Geologic units in Loudoun county, Virginia

Additional scientific data in this geographic area

Catoctin Formation - Metabasalt (Proterozoic Z-Cambrian) at surface, covers 15 % 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).

Marshall Metagranite (Proterozoic Y) at surface, covers 12 % of this area

Medium-to dark-gray, fine- to medium-grained, mostly equigranular, but rarely inequigranular granite. Principal minerals are bluish-gray quartz, oligoclase, microcline, and biotite, with lesser amounts of muscovite, opaque minerals, epidote, chlorite, and rare garnet. Gneissic layering, commonly absent, is well developed locally. This unit commonly occurs as dikes intruding porphyroblastic granite gneiss (Ybp), and contains xenoliths of Ybp. U-Pb zircon data indicate crystallization ages of 1110±4 Ma and 1112±3 Ma (Aleinikoff and others, 1993). Single crystals of monazite give an age of 1051±3 Ma, interpreted to be a metamorphic age related to intrusion of adjacent granites that yield ages ranging from 1055 to 1070 Ma.

Newark Supergroup; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 11 % of this area

Sandstone, very fine- to coarse-grained, reddish-brown to gray, micaceous, minor conglomerate beds. Siltstone, reddish-brown to gray, micaceous. Shale, reddish-brown, greenish-gray, gray, yellowish-brown, laminated, fossiliferous. Upward-fining sequences, discontinuous vertically and horizontally.

Diabase (Jurassic) at surface, covers 9 % of this area

Fine- to coarsely-crystalline, subaphanitic or porphyritic with aphanitic margins; dark-gray mosaic of plagioclase laths and clinopyroxene, with some masses characterized by olivine or bronzite, others granophyric. Also occurs as dikes and sills in the Valley and Ridge, Piedmont, and Blue Ridge physiographic provinces.

Garnetiferous Leucocratic Metagranite (Proterozoic Y) at surface, covers 8 % of this area

Leucocratic, medium- to fine-grained, equigranular to granoblastic monzogranite contains very-light-gray to light-gray feldspar, medium-gray quartz as much as 0.5 cm in diameter, and dusky-red euhedral to anhedral almandine garnet as much as 1 cm in diameter. Mineralogy includes quartz, microperthite, microcline, myrmekite, plagioclase, symplectitc biotite and minor chlorite, ilmenite, zircon, titanite, epidote, leucoxene, and clinozoisite. Modal composition ranges from 28 to 29 percent quartz, 35 to 40 percent potassium feldspar, 28 to 32 percent plagioclase feldspar, 2 to 5 percent almandine garnet. U-Pb zircon data suggest a crystallization age of approximately 1070 Ma (Aleinikoff and others, 1993). In northern Virginia there are numerous localities where dikes of this unit cut porphyroblastic granite gneiss (Ybp).

Newark Supergroup; Triassic shale and siltstone (Triassic) at surface, covers 6 % of this area

Shale, light-greenish gray, light- to dark-gray, carbonaceous, and reddish-brown in cyclic sequences, laminated, silty to sandy, fossiliferous. Siltstone, typically reddish-brown to gray, sandy, micaceous, with minor fine-grained sandstone beds.

Leucocratic Metagranite (Proterozoic Y) at surface, covers 5 % of this area

Leucocratic metagranite. White to light-olive-gray, to pink, fine- to medium-grained, massive monzogranite composed of 22 to 38 percent quartz (white, clear, or blue), 33 to 34 percent orthoclase, microcline, and rod and bleb microperthite, and 38 to 44 percent oligoclase and albite, and minor biotite. Locally, potassium feldspar porphyroblasts are 1 to 2 cm in diameter. In Loudoun County this unit becomes coarser-grained and richer in biotite from west to east across the outcrop belt. U-Pb zircon data from two localities in Loudoun County indicate crystallization ages of 1058±3 Ma and 1060±2 Ma (Aleinikoff and others, 1993).

