Geologic units in Campbell county, Virginia

Additional scientific data in this geographic area

Fork Mountain Formation (Proterozoic Z-Cambrian) at surface, covers 20 % of this area

Fork Mountain Formation (Conley and Henika, 1973; Conley, 1985). Light- to medium-gray, fine- to medium grained, polydeformed and polymetamorphosed porphyroblastic aluminosilicate-mica schist, interlayered with medium-gray irregularly-layered garnetiferous biotite gneiss, migmatitic in part; calcsilicate granofels; amphibolite; rare white marble; and, coarse calc-quartzite lenses. Complex schistosity, multiple crenulation cleavages, and partly-retrograded, polymetamorphic aluminosilicate and garnet porphyroblasts are diagnostic of Fork Mountain schists. Primary sedimentary structures rarely are preserved. A spectacular polymictic breccia bed that can be traced along strike for several miles within the Fork Mountain near Stuart is a notable exception. Medium- to coarse-granular, blue quartz lenses, angular to rounded inclusions of boudinaged fine-grained, color-laminated, calc-silicate rock, and thick beds of coarse, clast-supported, epidotized lithic breccia are typical of the Fork Mountain biotite gneiss. Prograde regional metamorphic mineral assemblages: (1) quartz + muscovite + biotite + garnet + staurolite + magnetite- ilmenite + rutile; (2) quartz + muscovite + paragonite + plagioclase + garnet + staurolite + sillimanite + magnetite-ilmenite + rutile; (3) quartz + biotite + sillimanite + potassium feldspar + plagioclase + garnet + magnetite-ilmenite; (4) quartz + plagioclase + biotite + muscovite + sillimanite + garnet + tourmaline; (5) quartz + plagioclase + potassium feldspar + biotite + hornblende + epidote + ilmenite; (6) quartz + plagioclase + potassium feldspar + muscovite + biotite + sillimanite + magnetite-ilmenite + garnet + kyanite. Retrograde metamorphic mineral assemblages: (1) quartz + muscovite + chlorite; (2) quartz + muscovite + chloritoid + chlorite; (3) quartz + muscovite + staurolite + chloritoid; (4) quartz + muscovite + kyanite. Contact metamorphic mineral assemblages: (1) andalusite + sillimanite + kyanite + corundum; (2) corundum + spinel + magnetite + kyanite. Geophysical signature: The Fork Mountain has a characteristic "curly maple" pattern on magnetic contour maps. This pattern is the result of isolated concentrations of highly magnetic minerals that produce rounded, high-intensity, positive and negative anomalies. The aluminosilicate-mica schist is the upper part of the Fork Mountain Formation and forms a series of northeastward-trending ridges along the northwest side of the Smith River allochthon. The garnetiferous biotite gneiss is at a lower structural level of the Fork Mountain Formation near Martinsville where lower strata have been intruded by the Martinsville igneous complex, and the remaining metasedimentary rocks contain extensive thermal meta mor phic zones locallized along the intrusive contacts (Conley and Henika, 1973). Biotite gneiss in the Fork Mountain Formation has been interpreted to be a highly metamorphosed diamictite (Rankin, 1975; Conley, 1985; and Pavlides, 1989). At the northeastern limit of the Fork Mountain outcrop belt, in Appomattox and Buckingham counties, the dominant lithologies are polydeformed yellowish-gray chloritoid-chlorite- muscovite quartzose phyllite and quartz-rich mica schist. Tightly-folded, transposed pinstriped segregation layering at a high angle to the penetrative schistosity defined by phyllosilicate minerals is characteristic; polycrystalline quartz-rich boudins are abundant. These rocks are lithologically indistinguishable from those along the highly-tectonized western margin of the metagraywacke, quartzose schist, and melange (CZpm) outcrop belt; current interpretation is that the Fork Mountain is correlative to some degree with CZpm.

