Geologic units in York county, Pennsylvania

Octoraro Formation (Probably lower Paleozoic) at surface, covers 28 % of this area

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

Marburg Schist (Probably lower Paleozoic) at surface, covers 12 % of this area

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

Gettysburg Formation (Triassic) at surface, covers 12 % of this area

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

New Oxford Formation (Triassic) at surface, covers 10 % of this area

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

Antietam and Harpers Formations, undivided (Cambrian) at surface, covers 10 % 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

Chickies Formation (Cambrian) at surface, covers 6 % of this area

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

Diabase (Jurassic) at surface, covers 5 % 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.

Conestoga Formation (Ordovician and Cambrian) at surface, covers 3 % 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.

Gettysburg conglomerate (Triassic) at surface, covers 3 % of this area

Gray quartz conglomerate, sandstone, and red siltstone and mudstone.

Kinzers Formation (Cambrian) at surface, covers 2 % 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.

Peters Creek Schist (Probably lower Paleozoic) at surface, covers 2 % of this area

Chlorite-sericite schist containing interbedded quartzite.

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

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

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

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

Sams Creek Metabasalt (Probably lower Paleozoic) at surface, covers 0.8 % of this area

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

Heidlersburg Member of Gettysburg Formation (Triassic) at surface, covers 0.5 % of this area

Red, green, and gray shale and argillite, and minor thin beds of gray arkosic sandstone; some quartz conglomerate and limestone conglomerate.

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

Massive dolomite containing thin shaly interbeds.

Limestone fanglomerate (Triassic) at surface, covers 0.3 % 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.

Metarhyolite (Precambrian) at surface, covers 0.3 % of this area

Light gray, green, and purple; commonly schistose, containing quartz augen and veins.

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

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

New Oxford conglomerate (Triassic) at surface, covers 0.2 % of this area

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

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

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

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

Gray, buff-weathering quartzite.

Peach Bottom Slate and Cardiff Conglomerate, undivided (Probably lower Paleozoic) at surface, covers 0.2 % of this area

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

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

Gray, buff-weathering quartzite and quartz schist.

Quartz fanglomerate (Triassic) at surface, covers 0.2 % of this area

Well-rounded quartzite pebbles, cobbles, and rare boulders set in a reddish-brown, sandy matrix.

Montalto Member of Harpers Formation (Cambrian) at surface, covers 0.2 % of this area

Gray quartzite.

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

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

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

Light gray, coarsely crystalline; contains graphite flakes in places.

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

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.

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

Fine to medium grained, light to medium green; includes probable metavolcanic rocks.

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

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

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.

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

St. Paul Group (Ordovician) at surface, covers < 0.1 % of this area

Very finely crystalline, "birdseye" limestone at top and base, granular fossiliferous limestone, black chert, and dolomite in middle.

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.

Wissahickon Formation (Undivided) (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Muscovite-chlorite-albite schist, muscovite-chlorite schist, chloritoid schist, and quartzite; intensely folded and cleaved.

Marburg Schist (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Bluish-gray to silvery-green, fine-grained, muscovite-chlorite-albite-quartz schist; intensely cleaved and closely folded; contains interbedded quartzites.

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

Interbedded red to purple shale and sandstone and some beds of dolomite and impure limestone.

Snitz Creek and Buffalo Springs Formations, undivided (Cambrian) at surface, covers < 0.1 % 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.

Wissahickon Formation; Metagraywacke (Late Precambrian (?)) at surface, covers < 0.1 % of this area

(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.

Peach Bottom Slate (Ordovician (?)) at surface, covers < 0.1 % of this area

Hard, bluish-black graphitic slate; thin beds of fine-grained black quartzite near base; apparent maximum thickness 1,000 feet.

Ultramafic Rocks (Early Paleozoic - Late Precambrian (?)) at surface, covers < 0.1 % of this area

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.

Cardiff Metaconglomerate (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Silvery-gray, well foliated, micaceous quartz-pebble metaconglomerate and quartzite; apparent maximum thickness 700 feet.

Sams Creek Metabasalt (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Grayish-green, massive to schistose, amygdaloidal metabasalt.

Wakefield Marble (Late Precambrian (?)) at surface, covers < 0.1 % of this area

White, fine-grained marble; subordinate white, green, and pink variegated marble; and blue marble.