Geologic units in Fairfax county, Virginia

Additional scientific data in this geographic area

Mather Gorge Formation - Schist (Proterozoic Z-Cambrian) at surface, covers 20 % 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.

Occoquan Granite (Cambrian-Ordovician) at surface, covers 10 % of this area

Occoquan granite (Lonsdale, 1927; Drake and Froelich, 1986). Light-gray, medium- to coarse-grained, muscovite-biotite monzogranite and lesser granodiorite and tonalite. In many places the rock has a strong quartz-rod lineation, and locally, two foliations. The Occoquan has been dated at about 560 Ma (U-Pb zircon; Seiders and others, 1975), 494±14 Ma (Rb-Sr whole-rock; Mose and Nagel, 1982), and 479±9 Ma (U-Pb single-crystal zircon; J.N. Aleinikoff, written communication, 1993).

Potomac Formation (Cretaceous) at surface, covers 10 % of this area

Potomac Formation (Lower and Upper(?) Cretaceous, McGee, 1886). Light-gray to pinkish- and greenish-gray quartzo-feldspathic sand, fine- to coarse-grained, pebbly, poorly sorted, commonly thick-bedded and trough cross-bedded. Sand is interbedded with gray to green, massive to thick-bedded sandy clay and silt, commonly mottled red or reddish-brown. Includes lesser amounts of clay-clast conglomerate and thin-bedded to laminated, carbonaceous clay and silt. In the inner Coastal Plain, unit was deposited mainly in fluvial-deltaic environments, intertongues eastward with thin glauconitic sands of shallow-shelf origin. Spore and pollen assemblages and leaf impressions of ferns and cycads indicate an Early Cretaceous age (Doyle and Robbins, 1977). In some downdip areas, uppermost part of unit may be of earliest Late Cretaceous age. Thickness ranges from a feather-edge at western limit of outcrop to more than 3500 feet in subsurface of outermost Coastal Plain.

Sykesville Formation (Cambrian) at surface, covers 7 % of this area

Sykesville Formation (Hopson, 1964; Drake, 1985). Light- to medium-gray, medium-grained metasedimentary melange consisting of a quartzofeldspathic matrix that contains quartz "eyes"and a heterogeneous suite of pebble- to boulder and larger-size olistoliths. These include: Mather Gorge Formation migmatite, phyllonite, and metagraywacke; also, ultramafic, metagabbroic, and felsic and mafic metavolcanic rocks, plagiogranite, and quartzite. The Sykesville is intruded by Occoquan Granite.

Popes Head Formation - Old Mill Branch Metasiltstone Member (Cambrian-Ordovician) at surface, covers 6 % of this area

Light-greenish-gray, pale-greenish-yellow- or yellowish-gray-weathering, medium to fine-grained graded micaceous metasiltstone and lesser fine grained micaceous metasandstone; fine-grained beds are phyllite and micaceous metasiltstone. Graded beds are 2 to 24 cm thick. Mineralogy: quartz + muscovite + biotite + plagioclase + chlorite + magnetite + epidote. The unit contains felsic metatuff (mineral assemblage quartz + plagioclase + epidote + muscovite + biotite + chlorite + green amphibole + magnetite) and mafic metatuff (mineral assemblage blue-green amphibole + plagioclase + titanite) layers up to 180 cm thick. Maximum thickness of unit is about 2300 feet; unit overlies with apparent unconformity the Mather Gorge, Sykesville, and Yorkshire Formations, the Annandale Group, and the Piney Branch Complex (Drake and Lyttle, 1981). The unit is intruded by Occoquan Granite.

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers 6 % 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.