Chilhowee Group (Cambrian) at surface, covers 4 % 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.

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers 4 % of this area

Fine- to coarse-grained, reddish-brown to gray, primary bedding features such as cross-beds, channel lags, and ripple marks , minor conglomerate, siltstone, and shale beds.

Layered Leucocratic Granite Gneiss (Proterozoic Y) at surface, covers 4 % of this area

Pale-red, pinkish- gray to light-gray leucocratic syenogranite with medium light- gray to greenish-gray melanocratic layers (0.25 to 1 cm thick) that are commonly migmatitic. Mineralogy consists of 26 to 39 percent quartz, 49 to 51 percent rod and bleb perthite, microcline, and orthoclase, 12 to 23 percent oligoclase, and minor garnet and biotite. Layering consists of 0.5- to 2-cm thick segregations of alkali feldspar, plagioclase, and quartz. Garnet and biotite are commonly restricted to melanocratic layers containing plagioclase and quartz. Isotopic data from multigrain fractions and single zircons are scattered, with Pb-Pb ages ranging from 1092 to 1139 Ma (Aleinikoff and others, 1993). Field relations are complex and suggest that the protolith for this unit may have been a composite of Ybp and younger granite(s) that was highly tectonized and homogenized during the Grenville orogeny.

Newark Supergroup; Conglomerate, greenstone clasts (Upper Triassic) at surface, covers 4 % of this area

Rounded to subrounded pebbles, cobbles, and boulders of predominantly metavolcanic Catoctin greenstone in a matrix of fine- to coarse-grained, silicified, ferruginous-cemented, greenish-gray to dark-green, clayey sandstone.

Hornblende Gneiss (Proterozoic Y) at surface, covers 3 % of this area

Gray-weathering, medium- to fine grained, massive to strongly-foliated hornblende-quartz-microcline- plagioclase gneiss with rare biotite and orthopyroxene. This lithology is very similar to layered pyroxene granulite (Ypg), and is considered equivalent.

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers 3 % of this area

Rounded to subangular pebbles, cobbles, and boulders of mixed lithologies including quartz, phyllite, quartzite, gneiss, schist, greenstone, and marble in a matrix of medium- to very-coarse-grained, reddish-brown to gray, locally arkosic, sandstone.

Porphyroblastic Granite Gneiss (Proterozoic Y) at surface, covers 2 % of this area

Dark-yellowish brown to moderate-yellowish-brown, medium-grained, granoblastic to megacrystic, mafic-rich monzogranite composed of 27 to 38 percent quartz, 28 to 39 percent orthoclase, rod and bleb perthite, microcline, and myrmekite, and 33 to 40 percent oligoclase and andesine. Porphyroblasts of potassium feldspar range from 1 to 10 cm while plagioclase and quartz are generally 2 cm or less in maximum dimension. Mafic minerals, including almandine, biotite, chlorite, hornblende, and opaque minerals, are, in places, concentrated in layers. Almandine (up to 1 cm in diameter) makes up as much as 3 percent of the mode, while hornblende, commonly 0.5 to 0.75 cm in length, constitutes less than 1 percent. The rock breaks along cleavage surfaces that are commonly rich in chlorite, giving the whole rock a light green color. A U-Pb zircon age from this rock is 1144±2 Ma; two different populations of monazite give ages of 1106±1 Ma and 1063±1 Ma, respectively (Aleinikoff and others, 1993). This unit is the oldest dated granitic rock in the northern Virginia Blue Ridge, and is very commonly intruded by dikes of the Marshall Metagranite (Ym) and garnetiferous leucocratic metagranite (Ygt), and less commonly by leucocratic metagranite (Yg). These field relations suggest that the monazite ages are not cooling ages but represent the times of metamorphic growth during subsequent intrusive events.

Swift Run Formation (Proterozoic Z) at surface, covers 2 % 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.

