally equivalent to the upper part of the Keefer Sandstone to the east and southwest of Craig County where the typical Wills Creek lithology is absent. Bloomsburg Formation (White, 1893): Sandstone, reddish-gray, fine-grained, thick-bedded with red mudstone interbeds. Thickness ranges from 35 to 400 feet between Frederick County and the northern Massanutten Mountains respectively. It grades into the Wills Creek Formation to the southwest, and is probably equivalent, in part, to the Keefer Sandstone southwest of Craig and Botetourt counties. McKenzie Formation (Stose and Swartz, 1912): Shale, medium-gray, yellowish weathering and interbedded sandstone, medium-gray, medium-grained, friable, thin-bedded and calcareous. Thickens northeastward from a few feet in Bath County to about 200 feet in Frederick County. It is probably equivalent in part to the Keefer Sandstone to the southwest and southeast and appears to be conformable with the Keefer Sandstone in northwestern Virginia.

Waynesboro Formation and Tomstown Dolomite (Cambrian) at surface, covers 3 % of this area

Waynesboro Formation (Stose, 1906). Largely dolostone and limestone with distinctive upper and lower sequences of interbedded red mudrock, red sandstone, and dolostone. Dolostone, light- and dark-gray, mottled, fine- to coarse-grained, thick-bedded, calcareous. Limestone, medium-gray, fine-grained, thick-bedded, locally with black chert nodules. Mudrock, grayish-red, locally fissile, interbedded with dolostone and sandstone. Sandstone, dark- grayish red, fine- to medium-grained, medium- to thin-bedded, forms low ridges and hills. The Waynesboro Formation is laterally equivalent to the Rome Formation and is only present northeast of Roanoke. It is well exposed in Botetourt County (Haynes, 1991) and in Clarke County (Gathright and Nystrom, 1974) where lower shale beds of the Elbrook Formation were incorrectly included in the Waynesboro as an upper member. It is conformable with the underlying Tomstown Dolomite and is between 1100 and 1200 feet thick. Tomstown Dolomite (Stose, 1906). Dolostone, limestone and minor chert. Dolostone (upper member), light- to dark gray, fine- to coarse-grained, medium- to thick-bedded, locally laminated with white chert nodules in uppermost beds; about 600 feet thick. Dolostone (high magnesian member), very-light-gray to yellowish-white, medium- to coarse-grained, very-thick-bedded, locally dark-gray, fine-grained and with white, coarse-grained, lenticular dolostone mottling; about 200 feet thick. Limestone, very-dark-gray, very-fine-grained, thin-bedded, partly dolomitic, with shaly partings; about 325 feet thick. The Tomstown is lithologically similar to, but thinner than the Shady Dolomite of southwestern Virginia and conformably overlies the Antietam Formation.

Edinburg Formation, Lincolnshire and New Market Limestones (Ordovician) at surface, covers 3 % of this area

New Market Limestones (northeast of Roanoke County). Edinburg Formation (Cooper and Cooper, 1946). Limestone and shale. Limestone, dark-gray to black, aphanic, thin-bedded with thin, black shale partings, locally contorted limestone beds, intraformational limestone breccias, and olistoliths interstratified with typical planar bedded limestone (Liberty Hall lithofacies). Limestone, medium- to light-gray, fine- to coarse-grained, nodular with very thin, black shale partings (Lantz Mills lithofacies). Limestone, light-gray, medium- to coarse-grained, thick-bedded (St Luke Limestone Member). Shale, black, graptolites common, basal unit in Augusta, eastern Rockingham, and southern Page counties. Thickness ranges from 400 feet at Strasburg to approximately 100 feet west of Lexington with a maximum of nearly 1500 feet near Harrisonburg. Lincolnshire Limestone (Cooper and Prouty, 1943). Limestone, light- to dark-gray, fine- to coarse-grained, with black chert nodules. Light-gray, coarse-grained limestone probably represents carbonate mounds ( Murat limestone). Upper contact is gradational; the lower contact is disconformable. Thickness ranges from 25 feet west of Front Royal to 280 feet northwest of Lexington (Cooper and Cooper, 1946). New Market Limestone (Cooper and Cooper, 1946). Limestone, medium- to dark-gray, aphanic to fine-grained. The upper portion of the New Market, the major quarry rock of northern Virginia, is massive micrite that weathers to fluted ledges. The lower portion is dolomitic with scattered lenticular, black, pyritic limestone, locally conglomeratic at the base. Upper contact is disconformable and the lower contact is a locally angular unconformity. The thickness ranges from 0 near Staunton to 250 feet west of Edinburg.

