Includes albite-chlorite schist, phyllite, some hornblende gneiss, and granitized members.
Gray-green phyllite, mica-chlorite schist, and conglomerate.
Reddish-brown to maroon, silty mudstone and shale containing thin red sandstone interbeds; several thin beds of impure limestone.
Light-gray to buff, commonly arkosic sandstone interbedded with red shale, mudstone, and fine-grained sandstone
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
Light-gray, hard, massive, Scolithus-bearing quartzite and quartz schist; thin, interbedded dark slate at top; conglomerate (Hellam Member) at base.
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.
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.
Gray quartz conglomerate, sandstone, and red siltstone and mudstone.
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.
Chlorite-sericite schist containing interbedded quartzite.
Dark-gray, knotty, argillaceous dolomite; impure light-gray marble at base locally.
Light-gray, locally mottled, massive, pure, coarsely crystalline dolomite; siliceous in middle part.
Altered basaltic flows, some amygdaloidal; green, schistose, containing hornblende, epidote, chlorite, and quartz.
Red, green, and gray shale and argillite, and minor thin beds of gray arkosic sandstone; some quartz conglomerate and limestone conglomerate.
Massive dolomite containing thin shaly interbeds.
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.
Light gray, green, and purple; commonly schistose, containing quartz augen and veins.
Very finely crystalline, light-gray limestone interbedded with gray dolomite; coarsely crystalline limestone lenses present.
Quartz or quartzite pebbles, cobbles, and rare boulders set in a red, sandy, ferruginous matrix; some silica cement; some feldspar clasts.
Dark-gray, fine-grained intrusives; locally, mineralogy is altered and unit has greenish color.
Gray, buff-weathering quartzite.
Bluish-black slate (Peach Bottom); quartz conglomerate having a matrix of sericite and chlorite (Cardiff).
Gray, buff-weathering quartzite and quartz schist.
Well-rounded quartzite pebbles, cobbles, and rare boulders set in a reddish-brown, sandy matrix.
Predominantly greenish gray, gray, purple, and maroon shale, siltstone, and graywacke; includes some wildflysch having Martinsburg matrix.
Light gray, coarsely crystalline; contains graphite flakes in places.
In descending order: Weverton--gray to purplish-gray quartzite and quartzose conglomerate containing rounded pebbles. Loudoun--sericitic slate and purplish-gray, crumbly, poorly sorted, arkosic sandstone and conglomerate.
Fine to medium grained, light to medium green; includes probable metavolcanic rocks.
Dark-greenish-gray phyllite and schist containing thin quartzite layers.
In descending order: Hershey--dark-gray to black, thin-bedded, argillaceous limestone; Myerstown--medium- to dark-gray, platy, medium-crystalline limestone; carbonaceous at base.
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.
Very finely crystalline, "birdseye" limestone at top and base, granular fossiliferous limestone, black chert, and dolomite in middle.
Includes serpentine, steatite, and other products of alteration of peridotites and pyroxenites.
Muscovite-chlorite-albite schist, muscovite-chlorite schist, chloritoid schist, and quartzite; intensely folded and cleaved.
Bluish-gray to silvery-green, fine-grained, muscovite-chlorite-albite-quartz schist; intensely cleaved and closely folded; contains interbedded quartzites.
Interbedded red to purple shale and sandstone and some beds of dolomite and impure limestone.
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.
(Formerly mapped as Peters Creek Formation.) Rhythmically interbedded chlorite-muscovite meta-graywacke and fine-grained chlorite-muscovite schist; graded bedding locally preserved; thickness probably 2,000 to 3,000 feet.
Hard, bluish-black graphitic slate; thin beds of fine-grained black quartzite near base; apparent maximum thickness 1,000 feet.
Chiefly serpentinite with partly to completely altered dunite, peridotite, pyroxenite, and massive to schistose soapstone; talc-carbonate rock and altered gabbro are common in some bodies.
Silvery-gray, well foliated, micaceous quartz-pebble metaconglomerate and quartzite; apparent maximum thickness 700 feet.
Grayish-green, massive to schistose, amygdaloidal metabasalt.
White, fine-grained marble; subordinate white, green, and pink variegated marble; and blue marble.