Bassett Formation - Biotite gneiss and granite gneiss. (Proterozoic Z) at surface, covers 17 % of this area

Light-gray to black-and-white, fine to medium-grained, leucocratic biotite gneiss that is mostly segregation-layered, but locally is a medium-grained quartzfeldspar granofels. Contains interlayers of muscovite-biotite schist, quartz schist, and epidote quartzite. Mineral assemblages: (1) quartz + plagioclase + potassium feldspar + biotite + muscovite + magnetite-ilmenite + tourmaline ± kyanite ± epidote ± titanite ± hornblende ± garnet; (2) quartz + plagioclase + epidote + pyroxene. Porphyroclasts of zoned plagioclase in an equigranular, polygonal quartz-potassium feldspar groundmass and medium to thick bedding suggest a volcaniclastic protolith (Conley, 1985). Gneisses are migmatitic and cut by numerous granite dikes and sills near the contacts with the Martinsville igneous complex. Geophysical signature: potassium feldspar-bearing gneisses have positive radiometric, and generally flat magnetic signatures relative to adjacent amphibolite units. In the core of the Sherwill anticline (Campbell and Appomattox counties), the dominant rock-type is graded salt-andpepper metagraywacke, interbedded with lesser mica schist and graphite schist. This association bears lithologic affinity to the Lynchburg Group, which occupies the cores of structural domes to the west; this correlation has been made by several workers (Brown, 1958; Kaldy, 1977; Gates, 1987).

Alligator Back Formation - Feldspathic metagraywacke (Proterozoic Z-Cambrian) at surface, covers 13 % of this area

Heterogeneous assemblage of rock-types includes medium- to light-gray, laminated quartzofeldspathic to calcareous gneiss with thin mica schist partings; white and gray, fine- to coarse-grained, generally laminated marble; gray to greenish-gray fine-grained graphitic mica schist and quartzite; light-gray, medium- to fine-grained mica schist; massive quartzite and micaceous blue quartz granule metasandstone; and, dark-greenish-black actinolite schist. Mineralogy: (1) quartz + potassium feldspar + pla ioclase + biotite + muscovite + calcite + epidote + titanite + magnetite- ilmenite; (2) quartz + muscovite + chlorite + graphite + titanite + ilmenite; (3) quartz + albite + muscovite + biotite + titanite + ilmenite; (4) quartz + mus co vite + garnet + kyanite; (5) chlorite + tremolite + magnetite-ilmenite; (6) chlorite + actinolite-tremolite + talc + dolomite + magnetite-ilmenite; (7) quartz + albite + actinolite + biotite + epidote + magnetite. Units here mapped as Alligator Back Formation were previously mapped as the Evington Group (Espenshade, 1954; Brown, 1958; Redden, 1963; Gates, 1986; Patterson, 1987) and considered to be younger than the Lynchburg Group. Regional mapping by Henika (1991) and Scheible (1975) indicates that rocks assigned to Alligator Back Formation by Rankin and others (1973) are continuous with the upper part of the Lynchburg Group in the type section along the James River at Lynchburg (Jonas, 1927) and that the Alligator Back consistently dips southeast beneath the overlying Candler Formation from the Virginia-North Carolina border to the James River at Lynchburg. Sedimentary and structural facing criteria indicate that rock units immediately southeast of the Candler Formation in an outcrop belt from Stapleton on the James River, southwest to Leesville Dam on the Roanoke River, are older than the Candler (Henika, 1992). Although previously mapped as upper Evington Group (Espenshade, 1954; Brown, 1958; Redden, 1963; Patterson, 1987), these rocks are herein correlated with the Alligator Back Formation (upper Lynchburg Group), having been uplifted against the Candler Formation to the northwest along the Bowens Creek fault (Henika, 1992). Rocks in the same outcrop belt along strike to the southwest of the Leesville Reservoir were previously correlated with the Alligator Back Formation by Conley (1985). The sequence of lithologic units within the Alligator Back Formation southeast of the Bowens Creek fault is the same as that proposed by Brown (1951; 1958), and Espenshade (1954) for the formations in the Evington Group, that are structurally above the Candler Formation. The sequence is based on the detailed structural and stratigraphic relationships first established by Brown (1958) in the Lynchburg 15-minute quadrangle.

