Geologic units in Philadelphia county, Pennsylvania

Wissahickon Formation (Probably lower Paleozoic) at surface, covers 43 % 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.

Trenton Gravel (Quaternary) at surface, covers 30 % of this area

Gray or pale-reddish-brown, very gravelly sand interstratified with crossbedded sand and clay-silt beds; includes areas of Holocene alluvium and swamp deposits.

Pensauken and Bridgeton Formations, undifferentiated (Tertiary) at surface, covers 23 % 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.

Granitic gneiss and granite (Probably lower Paleozoic) at surface, covers 2 % of this area

Includes Springfield Granodiorite (granitized Wissahickon) in Philadelphia area.

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.

Chickies Formation (Cambrian) at surface, covers 0.3 % of this area

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

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.

Felsic gneiss (Precambrian) at surface, covers 0.2 % of this area

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

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.

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

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

Conestoga Formation (Ordovician and Cambrian) at surface, covers < 0.1 % 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.

Potomac Formation, unit 3 (Upper Cretaceous, lower Cenomanian) at surface, covers < 0.1 % of this area

Sand, fine- to coarse-grained, locally gravelly, crossbedded, light-colored, interbedded with white or variegated red and yellow, massive clay, and rarely dark-gray, woody clay. The Potomac Formation crops out only in the Delaware River valley where the river and its tributaries have eroded away the overlying formations. The Potomac has been mapped in a broad belt parallel to the inner edge of the Coastal Plain. Although mapped in a broad belt, the Potomac is very poorly exposed because of the widespread cover of surficial sediments. The best exposures occur where surficial material is mined away in the Camden area. Unit is about 45 m (148 ft) thick. Contact with the overlying Magothy Formation is difficult to pick where the basal Magothy also contains variegated clays. Most of the basal Magothy has more dark-colored clay, and the contact was drawn by using this criterion. The basal contact of the Potomac with the underlying crystalline rock is not exposed in New Jersey. Biostratigraphically, the Potomac has been separated into pollen zones I, II, and III (Doyle, 1969; Doyle and Robbins, 1977). Samples from the Potomac Formation in the Camden area and along the Delaware River nearby contain pollen assemblages of early Cenomanian age (Zone III) (Les Sirkin, written commun., 1988).

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.