Geologic units in Berks county, Pennsylvania

Shale and graywacke of Hamburg sequence (Ordovician) at surface, covers 20 % of this area

Shale containing zones of conspicuous graywacke.

Hamburg sequence rocks (Ordovician) at surface, covers 15 % of this area

Predominantly greenish gray, gray, purple, and maroon shale, siltstone, and graywacke; includes some wildflysch having Martinsburg matrix.

Felsic to mafic gneiss (Precambrian) at surface, covers 10 % of this area

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

Brunswick Formation (Triassic) at surface, covers 6 % 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.

Hammer Creek conglomerate (Triassic) at surface, covers 6 % of this area

Cobble and pebble quartz conglomerate interbedded with red sandstone.

Allentown Formation (Cambrian) at surface, covers 5 % of this area

Medium- to medium-dark-gray, thick-bedded dolomite and impure limestone; dark-gray chert stringers and nodules; laminated; oolitic and stromatolitic; some orange-brown-weathering calcareous siltstone at base.

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

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

Shawangunk Formation (Silurian) at surface, covers 3 % of this area

Light- to dark-gray, fine- to very coarse grained sandstone and conglomerate containing thin shale interbeds. Includes four members, in descending order: Tammany--conglomerate and sandstone; Lizard Creek--sandstone and red or green shale; Minsi--sandstone and conglomerate; Weiders--conglomerate. Tammany and Lizard Creek Members together are approximately equivalent to Clinton Group to the west; Minsi and Weiders Members together are equivalent to Tuscarora Formation to the west.

Hardyston Formation (Cambrian) at surface, covers 3 % of this area

Typically light-gray, fine- to medium-grained quartzite, and feldspathic sandstone; color ranges from nearly white to dark gray; massive bedded; Scolithus present in upper part; quartz-pebble conglomerate occurs at base.

Leithsville Formation (Cambrian) at surface, covers 3 % of this area

Medium- to dark-gray, crystalline dolomite, light-olive-gray in places, weathering to light gray and yellowish brown; massive bedded; oolitic; pink to gray, mottled chert and dark-gray chert; thin shale and dolomitic shale interbeds; scattered sand grains; upper part is very shaly.

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

Dark, medium grained; includes some rocks of probable sedimentary origin.

Diabase (Jurassic) at surface, covers 2 % 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.

Epler Formation (Ordovician) at surface, covers 2 % of this area

Very finely crystalline, light-gray limestone interbedded with gray dolomite; coarsely crystalline limestone lenses present.

Ontelaunee Formation (Ordovician) at surface, covers 2 % of this area

Light- to dark-gray, very finely to medium-crystalline dolomite containing interbeds of light-gray limestone; interbedded nodular, dark-gray chert at base.

Jacksonburg Formation (Ordovician) at surface, covers 2 % of this area

Dark-gray shaly limestone (cement rock) having slaty cleavage; basal medium- to thick-bedded limestone (cement limestone) increases in thickness eastward.

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

Gray dolomite, in part oolitic, interbedded with medium-gray limestone and dark-gray oolitic chert.

Martinsburg Formation (Ordovician) at surface, covers 1 % of this area

Dark-gray, slaty to phyllitic shale, calcareous at base. Probably only lower part of Martinsburg Formation is present; overlain by Hamburg sequence rocks.

Beekmantown Group (Ordovician) at surface, covers 1 % of this area

Includes, in descending order, the Ontelaunee (Oo), Epler (Oe), Rickenbach (Ori), and Stonehenge (Os) Formations.

Limestone of Hamburg sequence (Ordovician) at surface, covers 1 % of this area

Conspicuous limestone.

Hershey and Myerstown Formations, undivided (Ordovician) at surface, covers 1 % of this area

In descending order: Hershey--dark-gray to black, thin-bedded, argillaceous limestone; Myerstown--medium- to dark-gray, platy, medium-crystalline limestone; carbonaceous at base.

Martinsburg Formation (Ordovician) at surface, covers 1 % of this area

Gray to dark-gray shale and slate.