Shirley Formation (Quaternary) at surface, covers 5 % of this area

Shirley Formation (middle Pleistocene, Johnson and Berquist, 1989). Light- to dark-gray, bluish-gray and brown sand, gravel, silt, clay, and peat. Constitutes surficial deposits of riverine terraces and relict baymouth barriers and bay-floor plains (altitude 35-45 feet) inset below depositional surfaces of the Chuckatuck Formation (Johnson and Peebles, 1984). Upper part of unit is truncated on the east by the Suffolk and Harpersville scarps; locally, lower part occurs east and west of scarps. Fluvial-estuarine facies comprises (1) a lower pebble to boulder sand overlain by (2) fine to coarse sand interbedded with peat and clayey silt rich in organic material, including in-situ tree stumps and leaves and seeds of cypress, oak, and hickory, which grades upward to (3) medium- to thick-bedded, clayey and sandy silt and silty clay. Marginal-matrix facies in lower James River and lowermost Rappahannock River areas is silty, fine-grained sand and sandy silt containing Crassostrea virginica, Mulinia, Noetia, Mercenaria, and other mollusks. Astrangia from lower Rappahannock River area has yielded a uranium-series age of 184,000 ± 20,000 yrs B.P. (Mixon and others, 1982). Thickness is 0 to 80 feet.

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.

Diabase (Jurassic) at surface, covers 4 % 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.

Mather Gorge Formation - Metagraywacke (Proterozoic Z-Cambrian) at surface, covers 4 % 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.

Pliocene Sand and Gravel (Tertiary) at surface, covers 4 % of this area

Interbedded yellowish-orange to reddish-brown gravelly sand, sandy gravel, and fine to coarse sand, poorly to well-sorted, cross-bedded in part, in cludes lesser amounts of clay and silt in thin to medium beds. Commonly caps drainage divides (altitude 250-170 feet) in western part of Coastal Plain. Lower part of unit, showing flaser and lenticular bedding and containing rare to abundant Ophiomorpha nodosa represents deposition in marginal-marine environments and is, in part, a nearshore equivalent of the more downdip, marine facies of the Yorktown Formation. In the northern part of the Coastal Plain, the more poorly sorted and less cleanly washed upper part of unit, which lacks fossils, comprises fluvial-deltaic sediments that prograded eastward across the shelf during a regressive phase of the Yorktown. To the south, the upper part of unit is massively bedded clayey sand in places containing heavy mineral concentrations that average 8 percent or more; the sands are nearshore, beach and dune origin; interstitial clay was derived, in part, from in-situ weathering of feldspar sand. Thick ness is 0 to 50 feet.

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.

Indian Run Formation (Cambrian) at surface, covers 2 % of this area

Indian Run Formation (Drake, 1985). Poorly- to well foliated metasedimentary melange consisting of a medium grained quartz-plagioclase-muscovite-biotite-chlorite-garnet matrix containing quartz "eyes" and a heterogeneous suite of olistoliths. These include: foliated Accotink Schist and Lake Barcroft Metasandstone, and foliated ultramafic, metagabbroic, and felsic and mafic metavolcanic rocks. The Indian Run predates Occoquan Granite.

Piney Branch Complex (Proterozoic Z-Cambrian) at surface, covers 2 % of this area

Piney Branch Complex (Drake and Morgan, 1981). Heterogeneous assemblage of metamorphosed peridotite, pyroxenite, and gabbro; dominant rock-types include serpentinite, soapstone, and actinolite schist. The unit contains dikes and sheets of plagiogranite. Secondary unit decription from USGS Geologic Names lexicon (ref. VA004): It is a tectonic melange resulting from the deformation of a layered complex that contained repetitive cycles of ultramafic and mafic rocks that are now metamorphosed to serpentinite, soapstone, actinolite schist, and metagabbro, and intruded by dikes and sheet of plagiogranite. It has a discontinuous underlying border of precursory ophiolitic melange, the Yorkshire Formation. The allochthon consisting of the Piney Branch and Yorkshire was thrust upon the Peters Creek Schist, and with it form a composite allochthon that was emplaced on the Sykesville Formation perhaps by gravity sliding. It unconformably underlies the Popes Head Formation. Movement of the Piney Branch began in the Late Proterozoic after the metamorphism of the Peters Creek, and ended during the Taconic Orogeny. Age is Late Proterozoic and (or) Early Cambrian.