Marshall Metagranite (Proterozoic Y) at surface, covers 2 % of this area

Medium-gray to brownish-gray, medium- to coarse-grained monzogranite composed of 30 percent quartz (clear or blue), 28 percent rod and bleb perthite, microcline, and orthoclase, and 42 percent saussuritized oligoclase. Sheared rock commonly has as much as 20 percent biotite. Porphyroblastic augen, commonly 1 to 2 cm in length, consist of aggregates of potassium feldspar, plagioclase, and quartz. U-Pb zircon data indicate a crystallization age of 1127±7 Ma (Aleinikoff and others, 1993).

Newark Supergroup; Jurassic Interbedded Sandstone and Siltstone (Lower Jurassic) at surface, covers 1 % of this area

Interbedded fine- to coarse grained, pebbly, reddish-brown, and arkosic sandstone and reddish-brown siltstone; rhythmically interbedded with siltstone and shale unit (sh). Occurs only in the Culpeper basin.

Biotite Granite Gneiss (Proterozoic Y) at surface, covers 1 % of this area

Pink to gray, medium-grained, well-foliated or lineated biotite-plagioclase-quartz-microcline gneiss.

Newark Supergroup; Jurassic Basalt (Lower Jurassic) at surface, covers 1 % of this area

Fine- to medium-crystalline, equigranular, porphyritic, vesicular, or amygdaloidal; medium- to dark-gray subophitic intergrowths of plagioclase laths and clinopyroxene with amygdules of calcite, zeolites, and prehnite. Occurs only in the Culpeper basin as three principle basalt flows separated by sedimentary rocks.

Garnet-Graphite Paragneiss (Proterozoic Y) at surface, covers 0.9 % of this area

Light-brown-weathering, medium- to fine-grained graphite-biotite-garnet-plagioclase quartz paragneiss; includes quartz-chlorite-magnetite schist and carbonaceous phyllonite; graphite makes up 10 percent of the rock in places and garnet, up to 25 percent locally. The protolith of this unit is intepreted to be metasedimentary country rock that Grenville-age plutonic rocks intruded. The rock is similar in appearance to parts of the border gneiss (Ybr).

Quartzite and Quartz-Sericite Tectonite (Proterozoic Y) at surface, covers 0.5 % of this area

Light-gray to white, fine- to medium-grained, massive; contains rounded zircons, thin lenses of graphite, and pods of paragneiss (Yp); unit is considered part of a metasedimentary suite.

Metanorite and Metadiorite (Proterozoic Y) at surface, covers 0.5 % of this area

Gray-weathering, medium- to coarse-grained, massive to weakly foliated hornblende-orthopyroxene-plagioclase metanorite and medium- to fine-grained biotite-hornblende-plagioclase metadiorite. Occursas lenses and thin belts, commonly in proximity to garnet graphite paragneiss (Yp).

Charnockite (Proterozoic Y) at surface, covers 0.4 % 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).

Catoctin Formation - Metarhyolite (Proterozoic Z-Cambrian) at surface, covers 0.4 % of this area

Includes light-gray to medium-light gray, aphanitic to fine-grained metarhyolite containing grayish-yellow potassium feldspar phenocrysts; and, medium-gray to medium-dark-gray tuffaceous metarhyolite containing potassium feldspar phenocrysts and quartz-filled amygdules. Metarhyolite occurs as dikes up to 50 m thick, cutting Grenville basement and the Swift Run Formation (Southworth, 1991); also as cream-colored rhyolitic metatuff which is interbedded with metabasalt (CZc) stratigraphically near the base of that unit in Loudoun and northwestern Fauquier Counties (Nickelsen, 1956; Gathright and Nystrom, 1974). Metarhyolite at two localities in Loudoun County has been dated at 564 ± 9 Ma and 572 ± 5 Ma (U-Pb zircon; Aleinikoff and others, 1991).