Metagraywacke, Quartzose Schist, and Melange (Proterozoic Z-Cambrian) at surface, covers 9 % of this area

Metagraywackes are quartzose chlorite or biotite schists containing very fine to coarse granules of blue quartz; primary graded laminations have been transposed by shearing into elongate lozenges that give the rock a distinctive pin-striped appearance in weathered surfaces perpendicular to schistosity. A mylonitic fabric is superimposed in varying degrees, as are late-stage chevron-style folds. In Buckingham, Appomattox, and Campbell Counties, rocks in this unit are progressively more tectonized from east to west across the outcrop belt; in the western portion, the dominant lithology is a polydeformed, mylonitic mica schist with abundant quartz-rich boudins; transposed pinstriped lamination or segregation layering at a high angle to mylonitic schistosity is characteristic. Metagabbroic blocks ranging in size from 5 cm to 3 m across and larger have been identified at widely scattered locations throughout the outcrop belt. Mineralogy: quartz + albite + epidote + chlorite + muscovite + magnetite ± chloritoid ± calcite; biotite- and staurolite-bearing assemblages occur in Appomattox and Campbell Counties. Detrital minerals identifi ed in thin section include plagioclase, perthite, epidote, magnetite, tourmaline, and titanite. Lithic fragments include dacite tuff, gabbro, and monocrystalline quartz with zircon and biotite inclusions (Evans, 1984). The northern portion of the outcrop belt includes melange zone IV of the Mine Run complex of Pavlides (1989; 1990). In Albemarle and Fluvanna counties, CZpm includes the lower chlorite-muscovite unit of Smith and others (1964) and Hardware metagraywacke of Evans (1984). In Appomattox and Buckingham Counties, polydeformed quartzose mica schists in the western portion of the outcrop belt are lithologically indistinguishable from schists mapped as Fork Mountain Formation in structural blocks that occur to the west; these units are considered to be at least in part correlative. In Campbell County, polydeformed metagraywacke and mica schist is intruded by the Cambrian-age Melrose Granite (Cm).

Alligator Back Formation - Actinolite schist (Proterozoic Z-Cambrian) at surface, covers 7 % of this area

Dark-grayish-green chlorite-actinolite schist metabasalt. Mineralogy: actinolite + epidote + chlorite ± biotite + albite + quartz + magnetite-ilmenite. Geophysical signature: linear, positive magnetic anomaly. Schist commonly contains recognizable flow structures, deformed and mineralized pillow basalts, pyroclastic breccia, pink and white marble, and laminated metatuff. Massive to thin beds are interlayered with metamorphosed sedimentary and mafic to ultramafic rocks. This unit was previously mapped as the Catoctin Formation or the Slippery Creek Greenstone in the Lynchburg quadrangle (Brown 1958).

Bassett Formation - Amphibolite. (Proterozoic Z-Cambrian) at surface, covers 6 % of this area

Dark-greenish-gray to black-and-white, medium- to coarse-grained, layered to massive hornblende schist, hornblende gneiss, amphibolite, garnet-pyroxene granofels, and coarse uralitic hornblende metagabbro. Mafic rocks are interlayered with white to light-gray, medium- to coarse-grained quartz-feldspar granofels, and cut by alaskite and pegmatite dikes and sills. Ovoid masses of quartz, plagioclase, epidote and quartz that resemble flattened amygdules, and features that resemble graded bedding and cut-and-fill structures suggest a mixed volcanic-volcaniclastic protolith (Conley and Henika, 1970). Mineral assemblages: (1) hornblende + plagioclase + quartz + pyroxene + garnet + epidote + magnetite + titanite; (2) diopside + grossular + plagioclase + magnetite + quartz + epidote; (3) hornblende + plagioclase + potassium feldspar + quartz + epidote. Geophysical signature: Narrow, positive magnetic anomalies closely parallel amphibolite outcrops belts.

Melrose Granite (Cambrian) at surface, covers 5 % of this area

Light-greenish-gray to pink-banded, massive, medium- to coarse-grained biotite granite; ranges in composition from quartz monzonite to quartz diorite. Fabric shows a southeastward progression from protomylonite to mylonite to ultramylonite where the unit is cut by the Brookneal shear zone (Gates and others, 1986). Mineralogy: quartz + plagioclase + potassium feldspar + biotite + muscovite + chlorite + epidote + titanite + garnet + mag ne tite-ilmenite + calcite + zircon (Gates, 1981). Geophysical signature: positive radiometric, negative magnetic. The Melrose has been dated at 515 Ma (U-Pb zircon; Sinha and others, 1989).

