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Mineral Resources > Online Spatial Data > Geology > by state > Pennsylvania Geology

Geologic units in Chester county, Pennsylvania

Felsic to mafic gneiss (Precambrian) at surface, covers 2 % of this area

Felsic to mafic gneiss - Light, medium grained; predominantly quartz and feldspar of igneous origin.

Lithology: felsic gneiss; mafic gneiss; orthogneiss

Wissahickon Schist (Paleozoic) at surface, covers < 0.1 % of this area

Wissahickon Schist - Dense micaceous schist, gneiss and migmatite.

Lithology: mica schist; gneiss; migmatite

Conestoga Formation (Ordovician and Cambrian) at surface, covers 2 % of this area

Conestoga Formation - Light-gray, thin-bedded, impure, contorted limestone having shale partings; conglomeratic at base; in Chester Valley, includes micaceous limestone in upper part, phyllite in middle, and alternating dolomite and limestone in lower part.

Lithology: limestone; phyllite; conglomerate; dolostone (dolomite); shale

Felsic and intermediate gneiss (Precambrian) at surface, covers 6 % of this area

Felsic and intermediate gneiss - Light, medium grained; includes rocks of probable sedimentary origin.

Lithology: felsic gneiss; mafic gneiss; paragneiss

Felsic gneiss (Precambrian) at surface, covers 4 % of this area

Felsic gneiss - Light, medium grained; includes rocks of probable sedimentary origin.

Lithology: felsic gneiss; paragneiss

"Glenarm Wissahickon" formation (Probably lower Paleozoic) at surface, covers 24 % of this area

"Glenarm Wissahickon" formation - Lithologically similar to oligoclase-mica schist of the Wissahickon Formation (PZw), but also includes lenticular amphibolite bodies having ocean-floor basalt chemistry.

Lithology: mica schist; mafic gneiss; amphibolite;

Bryn Mawr Formation (Tertiary) at surface, covers < 0.1 % of this area

Bryn Mawr Formation - High-level terrace deposits; reddish-brown gravelly sand and some silt. Age uncertain.

Lithology: gravel; sand; silt

Stockton Formation (Triassic) at surface, covers 3 % of this area

Stockton Formation - Light-gray to buff, coarse-grained, arkosic sandstone; includes reddish-brown to grayish-purple sandstone, siltstone, and mudstone.

Lithology: arkose; siltstone; sandstone; mudstone

Pensauken and Bridgeton Formations, undifferentiated (Tertiary) at surface, covers < 0.1 % of this area

Pensauken and Bridgeton Formations, undifferentiated - Dark-reddish-brown, cross-stratified, feldspathic quartz sand and some thin beds of fine gravel and rare layers of clay or silt.

Lithology: sand; gravel; clay or mud; silt

Vintage Formation (Cambrian) at surface, covers 0.4 % of this area

Vintage Formation - Dark-gray, knotty, argillaceous dolomite; impure light-gray marble at base locally.

Lithology: dolostone (dolomite); marble

Hammer Creek Formation (Triassic) at surface, covers < 0.1 % of this area

Hammer Creek Formation - Gray and pale red, fine- to coarse-grained quartzose sandstone, siltstone, and mudstone.

Lithology: sandstone; siltstone; mudstone

Mafic gneiss (Probably lower Paleozoic) at surface, covers 0.8 % of this area

Mafic gneiss - Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to pCAmgh in places.

Lithology: mafic gneiss; paragneiss

Metagabbro (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Metagabbro - Ranges from fine- to medium-grained, altered gabbro to hornblende gneiss.

Lithology: mafic gneiss; gabbro

Hammer Creek conglomerate (Triassic) at surface, covers 1 % of this area

Hammer Creek conglomerate - Cobble and pebble quartz conglomerate interbedded with red sandstone.

Lithology: conglomerate; sandstone

Mafic gneiss (Precambrian) at surface, covers 0.9 % of this area

Mafic gneiss - Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to "PZmgp" in places.

Lithology: mafic gneiss; paragneiss

Diabase (Jurassic) at surface, covers 0.7 % of this area

Diabase - Medium- to coarse-grained, quartz-normative tholeiite; composed of labradorite and various pyroxenes; occurs as dikes, sheets, and a few small flows. Includes the dark-gray York Haven Diabase (high titanium oxide) and the slightly younger Rossville Diabase (low titanium oxide). In chilled margins, the Rossville is distinguished from the York Haven by its lighter gray color and distinctive, sparse, centimeter-sized calcic-plagioclase phenocrysts.

Lithology: diabase

Ultramafic rocks (Probably lower Paleozoic) at surface, covers 2 % of this area

Ultramafic rocks - Includes serpentine, steatite, and other products of alteration of peridotites and pyroxenites.

Lithology: serpentinite; pyroxenite; peridotite

Elbrook Formation (Cambrian) at surface, covers 1 % of this area

Elbrook Formation - Microcrystalline limestone and, in places, marble; includes subordinate dolomite containing abundant phyllitic layers; occurs in Chester and Montgomery Counties; relation to Elbrook of Cumberland Valley sequence is uncertain.

Lithology: limestone; dolostone (dolomite); phyllite; marble

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

Metabasalt - Dark-gray, fine-grained intrusives; locally, mineralogy is altered and unit has greenish color.

Lithology: meta-basalt

Felsic and intermediate gneiss (Precambrian) at surface, covers 8 % of this area

Felsic and intermediate gneiss - Medium grained, light pink to greenish gray; largely quartz, feldspar, and mica; commonly gneissic, containing alteration minerals; interfingers with gabbroic gneiss.