Fauquier Formation - Arkosic metasandstone (Proterozoic Z) at surface, covers 0.3 % of this area

Dark-gray, medium- to coarse-grained metasandstone contains quartz, plagioclase, perthitic potassium feldspar, and sericite, with minor biotite and epidote; thin beds of pebble conglomerate occur with coarse-grained metasandstone; commonly cross-bedded. Unit comprises the lowest part of the Fauquier; thickness is extremely variable. In the vicinity of Castleton, fine-grained volcanogenic rocks geochemically indistinguishable from nearby Battle Mountain Felsite (Zrbf) are interbedded with the basal Fauquier (Hutson, 1990).

Fauquier Formation - Meta-arkose and metasiltstone (Proterozoic Z) at surface, covers 0.2 % of this area

Alternating beds of dark-gray, very-fine-grained meta-arkose and metasiltstone; composed dominantly of angular quartz grains, with lesser plagioclase and potassium feldspar, and minor biotite. Crossbedding and graded bedding are present; thickness ranges from 300 to 500 meters.

Robertson River Igneous Suite - Cobbler Mountain alkali feldspar quartz syenite (Proterozoic Z) at surface, covers 0.2 % of this area

Light- to dark-gray, medium- to coarse-grained, porphyritic (mesoperthite phenocrysts) to seriate-equigranular alkali feldspar-quartz syenite composed of microcline mesoperthite, quartz, and plagioclase, with hastingsitic amphibole, biotite, stilpnomelane, zircon, allanite, fluorite, and rare aegirine-augite. Euhedral to subhedral feldspar phe nocrysts are diagnostic. Syenite locally displays miarolitic cavities contain ing quartz. The rock has been dated at 722± 3 Ma (U-Pb zircon; Tollo and Aleinikoff, in press).

Fauquier Formation - Metasiltstone and phyllite (Proterozoic Z) at surface, covers 0.2 % of this area

Medium- to dark-gray (fresh), very-pale-orange (weathered), very-fine grained, laminated metasiltstone, composed of alternating silty and micaceous layers on the order of a millimeter to several millimeters thick, and phyllite without discernable layers; major minerals are silt-size quartz and sericite; chlorite, biotite, and magnetite occur locally. Thiesmeyer (1939) described these rocks as "varved slates" interpreted as lacustrine deposits. Espenshade (1986) called this unit metarhythmite. The unit is on strike with, and in part equivalent to the Monumental Mills Formation of Wehr (1985), interpreted as de pos it ed in a delta front-slope environment.

Marble (Proterozoic Z) at surface, covers 0.2 % of this area

Includes white and light-gray to grayish-blue, fine-grained dolomitic marble and siliceous marble, dolomitic meta-arkose, dolomitic quartz-muscovite schist, and calcitic marble; may contain quartz, feldspar, muscovite, phlogopite, and tremolite. Marble is poorly exposed in discontinuous lenses either just below the top of the Fauquier Formation laminated metasiltstone. (Zfl ) or just above the base of the Catoctin (CZc); a lense of dolomitic marble occurs within the Swift Run Formation in Loudoun County. Along the Hazel River, a marble clast conglomerate with a biotite-rich feldspathic matrix occurs just below the base of the Catoctin Formation.

Weverton-Loudoun Formation (Cambrian) at surface, covers 0.1 % of this area

(Chilhowee Group) - tough quartzitic sandstones, conglomerates, and shale.

Metatrondhjemite (Proterozoic Y) at surface, covers < 0.1 % of this area

White, medium- to fine-grained, weakly to moderately well-foliated biotite-quartz-plagioclase gneiss; potassium feldspar is rare or absent.

Fauquier Formation - Metaconglomerate (Proterozoic Z) at surface, covers < 0.1 % of this area

Pebbles, cobbles, and occasional boulders of quartz, several varieties of granite, and feldspar, in a meta-arkosic matrix. Discontinuous lenses occur at or near the base of the Fauquier. Meta-arkose, metasiltstone, and metaconglomerate of the Fauquier (Zfs, Zfa, Zfc) are interpreted as non-marine, fluvial sediments, deposited unconformably on Grenville-age basement (Espenshade, 1986). These units have been mapped on a lithologic basis in metasedimentary outliers west of the principal Fauquier strike-belt, and include some rocks previously mapped as Mechums River Formation (Gooch, 1958). Stratigraphic and facies relations between Fauquier lithologic units and laterally equivalent Monumental Mills and other Lynchburg Group units are discussed by Wehr (1985), Wehr and Glover (1985), Conley (1989), and Kasselas (1993).

