Geologic units in Delaware county, Pennsylvania

Wissahickon Formation (Probably lower Paleozoic) at surface, covers 26 % 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.

Felsic and intermediate gneiss (Precambrian) at surface, covers 18 % of this area

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

Trenton Gravel (Quaternary) at surface, covers 10 % 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 9 % 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.

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

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

Glenarm Wissahickon"" formation (Probably lower Paleozoic) at surface, covers 7 % 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.

Mafic gneiss (Probably lower Paleozoic) at surface, covers 5 % of this area

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

Bryn Mawr Formation (Tertiary) at surface, covers 3 % of this area

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

Ultramafic rocks (Probably lower Paleozoic) at surface, covers 3 % of this area

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

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

Includes Springfield Granodiorite (granitized Wissahickon) in Philadelphia area.

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

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

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

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

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

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

Octoraro Formation (Probably lower Paleozoic) at surface, covers 0.6 % of this area

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

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

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

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).

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.

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

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