Falls Church Intrusive Suite (Cambrian-Ordovician) at surface, covers 2 % of this area

Falls Church Intrusive Suite (Drake and Froelich, 1986; in press). Tonalite, granodiorite, monzogranite, and trondhjemite. Tonalite, medium-dark-gray, medium- to coarse grained biotite-hornblende tonalite and biotite tonalite with abundant inclusions of mafic and ultramafic rock. Tonalite is typically well-foliated; in many places it has a strong quartz-rod lineation. Granodiorite, medium-gray, medium- to coarse grained, poorly- to moderately-well-foliated, biotite- and biotite-muscovite-bearing. Monzogranite, medium-grained, muscovite-biotite and garnet-muscovite-biotite-bearing. Trondhjemite, light-pink to light-gray, medium-grained, muscovite-bearing; strongly deformed and recrystallized. Tonalite from this suite has been dated at 481±11 Ma (U-Pb single-crystal zircon; J.N. Aleinikoff, written communication, 1993).

Newark Supergroup; Conglomerate, mixed clasts (Upper Triassic) at surface, covers 2 % 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.

Annandale Group - Accotink Schist (Proterozoic Z-Cambrian) at surface, covers 2 % of this area

Accotink Schist (Drake and Lyttle, 1981). Light-gray, yellowish-gray to moderate-brown to very-pale-orange-weathering schist and interbedded with micaceous metagraywacke. Locally, schist contains randomly scattered, discontinuous beds of meta-arenite. Schist intervals are 20 to 210 cm thick, but individual sedimentation units average about 1 cm. Mineralogy: quartz + muscovite + biotite + chlorite + plagioclase; accessory minerals include garnet, magnetite, epidote, apatite, zircon, and pyrite. Micaceous metagraywacke appears to be a more quartzofeldspathic element of a pelitic sedimentary sequence. Unit thickness is unknown because the base is nowhere exposed; the Accotink is intruded by Occoquan Granite.

Bacons Castle Formation (Tertiary) at surface, covers 1 % of this area

Tb1/Tb2 Bacons Castle Formation (upper Pliocene, Coch, 1965). Gray, yellowish-orange, and reddish-brown sand, gravel, silt, and clay; constitutes surficial deposits of high plain extending from Richmond, eastward to the Surry scarp. Unit is subdivided into two members: Tb1, massive to thick-bedded pebble and cobble gravel grading upward into cross-bedded, pebbly sand and sandy and clayey silt, and Tb2, predominantly thin-bedded and laminated clayey silt and silty fine-grained sand. Tb2 is characterized by flaser, wavy, and lenticular bedding and rare to common clay-lined burrows including Ophiomorpha nodosa. Thickness is 0 to 70 feet.

Annandale Group - Lake Barcroft Metasandstone (Proterozoic Z-Cambrian) at surface, covers 1 % of this area

Lake Barcroft Metasandstone (Drake and Lyttle, 1981). Light-greenish-gray to light-gray, medium-grained meta-arenite; and, very-light-gray, fine- to medium-grained metagraywacke. Meta-arenite beds are massive, up to 2 m thick, and probably result from sedimentary amalgamation; metagraywacke beds are regular, sharp, and flat based, containing graded and parallel-laminated intervals; thickness ranges from 10 to 15 cm. Mineralogy: (meta-arenite), quartz + epidote + plagioclase + chlorite + muscovite + magnetite; (metagraywacke), quartz + biotite + muscovite + plagioclase + garnet + epidote + magnetite. Unit is about 1200 feet thick; grades downward into Accotick schist.

