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

Geologic units in Montgomery county, Pennsylvania

Antietam and Harpers Formations, undivided (Cambrian) at surface, covers 0.2 % 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

Chickies Formation (Cambrian) at surface, covers 2 % 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

Octoraro Formation (Probably lower Paleozoic) at surface, covers 0.8 % of this area

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

Lithology: schist; phyllite; gneiss; granitoid

Patapsco(?) Formation (Cretaceous) at surface, covers 0.4 % of this area

Patapsco(?) Formation - Intensely colored, variegated, ferruginous clay and, in places, beds of sand; occurs in isolated patches.

Lithology: clay or mud; sand

Limestone fanglomerate (Triassic) at surface, covers < 0.1 % of this area

Limestone fanglomerate - Yellowish-gray to medium-gray, angular limestone and dolomite pebbles, cobbles, and fragments set in a red, very fine grained quartz matrix; a few shale-clast interbeds.

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

Quartz fanglomerate (Triassic) at surface, covers < 0.1 % of this area

Quartz fanglomerate - Well-rounded quartzite pebbles, cobbles, and rare boulders set in a reddish-brown, sandy matrix.

Lithology: conglomerate; quartzite; sand

Granitic gneiss and granite (Probably lower Paleozoic) at surface, covers 0.8 % of this area

Granitic gneiss and granite - Includes Springfield Granodiorite (granitized Wissahickon) in Philadelphia area.

Lithology: granitic gneiss; granite; granodiorite

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)

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

Wissahickon Formation - Includes oligoclase-mica schist, some hornblende gneiss, some augen gneiss, and some quartz-rich and feldspar-rich members due to various degrees of granitization.

Lithology: mica schist; mafic gneiss; augen gneiss; granitoid

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

Brunswick Formation (Triassic) at surface, covers 44 % 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

Mafic gneiss (Precambrian) at surface, covers 0.6 % 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 11 % 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

Stockton Formation (Triassic) at surface, covers 18 % 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 0.2 % 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

Stockton conglomerate (Triassic) at surface, covers < 0.1 % of this area

Stockton conglomerate - Quartz cobbles set in a poorly sorted, sandy matrix; includes conglomeratic sandstone.

Lithology: conglomerate; sand; sandstone

Bryn Mawr Formation (Tertiary) at surface, covers 0.2 % of this area

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

Lithology: gravel; sand; silt

Elbrook Formation (Cambrian) at surface, covers 0.8 % 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

Ultramafic rocks (Probably lower Paleozoic) at surface, covers 0.1 % of this area

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

Lithology: serpentinite; pyroxenite; peridotite

Diabase (Jurassic) at surface, covers 6 % 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

Mafic gneiss (Probably lower Paleozoic) at surface, covers < 0.1 % 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

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

"Glenarm Wissahickon" formation (Probably lower Paleozoic) at surface, covers < 0.1 % 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;

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