Newark Supergroup; Triassic Sandstone, Siltstone, and Shale (Upper Triassic) at surface, covers 4 % 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.

Ashe Formation - Biotite gneiss (Proterozoic Z) at surface, covers 3 % of this area

Medium- to light-gray, massive, conglomeratic biotite schist and gneiss, with feldspar, quartz, and granitic clasts; grades upwards into medium- to fine-grained, salt-and-pepper-textured two-mica plagioclase gneiss with very-light-gray mica schist interbeds. Quartzite, impure marble, calcareous gneiss and amphibolite occur locally. Some dark-gray to black, pyrite-bearing mica schist occurs at tops of thick, fining-upwards graded sequences. Mineralogy: (1) quartz + plagioclase + potassium feldspar + biotite + muscovite + chlorite + epidote + ilmenite; (2) quartz + plagioclase + biotite + muscovite + epidote-allanite + garnet + titanite + ilmenite; (3) quartz + calcite + plagioclase + biotite + muscovite + epidote + ilmenite + titanite; chlorite occurs as a secondary mineral. Unit is unconformable on Grenville basement and cut by Late Precambrian mafic and felsic dikes.

Candler Formation - Phyllite and schist (Cambrian) at surface, covers 3 % of this area

Medium- to dark-gray and greenish- gray mica phyllite and sandy laminated schist. Lenses and pods of feldspathic quartzite, metamorphosed quartzarenite, dolomitic marble, and dark-gray to medium-bluish-gray, laminated marble are common in the upper part. Mineralogy: quartz + albite + muscovite + chlorite + magnetite-ilmenite + epidote ± biotite ± chloritoid ± calcite. Chloritoid and magnetite porphyroblasts are common near the Bowens Creek fault. Geophysical signature: Low amplitude, linear magnetic highs are superimposed on a pronounced southeast-sloping magnetic gradient between Alligator Back units northwest of the Candler and a persistant linear magnetic trough localized along the trend of the Bowens Creek fault zone. Microstructural elements in the upper Candler indicate dextral transpression along a continuous shear zone (Bowens Creek fault zone) within the Candler outcrop belt from the Virginia-North Carolina boundary in Patrick County northeastward to at least the north end of Buffalo Ridge on the Amherst-Campbell County line. Conley and Henika (1970) and Gates (1986) hypothesized that the Bowens Creek fault is part of a major strike slip (wrench) system that is part or a continuation of the Brevard fault zone to the southwest . Northeast of the Scottsville Mesozoic basin, the Candler includes laminated metasiltstone (Ccas), ferruginous metatuff, dolomitic marble, and phyllite that are conformable above Catoctin metabasalt (Evans, 1984; Conley, 1989; Rossman, 1991); in Orange County, the Candler includes the True Blue formation of Pavlides (1989, 1990).

Interlayered Mafic and Felsic Metavolcanic Rocks - Foliated felsite (Cambrian) at surface, covers 3 % of this area

Foliated felsite; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Foliated felsite. Grayish-orange-pink to white, fine- to medium-grained, foliated to granular metavolcanic rocks range in composition from rhyolite to dacite. Includes muscovite feldspar- quartz schist, gneiss and granofels; massive crystal metatuff; welded ashflow tuff; and, inequigranular metavolcanic breccia. Relict primary volcanic textures are recognizable where metamorphic grade is low (Henika, 1975; 1977). This unit includes felsic gneiss with less common mafic and rare calcareous gneiss mapped by Tobisch (1972), in part the metamorphosed volcanic sequence of Gates (1981), and dominantly felsic-composition units mapped by Nelson (1992). The unit contains numerous granitic dikes, sills, and lit-par-lit injections where it occurs in close proximity to Shelton Formation (Ost). Felsites occur interlayered with amphibolite, amphibole gneiss and schist (Cmv), quartzofeldspathic biotite gneiss (Cbg), sillimanite-quartz-muscovite schist and gneiss (Csg), and ferruginous quartzite (Cfq).

