Geologic units in Lancaster county, Pennsylvania

Conestoga Formation (Ordovician and Cambrian) at surface, covers 13 % 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.

Octoraro Formation (Probably lower Paleozoic) at surface, covers 11 % of this area

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

Peters Creek Schist (Probably lower Paleozoic) at surface, covers 9 % of this area

Chlorite-sericite schist containing interbedded quartzite.

Ledger Formation (Cambrian) at surface, covers 7 % of this area

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

Hammer Creek Formation (Triassic) at surface, covers 7 % of this area

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

Cocalico Formation (Ordovician) at surface, covers 6 % of this area

Gray phyllitic shale, maroon shale, siltstone, and silty, siliceous shale; some interbedded argillaceous and quartzose sandstone; predominantly allochthonous, and probably closely related to Hamburg sequence, but includes some autochthonous elements.

Epler Formation (Ordovician) at surface, covers 6 % of this area

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

New Oxford Formation (Triassic) at surface, covers 5 % of this area

Light-gray to buff, commonly arkosic sandstone interbedded with red shale, mudstone, and fine-grained sandstone

Zooks Corner Formation (Cambrian) at surface, covers 4 % of this area

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

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

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

Antietam and Harpers Formations, undivided (Cambrian) at surface, covers 3 % 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

Stonehenge Formation (Ordovician) at surface, covers 3 % of this area

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

Buffalo Springs Formation (Cambrian) at surface, covers 3 % 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.

Felsic to mafic gneiss (Precambrian) at surface, covers 2 % of this area

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

Hammer Creek conglomerate (Triassic) at surface, covers 2 % of this area

Cobble and pebble quartz conglomerate interbedded with red sandstone.

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.

Snitz Creek and Buffalo Springs Formations, undivided (Cambrian) at surface, covers 2 % of this area

Includes, in descending order, the Snitz Creek (CAsc) and Buffalo Springs (CAbs) Formations. Snitz Creek Formation - thick-bedded. medium- to coarsely crystalline dolomite; in part oolitic, containing laminated limestone and sandstone interbeds. Buffalo Springs Formation - 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.

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.

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

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

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

Base--dark-brown shale; middle--gray and white spotted limestone and, locally, marble having irregular partings; top--sandy limestone which weathers to a fine-grained, friable, porous, sandy mass.

Antietam Formation (Cambrian) at surface, covers 0.9 % of this area

Gray, buff-weathering quartzite and quartz schist.

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

Reddish-brown to maroon, silty mudstone and shale containing thin red sandstone interbeds; several thin beds of impure limestone.

Ontelaunee Formation (Ordovician) at surface, covers 0.6 % 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.

New Oxford conglomerate (Triassic) at surface, covers 0.6 % of this area

Quartz or quartzite pebbles, cobbles, and rare boulders set in a red, sandy, ferruginous matrix; some silica cement; some feldspar clasts.

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

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

Banded mafic gneiss (Precambrian) at surface, covers 0.5 % of this area

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

Felsic and intermediate gneiss (Precambrian) at surface, covers 0.4 % of this area

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

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

Dark-greenish-gray phyllite and schist containing thin quartzite layers.

Ultramafic rocks (Probably lower Paleozoic) at surface, covers 0.4 % of this area

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

Graphitic felsic gneiss (Precambrian) at surface, covers 0.3 % 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.

Peach Bottom Slate and Cardiff Conglomerate, undivided (Probably lower Paleozoic) at surface, covers 0.3 % of this area

Bluish-black slate (Peach Bottom); quartz conglomerate having a matrix of sericite and chlorite (Cardiff).

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

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

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

Includes, in descending order, the Hershey and Myerstown Formations, undivided (Ohm) and Annville Formation (Oan).

Hornblende gneiss (Precambrian) at surface, covers 0.1 % of this area

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

Hershey and Myerstown Formations, undivided (Ordovician) at surface, covers 0.1 % 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.

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

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

Marburg Schist (Probably lower Paleozoic) at surface, covers 0.1 % of this area

Gray-green phyllite, mica-chlorite schist, and conglomerate.

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.

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

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

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.

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

Ranges from fine- to medium-grained, altered gabbro to hornblende gneiss.

Sams Creek Metabasalt (Probably lower Paleozoic) at surface, covers < 0.1 % of this area

Altered basaltic flows, some amygdaloidal; green, schistose, containing hornblende, epidote, chlorite, and quartz.

Martinsburg Formation (Ordovician) at surface, covers < 0.1 % 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.

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

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

Hardyston Formation (Cambrian) at surface, covers < 0.1 % 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.