Limestone fanglomerate (Triassic) at surface, covers 0.9 % 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.

Rickenbach Formation (Ordovician) at surface, covers 0.9 % of this area

Gray, very finely to coarsely crystalline, laminated dolomite; dark-gray chert in irregular beds, stringers, and nodules; bands of quartz sand grains in lower half.

Stockton Formation (Triassic) at surface, covers 0.8 % of this area

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

Graywacke and shale of Martinsburg Formation (Ordovician) at surface, covers 0.7 % of this area

Shale containing conspicuous graywacke; includes autochthonous sandstone and shale of Shochary Ridge.

Millbach Formation (Cambrian) at surface, covers 0.5 % of this area

Pink to white and gray, finely laminated limestone and interbedded finely crystalline dolomite; numerous stromatolitic limestone beds.

Lower (Middle?) Cambrian rocks, undivided (Cambrian) at surface, covers 0.5 % of this area

Lower Cambrian of Berks County includes tectonic slices of many of the following rock units: Zooks Corner (CAzc), Ledger (CAl), Kinzers (CAk), Vintage (CAv), Antietam (CAa, CAah), and Harpers (CAh, CAah) Formations.

Graphitic felsic gneiss (Precambrian) at surface, covers 0.5 % 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.

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

Light-gray to pinkish-gray, finely to coarsely crystalline limestone and interbedded dolomite; numerous siliceous and clayey laminae; stromatolitic limestone beds near top; some thin sandy beds.

Stonehenge Formation (Ordovician) at surface, covers 0.3 % of this area

Gray, finely crystalline limestone containing dark-gray silty laminations; numerous edgewise conglomerate beds.

Annville Formation (Ordovician) at surface, covers 0.2 % of this area

Light-gray, massive, high-calcium limestone; mottled at base.

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

Medium-gray, finely crystalline dolomite; silty to sandy, containing numerous siliceous to argillaceous laminae.

Bloomsburg Formation (Silurian) at surface, covers 0.1 % of this area

Grayish-red and greenish-gray shale, siltstone, and very fine to coarse-grained sandstone; some calcareous mudstone in central Pennsylvania; thins to west and is replaced by Mifflintown beds; thickens eastward, replacing overlying Wills Creek and Tonoloway Formations and underlying Mifflintown Formation.

Felsic and intermediate gneiss (Precambrian) at surface, covers 0.1 % of this area

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

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

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

Antietam and Harpers Formations, undivided (Cambrian) at surface, covers < 0.1 % 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

Graywacke of Hamburg sequence (Ordovician) at surface, covers < 0.1 % of this area

Predominantly graywacke.

Millbach and Schaefferstown Formations, undivided (Cambrian) at surface, covers < 0.1 % of this area

Includes, in descending order, the Millbach Formation (CAm), and the Schaefferstown Formation--gray limestone containing siliceous and argillaceous laminae; thin bedded.

Sedimentary strata at Jacksonwald and Aspers (Jurassic) at surface, covers < 0.1 % of this area

Arkosic sandstone; fossiliferous, gray to black shale and limestone; ripple-cross-laminated siltstone; and boulder conglomerate.

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

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

Juniata and Bald Eagle Formations, undivided (Ordovician) at surface, covers < 0.1 % of this area

Includes, in descending order, the Juniata (Oj) and Bald Eagle (Obe) Formations.

Lockatong Formation (Triassic) at surface, covers < 0.1 % 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.

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.

Graphitic felsic gneiss (Precambrian) at surface, covers < 0.1 % of this area

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

Stonehenge Formation (Ordovician) at surface, covers < 0.1 % of this area

Medium-light-gray to medium-gray, finely crystalline, thick-bedded limestone, containing dark siliceous laminae, edgewise conglomerate beds, and fossil-fragment lenses; dolomite beds increase in number eastward.

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.

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

Thick-bedded, medium- to coarsely crystalline dolomite, in part oolitic, containing laminated limestone and sandstone interbeds.