Geologic units in Montgomery county, Pennsylvania

Brunswick Formation (Triassic) at surface, covers 44 % 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 18 % of this area

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

Lockatong Formation (Triassic) at surface, covers 11 % 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.

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

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

Diabase (Jurassic) at surface, covers 6 % 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.

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

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

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.

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

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

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

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

Elbrook Formation (Cambrian) at surface, covers 0.8 % 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.

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

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

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

Includes Springfield Granodiorite (granitized Wissahickon) in Philadelphia area.

Mafic gneiss (Precambrian) at surface, covers 0.6 % of this area

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

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

Intensely colored, variegated, ferruginous clay and, in places, beds of sand; occurs in isolated patches.

Pensauken and Bridgeton Formations, undifferentiated (Tertiary) at surface, covers 0.2 % 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.

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

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

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

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

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

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

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

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

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

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

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

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.

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

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

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

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