Marcellus Shale and Needmore Formation (Devonian) at surface, covers 0.8 % of this area

Marcellus Shale (Hall, 1839; Butts and Edmundson, 1966). Shale, dark-gray to black, more or less fissile, pyritic. Thickness estimated to be 500 feet in Frederick County and 350 to 400 feet in the Massanutten synclinorium (Rader and Biggs, 1976). Needmore Formation (Willard, 1939). Shale, dark or greenish gray, with thin beds or nodules of black, argillaceous limestone and the Tioga metabentonite beds (Dennison and Textoris, 1970), generally present with the Millboro or Marcellus Shale and is disconformable with the underlying Ridgeley Sand tone. Thickness ranges from 0 to 160 feet and is replaced to the southwest by the Huntersville Chert.

Layered Pyroxene Granulite (Proterozoic Y) at surface, covers < 0.1 % of this area

Medium- to dark greenish-gray, fine- to medium-grained, segregation-layered quartzofeldspathic granulite. Major minerals are quartz, plagioclase, potassium feldspar (includes assemblages with one alkali feldspar), orthopyroxene and clinopyroxene, and magnetite-ilmenite; garnet, hornblende, and reddish-brown biotite are widespread minor constituents. Apatite and zircon are accessory minerals. Color index ranges from 15 to 35. Quartz and feldspars are granoblastic; ferromagnesian minerals define dark layers on the order of 1 to 3 mm thick, giving the rock a characteristic pinstriped appearance. Migmatitic leucosomes locally cut segregation layering. Geophysical signature: positive magnetic anomalies relative to adjacent biotite granulite and layered gneiss (Ygb). This unit pre-dates charnockite, alkali feldspar leucogranite, and other plutonic rocks on basis of cross-cutting relations, and is generally considered pre-Grenville-age country rock that was metamorphosed under granulite-facies metamorphic conditions and intruded by plutonic rocks during the Grenville orogeny. The unit includes Lady Slipper granulite gneiss (1130 Ma, U-Pb zircon, Sinha and Bartholomew, 1984), and Nellysford and Hills Mountain granulite gneisses of Bartholomew and others (1981).

Swift Run Formation (Proterozoic Z) at surface, covers < 0.1 % of this area

Swift Run Formation (Jonas and Stose, 1939; King, 1950; Gathright, 1976). Heterogeneous assemblage includes: pebbly to cobbly quartzite and feldspathic metaconglomerate; gray, grayish-pink, or grayish-green, feldspathic quartzite and metasandstone, locally crossbedded; greenish-gray, silvery quartz-sericite-chlorite sandy schist; and, greenish-gray to grayish-red-purple chlorite-sericite tuffaceous phyllite and slate. In Loudoun County, contains pinkish-gray and yellowish-gray to light brownish-gray, fine-grained dolomitic marble (Southworth, 1991). Individual lithologies are laterally discontinuous; formation ranges up to 350 feet in total thickness, but is locally very thin or absent (Gathright, 1976). The Swift Run was originally defined on the northwest limb of the Blue Ridge anticlinorium (Stose and Stose, 1946), where the unit rests unconformably on Grenville-age rocks, and is overlain conformably by the Catoctin Formation; the upper contact is mapped at the bottom of the lowest massive metabasalt. In places Swift Run lithologies are interbedded with Catoctin metabasalts, and the contact between the two units is gradational (Gathright, 1976). Swift Run metasedimentary rocks on the northwest limb have been interpreted as deposited in alluvial fan, floodplain, and lacustrine environments (Schwab, 1986); these are interbedded with metamorphosed tuffaceous and volcanoclastic units (Gathright, 1976; Bartholomew, 1977). Although the Swift Run has been interpreted as a thin western equivalent of the Lynchburg Group in the southeastern Blue Ridge (Stose and Stose, 1946; Brown, 1970), some workers have correlated the Swift Run with discontinuous lenses of feldspathic sandstone interbedded with felsic metatuff that occur immediately below the Catoctin on the southeast limb of the anticlinorium (Nelson, 1962; Conley, 1978; 1989; Wehr, 1985). On the Geologic Map of Virginia (1993), the Swift Run is terminated along an east-west-trending normal fault just west of Leesburg, and is not mapped farther southwest on the southeast limb of the Blue Ridge anticlinorium.