Geologic units in Rockingham county, North Carolina

Biotite Gneiss and Schist (Cambrian/Late Proterozoic) at surface, covers 47 % of this area

Inequigranular and megacrystic; abundant potassic feldspar and garnet; interlayered and gradational with calc-silicate rock, sillimanite-mica schist, mica schist, and amphibolite. Contains small masses of granitic rock.

Newark Supergroup, Dan River Group; Stoneville Formation (Triassic) at surface, covers 12 % of this area

Conglomerate, sandstone, and mudstone, lenticular and laterally-gradational bedding.

Felsic Mica Gneiss (Cambrian/Late Proterozoic) at surface, covers 10 % of this area

Interlayered with biotite and hornblende gneiss and schist.

Metagraywacke and Muscovite-Biotite Schist (Cambrian/Late Proterozoic) at surface, covers 9 % of this area

Metagraywacke (biotite gneiss) interlayered and gradational with muscovite-biotite schist; minor marble and granitic rock.

Granitic Rock (Permian/Pennsylvanian) at surface, covers 6 % of this area

Megacrystic to equigranular. Churchland Plutonic Suite (Western group) - Churchland, Landis, and Mooresville intrusives.

Newark Supergroup, Dan River Group; Pine Hall Formation (Triassic) at surface, covers 4 % of this area

Sandstone, mudstone, and conglomerate, yellowish orange to brown.

Mica Schist (Cambrian/Late Proterozoic) at surface, covers 4 % of this area

Garnet, staurolite, kyanite, or sillimanite occur locally; lenses and layers of quartz schist, micaceous quartzite, calc-silicate rock, biotite gneiss, amphibolite, and phyllite.

Newark Supergroup, Dan River Group; Cow Branch Formation (Triassic) at surface, covers 3 % of this area

Mudstone with minor sandstone, gray, laterally-continuous bedding. Intertongues with Stoneville and Pine Hall formations.

Shelton Granite Gneiss (Silurian) at surface, covers 2 % of this area

Poorly foliated; lineated granitic to quartz monzonitic gneiss.

Mafic Metavolcanic Rock (Cambrian/Late Proterozoic) at surface, covers 2 % of this area

Metamorphosed basaltic flows and tuffs, dark green to black; interbedded with felsic and intermediate metavolcanic rock and metamudstone.

Metamorphosed Granitic Rock (Late Proterozoic) at surface, covers 0.3 % of this area

Massive to foliated, locally mylonitic.

Amphibolite (Cambrian/Late Proterozoic) at surface, covers 0.2 % of this area

Metamorphosed mafic extrusive and intrusive rock; includes hornblende gneiss, thin layers of mica schist, calc-silicate rock, and, rarely, marble. Also includes small masses of metadiorite and metagabbro.

Metagraywacke, Amphibolite, and Kyanite Schist (Cambrian/Late Proterozoic) at surface, covers 0.2 % of this area

Metagraywacke (biotite gneiss) interlayered and gradational with amphibolite and kyanite schist; minor ultramafic and granitic rock.

Metamorphosed Granitic Rock (Ordovician/Cambrian) at surface, covers 0.2 % of this area

Equigranular to megacrystic, foliated to massive. Includes Toluca Granite.

Phyllite and Schist (Cambrian/Late Proterozoic) at surface, covers 0.1 % of this area

Minor biotite, pyrite, and sillimanite; includes minor quartzite.

Rich Acres Formation (Cambrian) at surface, covers < 0.1 % of this area

Rich Acres Formation (Conley and Henika, 1973; Conley, 1985). Dikes, sills, and irregularly-shaped plutons of dark-greenish-gray, medium-grained, locally porphryitic, biotite-hornblende gabbro. Mineralogy: plagioclase + clinopyroxene + orthopyroxene + hornblende + biotite + magnetite + quartz + rutile + apatite + zircon + epidote + calcite + pyrite + titanite; plagioclase is altered to epidote; pyroxenes are altered to uralite. Outer parts of some plutons are injected with thin veins composed of hornblende + plagioclase, and hornblende + pyroxene + plagiocase, and with quartz-microcline-oligoclase pegmatite. The unit includes small, irregularly-shaped plutons of porphyritic norite composed of 1- to 4-cm orthopyroxene and clinopyroxene and 1-cm plagioclase laths in ophitic texture, hornblende, biotite, and olivine. The Rich Acres is part of the Martinsville igneous complex of Ragland (1974).

Felsic Volcanic and Volcaniclastic Rocks (Proterozoic Z) at surface, covers < 0.1 % of this area

Dark-gray to black, biotite- and pyrite-rich volcaniclastic rock interbedded with medium-gray, fine-grained rocks with numerous quartzfilled vesicles; upper part of the unit consists of medium dark-gray, fine-grained felsic rock with numerous clasts of fine-grained, white-weathering, vesiculated felsite interlayered with fine-grained, clast-free felsic rock. Dikes of clast-free felsic rock cut nearby Middle Proterozoic granitic gneiss (Ybp); conformably overlain by feldspathic metasandstones of the Fauquier Formation.

Fork Mountain Formation (Proterozoic Z-Cambrian) at surface, covers < 0.1 % 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 - Amphibolite. (Proterozoic Z-Cambrian) at surface, covers < 0.1 % 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.