Newark Supergroup; Triassic shale and siltstone (Triassic) at surface, covers 2 % 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.

Arvonia Formation - Porphyroblastic garnet-biotite schist (Ordovician) at surface, covers 1 % of this area

Porphyroblastic mica schist, characterized by 1- to 2-mm garnet porphyroblasts in an anastomosing, greenish-black biotite-rich, schistose matrix. Most exposures show complex microstructures suggestive of polyphase deformation and superimposed shearing. In Appomattox and Campbell counties, and locally elsewhere,this unit includes quartzose muscovite schist along the western and eastern margins of the outcrop belt (presumed base of the stratigraphic section); locally the unit contains thin interbeds of calcareous mica schist and marble. Mineralogy: biotite + garnet + muscovite + quartz + plagioclase + magnetite ± kyanite ± calcite. Geophysical signature: characterized by elongate positive magnetic and radiometric anomalies. This unit was mapped in strike-belts southwest of, and not physicaly connected to the type section at Arvonia.

Alligator Back Formation - Banded marble (Proterozoic Z-Cambrian) at surface, covers 1 % of this area

Light- and dark-gray, laminated fine to medium-grained marble, calcareous gneiss, and schist. Mineralogy: calcite + quartz + biotite + muscovite + plagioclase + pyrite + magnetite-ilmenite. Thick to thin beds of marble are interlayered with graphitic phyllite and mica schist; the lithology grades from impure marble to calcareous metagraywacke depending on per cent age of detrital calcite present. The unit includes the Arch Marble of Brown (1958) and the Archer Creek Formation of Espenshade (1954).

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

Dominantly leucocratic to mesocratic, medium- to coarse-grained, strongly-lineated granite gneiss; includes porphyritic granite gneiss. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + muscovite ± garnet. Geophysical signature: western portion coincides with strong positive radiometric anomaly. This unit likely includes more than one intrusive body; portions may be derivative of felsic volcanogenic rocks. These rocks intrude and are interlayered with metavolcanic rocks of probable Cambrian age. The unit includes granite gneiss of Nelson (1992), and may be correlative in part with the Shelton Formation (Ost).

Metavolcanic and Metasedimentary Rocks - Felsic metatuff, mica schist, and gneiss. (Cambrian) at surface, covers 0.9 % of this area

Includes light-gray to grayish-pink, very-fine-grained to aphanitic, thin-bedded dacitic metatuff, locally containing subhedral to euhedral plagioclase phenocrysts; very-light-gray pinstriped quartz-rich, muscovite and biotite-muscovite schist; and, medium-gray fine-grained salt-and-pepper biotite-muscovite gneiss. Mineralogy: quartz + plagioclase + muscovite ± biotite + epidote + magnetite. These lithologies are interlayered with subordinate amounts of greenstone metabasalt and amphibole gneiss.

Metavolcanic and Metasedimentary Rocks - Greenstone or amphibole gneiss. (Cambrian) at surface, covers 0.9 % of this area

Includes dark- to dusky-green, schistose actinolite-chlorite metabasalt with epidote- quartz segregations; and, layered hornblende-plagioclase gneiss. These lithologies are interlayered with subordinate amounts of dacitic metatuff, quartz-muscovite schist, and fine-grained salt-and-pepper biotite-muscovite gneiss.

Mylonite, Mylonite Gneiss, and Cataclastic Rocks (Proterozoic - Paleozoic ?) at surface, covers 0.8 % of this area