Alluvium (Quaternary) at surface, covers 1 % of this area

Fine to coarse gravelly sand and sandy gravel, silt, and clay, light- to medium-gray and yellowish-gray. Deposited mainly in channel, point-bar, and flood-plain environments; includes sandy deposits of narrow estuarine beaches, and mud, muddy sand, and peat in swamps and in fresh- and brackish-water marshes bordering tidewater rivers. Grades into colluvium along steeper valley walls at margins of unit. Mostly Holocene but, locally, includes low-lying Pleistocene (?) terrace deposits. As much as 80 feet thick along major streams.

Popes Head Formation - Station Hills Phyllite Member (Cambrian-Ordovician) at surface, covers 1 % of this area

Light-greenish-gray, dusky yellow-weathering phyllite and lesser amounts of very-fine grained metasiltstone. Beds are 2 to 12 cm thick; many have thin basal intervals of graded siltstone. Mineralogy: muscovite + quartz + biotite + chlorite + plagioclase + magnetite + epidote. The top of the unit is nowhere exposed; maximum known thickness is 300 m. Some chlorite-rich phyllite is probably mafic metatuff; unit grades down into Old Mill Branch Metasiltstone.

Miocene Sand and Gravel (Tertiary) at surface, covers 1 % of this area

Fine- to coarse-grained sand, sandy gravel, silt, and clay, gray to light-yellowish-gray, commonly oxidized to yellowish-orange and yellowish-brown; pebbles and cobbles are deeply etched. Commonly caps interfluves at northwestern edge of Coastal Plain and constitutes thin Coastal Plain outliers in easternmost Piedmont where deposits directly overlie weathered crystalline rocks. In part, may represent a fluvial to marginal-marine facies of the Choptank Formation. Thickness is 0 to 30 feet.

Ultramafic Rocks (Proterozoic) at surface, covers 0.3 % of this area

Ultramafic rocks as exotic blocks within melange units

Windsor Formation (Tertiary-Quaternary) at surface, covers 0.3 % of this area

Windsor Formation (lower Pleistocene or upper Pliocene, Coch, 1968). Gray and yellowish- to reddish-brown sand, gravel, silt, and clay. Constitutes surficial deposits of extensive plain (altitude 85-95 feet.) seaward of Surry scarp and of coeval, fluvial-estuarine terraces west of scarp. Fining upward sequence beneath plain consists of a basal pebbly sand grading upward into cross-bedded, quartzose sand and massive, clayey silt and silty clay; lower and upper parts of sequence were deposited, respectively, in shallow-marine or open-bay and restricted-bay or lagoonal environments. In terraces west of Surry scarp, fluvial-estuarine deposits comprise muddy, coarse, trough cross-bedded sand and gravel grading upward to sandy silt and clay. Thickness is 0 to 40 feet.

Quaternary and Tertiary Deposits (Tertiary-Quaternary) at surface, covers 0.2 % of this area

Quaternary and Tertiary deposits, undifferentiated. Tabb through Windsor Formations and alluvial/tidal prism deposits.

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

Schist as exotic blocks within melange units

Chopawamsic Formation - Interlayered felsic and mafic metavolcanic rocks (Cambrian) at surface, covers 0.2 % of this area

Chopawamsic Formation, undivided, (Pavlides, 1981). Includes laterally discontinuous lenses and tongues of metamorphosed felsic, intermediate, and mafic volcanic flows and volcanoclastic rocks, with interlayered quartzite, quartzose graywacke, schist, and phyllite. Volcanic flows are locally highly vesicular; fragmental breccia and tuff are common. Felsic flows are typically light-gray aphanitic rocks with phenocrysts of quartz and feldspar; intermediate flows are dark-green amphibole-bearing rocks with fine-grained quartz-feldspar matrix; greenstone metabasalts contain blue green amphibole, chlorite, albitic plagioclase, and quartz. Geophysical signature: linear strike-elongate pattern of elevated magnetic anomalies. The Chopawamsic is correlated with the James Run Formation in Maryland; the James Run has been dated at 570 to 530 Ma (U-Pb zircon; Tilton and others 1970). The Chopawamsic is unconformably overlain by the Late Ordovician Arvonia and Quantico Formations. Pavlides (1981 and subsequent works) has made the interpretation on the basis of geologic and geochemical data that the Chopawamsic and related plutons represent an ancient is land-arc sequence.