Tomstown Dolomite (Cambrian) at surface, covers < 0.1 % of this area

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 south western Virginia and conformably overlies the Antietam Formation.

Catoctin Formation - Metasedimentary rocks (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Quartzite, feldspathic metasandstone, metasiltstone, and phyllite; occurs in discontinuous beds that are generally less than 50 feet thick, interbedded with Catoctin metabasalt (CZc).

Catoctin Formation - Metabasalt breccia (high titanium) (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Metabasalt breccia (high-titanium) (Espenshade, 1986). Dark-green, amygdular ellipsoids ranging from about 5 to 40 cm in length in a matrix of dark-green, dense angular fragments about 0.5 to 5 cm across; epidote is commonly abundant in the matrix; ellipsoidal amygdules contain quartz and epidote. Titanite constitutes several percent of the mode. This unit, where present, may be as thick as 2800 feet, and occurs stratigraphically above the low-titanium breccia (CZlb).

Mather Gorge Formation - Schist (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Schist, greenish-gray to gray, reddish-brown-weathering, fine- to coarse-grained, lustrous, quartz-rich; and much lesser mica gneiss; contains interbedded metagraywacke and some calc-silicate rock; also contains abundant mafic and ultramafic rock debris. Typical mineral assemblages from west to east and from low to high metamorphic grade are: (1) quartz + muscovite + chlorite + plagioclase + epidote + magnetite-hematite; (2) quartz + muscovite + biotite + garnet + staurolite + plagioclase + magnetite ± andalusite; (3) quartz + muscovite + garnet + kyanite + plagioclase + staurolite + magnetite; and (4) quartz + biotite + plagioclase + sillimanite ± microcline + magnetite. Higher-grade schists are migmatitic, and in many places show effects of a retrograde metamorphic over print.

Frederick Limestone (Cambrian) at surface, covers < 0.1 % of this area

Medium-gray to medium-bluish-gray laminated, thinly-bedded limestone.

Metaperidotite, Metapyroxenite, and Hornblende Metagabbro (Proterozoic Y) at surface, covers < 0.1 % of this area

Metaperidotite, greenish-black, dark-yellowish brown weathering, medium-grained, massive; consists of serpentine, amphibole, dark chlorite, and magnetite; serpentine replaces subhedral olivine; non-pleochroic amphibole occurs as single crystals, presumably pseudomorphic after subhedral to anhedral pyroxene; some amphibole occurs as large poikilitic crystals. Hornblende metagabbro, greenish-black, medium grained, speckled-white-weathering, massive, with a weak foliation and well-preserved primary igneous fabric; subhedral plagioclase is replaced by clinozoisite and albite; anhedral pyroxene is replaced by fibrous tremolite; brown hornblende occurs as subhedral single crystals. Metapyroxenite, dark-greenish-gray to greenish-black, light-greenish-gray weathering, medium-grained, weakly-foliated actinolite-rich rock contains lesser chlorite; amphibole is pseudomorphic after pyroxene.

Chilhowee Group; Weverton Formation (Late Precambrian - Cambrian) at surface, covers < 0.1 % of this area

Interbedded white to dark gray, thin-bedded, micaceous, ferruginous, and sericitic quartzites, phyllites, and white, thick-bedded, ledge-making quartzites; some gray to brown ferruginous quartz conglomerate and purple-banded phyllite; thickness approximately 100 feet in south, increases to 425 feet in north.