Mylonite. Includes protomylonite, mylonite, ultramylonite, and cataclastic rocks. Lithology highly variable, depending on the nature of the parent rock, and on intensive parameters and history of deformation. In most mapped belts of mylonite and cataclastic rock (my), tectonized rocks anastomose around lenses of less-deformed or undeformed rock. In the Blue Ridge, some of these lenses are large enough to show at 1:500,000 scale. In many places mylonitic and cataclastic rocks are gradational into less deformed or undeformed adjacent rocks, and location of contacts between tectonized rocks (my) and adjacent units is approximate or arbitrary. These boundaries are indicated on the map by color-color joins with superimposed shear pattern. Most mapped belts of mylonite represent fault zones with multiple movement histories. In the Blue Ridge, Paleozoic age contractional deformation fabrics are superimposed on Late Precambrian extensional fabrics (Simpson and Kalaghan, 1989; Bailey and Simpson, 1993). Many Piedmont mylonite zones contain dextral-transpressional kinematic indicators that formed during Late Paleozoic collision al tectonics (Bobyarchick and Glover, 1979; Gates and others, 1986). Paleozoic and older faults were reactivated in many places to form extensional faults during the Mesozoic (Bobyarchick and Glover, 1979).

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers 0.7 % 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.

Interlayered Mafic and Felsic Metavolcanic Rocks (Cambrian) at surface, covers 0.5 % of this area

Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Quartzofeldspathic biotite gneiss. Heterogeneous layered sequence consists of salt-and-pepper and segregation layered biotite granite gneiss interlayered with biotite schist; dark-gray to black, fine- to coarse-grained, thin- to thickly-laminated hornblende gneiss and schist; lesser quartz-muscovite schist; and, locally, gray to green, medium-grained, calcareous gneiss and calc-silicate granofels (Tobish and Glover, 1969). This unit includes the upper and lower felsic gneiss units and intermediate volcanic rocks in the Hyco Formation as used by Baird (1989, 1991); and biotite gneiss and interlayered gneiss of Kreisa (1980), correlative with the biotite gneiss unit of Marr (1980a; 1980b). Mineralogy: (quartzofeldspathic rocks), (1) quartz + albite + potassium feldspar + muscovite + chlorite + actinolite + epidote + calcite + magnetite + zircon; (2) quartz + oligoclase + muscovite + biotite + garnet + hornblende + magnetite + epidote + rutile + calcite + zircon; (mafic rocks), (1) quartz + albite + chlorite + epidote + actinolite + titanite + magnetite ilmenite. (2) quartz + oligoclase + andesine + hornblende + microcline + biotite + garnet + cordierite + magnetite + rutile + titanite + scapolite; (pelitic rocks), (1) quartz + albite + muscovite + chlorite + epidote + magnetite-ilmenite; (2) quartz + muscovite + biotite + kyanite + oligoclase + potassium feldspar + epidote + magnetite-ilmenite + garnet; (3) quartz + muscovite + sillimanite + magnetite-ilmenite; (calcareous rocks), (1) quartz + calcite + biotite + epidote + chlorite + tremolite + ilmenite; (2) calcite+ quartz + epidote + hornblende + pyroxene + scapolite. Geophysical signature: felsic rocks are delineated by strike-elongate positive radiometric anomalies (Henika and Johnson, 1980); mafic metavolcanic rocks and metasedimentary units are characterized by closed strike-elongate radiometric lows and closed strike-elongate aeromagnetic highs.

Metagabbro (Proterozoic) at surface, covers 0.4 % of this area

Metagabbro. (southwestern Piedmont, Conley, 1985). Dark-green and white, massive to layered, coarse-grained equigranular metagabbro. Mineralogy: clinopyroxene + orthopyroxene ± garnet + hornblende + plagioclase + magnetite. Garnets are surrounded by thin coronas composed of plagioclase and symplectic intergrowths of plagioclase-pyroxene. Forms elongate concordant bodies intrusive into the Bassett Formation. (northern Piedmont, Drake and Froelich, in press). Greenish-gray, fine- to coarse-grained, well-foliated epidote plagioclase-hornblende amphibolite; locally contains relict clinopyroxene. Occurs as sills in the Mather Gorge Formation. Fisher (1971) reports that zircons from an amphibolite body on an island in the Potomac River have a Pb-Pb age of 525+/- 60 Ma, and a concordia age of 550 Ma, assuming continuous diffusion; a Cambrian age for metagabbroic sills in the Mather Gorge would be consistent with regional geologic relations.