Yorkshire Formation (Cambrian) at surface, covers 0.2 % of this area

Yorkshire Formation (Drake and Morgan, 1981). Polygenetic melange consisting of a dark quartz-plagioclase chlorite matrix that contains chips, fragments, and small blocks of quartz, ultramafic rock, metagabbro, plagiogranite, mafic volcanic rock, and other exotic blocks. Characterized by abundant light-colored feldspar grains that contrast with the dark-colored phyllosilicate component. The Yorkshire forms thin, discontinuous lenses at the base of the Piney Branch Complex.

Charles City Formation (Quaternary) at surface, covers 0.2 % of this area

Charles City Formation (lower Pleistocene (?), Johnson and Berquist, 1989). Light- to medium-gray and light-to dark yellowish and reddish-brown sand, silt, and clay composing surficial deposits of riverine terraces and coast-parallel plains at altitiudes of 70 to 80 feet. Unit is adjacent to, and inset below, the Windsor Formation and older deposits. Bay or shallow shelf facies of the Charles City (Johnson and Peebles, 1984), present beneath flat to gently seaward-sloping plain in Suffolk area, includes a thin, basal, gravelly sand grading upward into fine- to medium-grained sand and an uppermost clayey and sandy silt; lower and middle parts of unit contain clay-lined, sand-filled burrows. Fluvial-estuarine facies in terrace remnants along major rivers consists of cross-bedded gravelly sand and clayey silt. Thickness is 0 to 55 feet, or more.

Quantico Formation - Slate and porphyroblastic schist (Ordovician) at surface, covers 0.1 % of this area

Gray to black, graphitic, pyritic phyllite and slate (northern Piedmont); metamorphic grade increases to the southwest to produce porphyroblastic staurolite-, kyanite-, and garnet-biotite muscovite schists. Locally the unit contains felsic metatuff, metagraywacke, and micaceous quartzite interbeds; thickness has been estimated at as much as 3000 feet (Pavlides, 1980). Mineralogy: quartz + muscovite + biotite ± garnet ± staurolite ± kyanite + opaque minerals; chlorite is a common secondary mineral. Geophysical signature: strike-elongated positive linear magnetic and radiometric anomalies. The unit was originally named Quantico Slate by Darton (1894), and modified to Quantico Formation by Pavlides (1980). An Ordovician age for the Quantico is indicated by fossils collected by Watson and Powell (1911) and more recently by Pavlides and others (1980). The Quantico unconformably overlies older units in the northeastern Pied mont, and is correlated with the Arvonia Formation to the southwest.

Georgetown Intrusive Suite (Cambrian) at surface, covers 0.1 % of this area

Georgetown Intrusive Suite (Flemming and others, in press). Tonalite, quartz gabbro, quartz diorite, metapyroxenite, and hornblendite. Tonalite, medium- to coarse-grained, massive to strongly-foliated, biotite-, biotite-hornblende-, and garnet-biotite-hornblende-bearing; color index 20 to 40; contains abundant mafic inclusions. Quartz gabbro and quartz diorite, mostly medium- to coarse-grained quartz-augite-hornblende metagabbro, lesser quartz diorite, and much lesser quartz norite; commonly contains thin cumulus layers of metapyroxenite and augite-hornblende fels. Forms small to medium-sized layered sills and localized border zones of tonalite plutons. Metapyroxenite and hornblendite, dark-green to black, massive to well-foliated, medium- to coarse-grained; includes serpentinite, soapstone, and talc schist. Forms small pods and xenolith swarms within or along the borders of larger tonalite and quartz gabbro plutons.