Catoctin Formation (Cambrian/Precambrian) at surface, covers < 0.1 % of this area

Predominantly a greenstone. Basic lava flows, schist and gneiss, containing chlorite, plagioclase, amphibolite, and epidote. Includes minor quantities of arkose and thin clastics. Western subsurface extension presently unknown.

Porphyoblastic Biotite-Plagioclase Augen Gneiss (Proterozoic Y) at surface, covers < 0.1 % of this area

Mesocratic, medium- to coarse-grained, biotite-rich quartzofeldspathic gneiss con tains prominent subhedral to euhedral monocrystalline feldspar augen. The ratio plagioclase: potassium feldspar may be as high as 10:1; color index ranges from 30 to 50. Apatite, epidote, muscovite, ilmenite, and titanite are ubiquitous accessories. Plagioclase contains abundant prismatic epidote and white mica; ilmenite is rimmed with masses of anhedral titanite; subhedral hornblende and subhedral to euhedral almandine-grossular garnet occur locally. In the vicinity of adjacent charnockite, anhedral actinolitic amphibole pseudomorphs after pyroxene or rims thoroughly uralitized relict pyroxene. Rock fabric is gradational from granofels to mylonite gneiss. Geophysical signature: negative magnetic signature relative to adjacent charnockite. In northern Virginia, this unit strongly resembles prophyroblastic granite gneiss (Ybp); however, the augen in Ybp are more commonly polycrystalline aggregates rather than single-crystal porphyroblasts. This unit is widespread in the central and southeastern Blue Ridge, encompassing a number of lithologically similar metaplutonic entities: the "biotitic facies"of the Roses Mill and Turkey Mountain ferrodiorites of Herz and Force (1987), the Archer Mountain quartz monzonite of Bartholomew and others (1981), biotite granofels and augen gneiss of Evans (1984, 1991), biotite augen gneiss of Conley (1989), and augen-bearing gneiss of Lukert and Halladay (1980), and Lukert and Nuckols (1976). Historically, most workers have interpreted these rocks as Grenville-age plutons in which the present-day biotite-rich mineral assemblage is a primary igneous assemblage that crystallized from a melt (for example, Bartholomew and others, 1981). Herz and Force (1987) and Evans (1991) presented evidence that these biotite gneisses were derived from charnockite plutons by retrograde hydration reactions. Pettingill and others (1984) reported ages of 1009±26 Ma (Rb-Sr whole-rock) and 1004±36 Ma (Sm-Nd whole-rock) for ferrodiorite to quartzmonzonite in the Roseland district. Where this unit has been mapped in the Upperville quadrangle (A.E. Nelson, unpublished data), U-Pb zircon data suggest a crystallization age of 1055±2 Ma (Aleinikoff and others, 1993).

New Oxford Formation (Triassic) at surface, covers < 0.1 % of this area

Red, maroon, and gray sandstone, siltstone, and shale; basal conglomerate member: From vicinity of Maryland Rte. 73 and southward, limestone conglomerate with red and gray calcareous matrix; northward, quartz conglomerate with red sandy matrix; estimated total thickness 4,5000 feet.

Catoctin Metabasalt (Late Precambrian) at surface, covers < 0.1 % of this area

Thick-bedded metabasalt with amygdaloidal layers and secondary veins of quartz, calcite, and epidote; interbedded green tuffaceous phyllite and blue amygdaloidal metaandesite.

Mather Gorge Formation - Metagraywacke (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Metagraywacke, light- to medium-gray, yellowish- to red dish-brown-weathering, fine- to medium-grained, generally well-bedded, and lesser semi-pelitic schist; contains interbedded quartzose schist and some calc-silicate rock; mineral assemblages as in schist (CZms). Beds range from about 3 cm to 3 m, averaging about 20 cm; graded bedding, sole marks, and slump features are abundant. Mather Gorge is unconformable beneath Popes Head Formation, which is intruded by Occoquan Granite; includes rocks previously mapped in northern Virginia as Peters Creek Schist.