Lithology: felsic gneiss; gneiss

Banded mafic gneiss (Precambrian) at surface, covers 3 % of this area

Banded mafic gneiss - Dark, fine to medium grained; includes rocks of probable sedimentary origin; may be equivalent to "PZmgh."

Lithology: mafic gneiss; paragneiss

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

Ledger Formation - Light-gray, locally mottled, massive, pure, coarsely crystalline dolomite; siliceous in middle part.

Lithology: dolostone (dolomite)

Amphibolite (Paleozoic) at surface, covers < 0.1 % of this area

Amphibolite - amphibolite

Lithology: amphibolite

Cockeysville Marble (Probably lower Paleozoic) at surface, covers 1 % of this area

Cockeysville Marble - White to light bluish gray, finely to coarsely crystalline.

Lithology: marble

Brunswick Formation (Triassic) at surface, covers 4 % of this area

Brunswick Formation - Reddish-brown mudstone, siltstone, and shale, containing a few green and brown shale interbeds; red and dark-gray, interbedded argillites near base. Youngest beds in Brunswick may be Jurassic in age.

Lithology: mudstone; siltstone; shale; argillite

Metadiabase (Precambrian) at surface, covers 0.1 % of this area

Metadiabase - Dark-gray, fine-grained intrusives; locally, mineralogy is altered and unit has greenish color.

Lithology: mafic metavolcanic rock

Mafic gneiss (Precambrian) at surface, covers 0.1 % of this area

Mafic gneiss - Dark, medium grained; includes rocks of probable sedimentary origin; may be equivalent to "PZmgh" in places.

Lithology: mafic gneiss; paragneiss

Lockatong Formation (Triassic) at surface, covers 0.6 % of this area

Lockatong Formation - Dark-gray to black, thick-bedded argillite containing a few zones of thin-bedded black shale; locally has thin layers of impure limestone and calcareous shale.

Lithology: argillite; shale; limestone; black shale

Kinzers Formation (Cambrian) at surface, covers 0.1 % of this area

Kinzers Formation - Base--dark-brown shale; middle--gray and white spotted limestone and, locally, marble having irregular partings; top--sandy limestone which weathers to a fine-grained, friable, porous, sandy mass.

Lithology: limestone; marble; shale; sand

Pegmatite (Paleozoic) at surface, covers < 0.1 % of this area

Pegmatite - Pegmatite

Lithology: pegmatite

Pegmatite (Probably lower Paleozoic) at surface, covers 0.2 % of this area

Pegmatite - Coarse to medium grained, granitic; contacts range from sharp to narrowly gradational; some zoning in places.

Lithology: pegmatite

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

Antietam Formation - Gray, buff-weathering quartzite and quartz schist.

Lithology: quartzite; schist

Franklin Marble (Precambrian) at surface, covers < 0.1 % of this area

Franklin Marble - White, coarsely crystalline; disseminated graphite flakes.

Lithology: marble

Graphitic felsic gneiss (Precambrian) at surface, covers 5 % of this area

Graphitic felsic gneiss - Includes Pickering Gneiss and small areas of marble; dominantly quartz and feldspar with varying amounts of graphite and various metamorphic minerals; medium grained, light to dark gray and greenish gray; sedimentary origin.

Lithology: gneiss; marble; paragneiss

Antietam and Harpers Formations, undivided (Cambrian) at surface, covers 0.7 % of this area

Antietam and Harpers Formations, undivided - Includes, in descending order, the Antietam (CAa) and Harpers (CAh) Formations. Antietam Formation - gray, buff-weathering quartzite. Harpers Formation - Dark-greenish-gray phyllite and schist containing thin quartzite layers; includes Montalto Member (CAhm) - gray quartztite

Lithology: quartzite; schist; phyllite

Octoraro Formation (Probably lower Paleozoic) at surface, covers 6 % of this area

Octoraro Formation - Includes albite-chlorite schist, phyllite, some hornblende gneiss, and granitized members.

Lithology: schist; phyllite; gneiss; granitoid

Setters Quartzite (Probably lower Paleozoic) at surface, covers 2 % of this area

Setters Quartzite - Includes white feldspathic quartzite, gray mica gneiss, and mica schist.

Lithology: quartzite; mica gneiss; mica schist

Peters Creek Schist (Probably lower Paleozoic) at surface, covers 9 % of this area

Peters Creek Schist - Chlorite-sericite schist containing interbedded quartzite.

Lithology: schist; quartzite

Graphitic felsic gneiss (Precambrian) at surface, covers 5 % of this area

Graphitic felsic gneiss - Medium grained, medium to dark gray; locally gneissic, predominantly feldspar and quartz, plus dark accessories and various alteration minerals.

Lithology: felsic gneiss; mafic gneiss

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

Harpers Formation - Dark-greenish-gray phyllite and schist containing thin quartzite layers.

Lithology: phyllite; schist; quartzite

Chickies Formation (Cambrian) at surface, covers 4 % of this area

Chickies Formation - Light-gray, hard, massive, Scolithus-bearing quartzite and quartz schist; thin, interbedded dark slate at top; conglomerate (Hellam Member) at base.

Lithology: quartzite; schist; slate; conglomerate

Anorthosite (Precambrian) at surface, covers 2 % of this area

Anorthosite - Medium to coarse grained, light to dark bluish gray, predominantly plagioclase; local alteration minerals.

Lithology: anorthosite