Geologic units in Chester county, Pennsylvania

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

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

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

Chlorite-sericite schist containing interbedded quartzite.

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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.

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

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

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

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

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

Includes white feldspathic quartzite, gray mica gneiss, and mica schist.

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

Light, medium grained; predominantly quartz and feldspar of igneous origin.

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

Cobble and pebble quartz conglomerate interbedded with red sandstone.

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

White to light bluish gray, finely to coarsely crystalline.

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

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.

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

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

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

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

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

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

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

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.

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

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.

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

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

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

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

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

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

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

Gray, buff-weathering quartzite and quartz schist.

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

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.

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

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

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

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

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

White, coarsely crystalline; disseminated graphite flakes.

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

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

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

Dense micaceous schist, gneiss and migmatite.

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

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

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

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

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

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

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

Amphibolite

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

Pegmatite

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

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