Chuckatuck Formation (Quaternary) at surface, covers < 0.1 % of this area

Chuckatuck Formation (middle(?) Pleistocene, Johnson and Berquist, 1989). Light- to medium-gray, yellowish-orange, and red dish-brown sand, silt, and clay and minor amounts of dark-brown and brownish-black peat. Comprises surficial deposits of mid-level coast-parallel plains (altitude 50-60 feet) and equivalent riverine terraces. Eastward, unit is truncated by the Suffolk scarp; westward, along major stream drainages, unit is separated from the younger topographically lower Shirley Formation by the Kingsmill scarp and equivalent estuarine scarps. Fluvial-estuarine facies includes, from bottom to top, (1) channel-fill deposits of poorly sorted, cross-bedded, pebbly and cobbly sand interbedded, locally, with peat and sandy silt rich in organic matter, (2) moderately well-sorted, cross-bedded to planar bedded, fine- to medium-grained sand grading up ward into (3) clayey silt and sandy and silty clay. Bay facies of coastwise plain includes a basal gravelly sand filling shallow paleochannels, a thin but extensive pebbly sand containing heavy mineral laminae and Ophiomorpha burrows, and an upper, relatively thick, medium- to fine-grained silty sand and sandy silt. Thickness is 0 to 26 feet.

Phyllite (Cambrian-Ordovician) at surface, covers < 0.1 % of this area

Phyllite (Pavlides, 1990): Mostly gray-to-green phyllite with lesser metasiltstone. Mylonitic rocks composed commonly of schist or phyllite with elongate granules of quartz, occur in the southern part of the outcrop belt. These are interpreted as highly tectonized parts of this formation. Alternatively, these rocks may be part of a separate terrane.

Bear Island Granodiorite (Ordovician) at surface, covers < 0.1 % of this area

Bear Island Granodiorite (Cloos and Cooke, 1953; Drake and Lee, 1989). Leucocratic, fine-grained, muscovite biotite granodiorite and related quartz-albite-microcline pegmatite. Unit comprises small- to moderate-sized sheets and crosscutting bodies. Muscovite from pegmatite dated at 469±20 Ma, interpreted as time of cooling below 5000 C (Rb-Sr; Muth and others, 1979).

Dalecarlia Instrusive Suite (Cambrian-Ordovician) at surface, covers < 0.1 % of this area

Dalecarlia Intrusive Suite (Drake and Flemming, in press). Medium- to coarse-grained, massive to well-foliated, biotite monzogranite and lesser granodiorite; locally contains plagioclase phenocrysts. Mapped bodies contain widespread leucocratic biotite-muscovite monzogranite.

Metagabbro (Proterozoic) at surface, covers < 0.1 % 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.

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

Meta-argillite is metagraywacke, exotic blocks within melange units

Wissahickon Formation (Undivided) (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Muscovite-chlorite-albite schist, muscovite-chlorite schist, chloritoid schist, and quartzite; intensely folded and cleaved.

Wissahickon Formation; Boulder Gneiss (Late Precambrian (?)) at surface, covers < 0.1 % of this area

(Formerly mapped as Sykesville and Laurel Formations.) Thick-bedded to massive, pebble- and boulder-bearing, arenaceous to pelitic metamorphic rock; typically a medium-grained, garnet-oligoclase-mica-quartz gneiss; locally an intensely foliated gneiss or schist; apparent thickness 15,000 feet.

Tabb Formation; Undifferentiated (Quaternary) at surface, covers < 0.1 % of this area

Tabb Formation, undifferentiated (upper Pleistocene, Johnson, 1976). Sand, silt, and peat of coast-parallel plains seaward of the Suffolk and Harpersville scarps, includes coeval terrace deposits along major river valleys west to Fall Line. Subdivided into three members (Johnson, 1976).