Interlayered Mafic and Felsic Metavolcanic Rocks - Amphibolite, hornblende-biotite gneiss, and schist. (Cambrian) at surface, covers 0.1 % of this area

Amphibolite, hornblende-biotite gneiss, and schist.; Heterogenous layered assemblage correlates with the Chopawamsic Formation and Ta River Metamorphic Suite, on strike to the northeast, and in traceable into the Milton belt in North Carolina (Geologic Map of North Carolina, 1985). Amphibolite, hornblende-biotite gneiss and schist. Black to moderate-olive-brown, medium- to coarse-grained, lineated and foliated; light-greenish-gray quartz-epidote stringers are common. Mineralogy: hornblende + tremolite-actinolite + oligoclase + biotite + epidote + garnet. Includes Blackwater Creek Gneiss and Catawba Creek amphibolite member of Hyco Formation of Baird (1989), hornblende gneiss of LeGrand (1960), gneiss unit of Kreisa (1980), and dominantly mafic-composition units mapped by Nelson (1992). Amphibolite is interlayered with biotite gneiss, as discussed above.

Leatherwood Granite (Cambrian) at surface, covers < 0.1 % of this area

Leatherwood Granite (Jonas, 1928; Pegeau, 1932; Conley, 1985). Light-gray, medium- to coarse-grained, porphyritic biotite granite generally shows rapakivi texture. Mineralogy: quartz + potassium feldspar + plagioclase + biotite + muscovite + epidote + apatite + titanite + zircon + mag ne tite. Geophysical signature: positive radiometric, negative magnetic. The major part of the Leatherwood occurs as sheets at the top of the Martinsville igneous complex. Leatherwood Granite and associated Rich Acres gabbro are cut by dikes of dark-gray, coarse-grained, porphyritic olivine norite. The Leatherwood was dated at 450 Ma (U-Pb zircon; Rankin, 1975); 464±20 Ma (Rb-Sr whole-rock, Odom and Russell, 1975); and 516 Ma (U-Pb zircon; Sinha and others, 1989).

Ultramafic Rocks (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Grayish-green to light-gray talc chlorite-actinolite or talc-tremolite schist. Mineralogy: (1) chlorite + actinolite + talc + dolomite + ilmenite + magnetite; (2) serpentine (antigorite) + talc + chlorite ± olivine ± augite; (3) tremolite + cummingtonite + chlorite + talc + magnetite-ilmenite ± quartz. Geophysical signature: elongate positive magnetic anomaly. Elongate, lenticular bodies generally trend parallel to schistosity of enclosing rocks and are concordant at variable stratigraphic levels within the Lynchburg Group.

Mafic Igneous Complex Undivided (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Metamorphosed stratiform mafic and ultramafic rocks include: greenish-gray, locally layered, coarse-grained metagabbro; dark-greenish-black schistose metabasalt; and, gray to grayish-green talc-chlorite-tremolite schist. Mineralogy: (1) chlorite + epidote + plagioclase + quartz + titanite + ilmenite; (2) chlorite + actinolite + biotite + epidote + titanite + plagioclase + quartz; (3) chlorite + actinolite + talc + dolomite + magnetite-ilmenite; (4) tremolite + chlorite + magnetite-ilmenite; (5) serpentine + talc + chlorite + actinolite ± olivine ± augite. Geophysical signature: strike-elongate positive magnetic anomaly. Metamorphosed mafic and ultramafic complexes are generally sheet-like bodies, concordant near the base of the Alligator Back and Charlottesville Formations. In Nelson County, these rocks are cut by metagabbroic dikes (CZmd) that are likely related to the Catoctin. Hess (1933) reports a stratified association of ultramafic, mafic, and minor silicic lithologies at Schuyler which he attributes to in situ differentiation of a sheet-like concordant intrusion. Glover and others (1989) report a non-tectonized intrusive contact between Charlottesville Formation metasiltstone and ultramafic schist at Schulyer. In contrast, Conley (1985) presents evidence that ultramafic-mafic complexes in Franklin County are tectonicly-emplaced slices of oceanic crust (ophiolites). Tectonic setting and mode of emplacement for these rock assemblages remain enigmatic; correlation of complexes in the southwestern Piedmont with those in the central Blue Ridge may ultimately prove invalid.