Geologic units in Carroll county, Maryland

Wissahickon Formation (Undivided) (Late Precambrian (?)) at surface, covers 43 % 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 21 % of this area

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

New Oxford Formation (Triassic) at surface, covers 15 % of this area

Red, maroon, and gray sandstone, siltstone, and shale; basal conglomerate member: From vicinity of Maryland Rte. 73 and southward, limestone conglomerate with red and gray calcareous matrix; northward, quartz conglomerate with red sandy matrix; estimated total thickness 4,5000 feet.

Sams Creek Metabasalt (Late Precambrian (?)) at surface, covers 8 % of this area

Grayish-green, massive to schistose, amygdaloidal metabasalt.

Ijamsville Formation (Late Precambrian (?)) at surface, covers 4 % of this area

Blue, green, or purple phyllite and phyllitic slate, with interbedded metasiltstone and metagraywacke; flattened pumiceous blebs occur locally.

Wissahickon Formation; Boulder Gneiss (Late Precambrian (?)) at surface, covers 2 % of this area

(Formerly mapped as Sykesville and Laurel Formations.) Thick-bedded to massive, pebble- and boulder-bearing, arenaceous to pelitic metamorphic rock; typically a medium-grained, garnet-oligoclase-mica-quartz gneiss; locally an intensely foliated gneiss or schist; apparent thickness 15,000 feet.

Wakefield Marble (Late Precambrian (?)) at surface, covers 2 % of this area

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

New Oxford Formation, basal quartz conglomerate (Triassic) at surface, covers 1 % of this area

From vicinity of Maryland Rte. 73 northward, quartz conglomerate with red sandy matrix

Silver Run Limestone (Late Precambrian (?)) at surface, covers 0.9 % of this area

Blue, thin-bedded, finely crystalline schistose limestone and calcareous slate.

Wissahickon Formation; Lower Pelitic Schist (Late Precambrian (?)) at surface, covers 0.6 % of this area

(Formerly mapped as oligoclase facies of Wissahickon Formation.) Medium- to coarse-grained biotite-oligoclase-muscovite-quartz schist with garnet, staurolite, and kyanite; fine- to medium-grained semipelitic schist; and fine-grained granular to weakly schistose psammitic granulite; psammitic beds increase upward; apparent thickness 5,500 feet or more.

Gettysburg Shale (Triassic) at surface, covers 0.6 % of this area

Red shale and soft red sandstone and siltstone; estimated thickness less than 5,000 feet.

Chilhowee Group; Harpers Formation (Cambrian) at surface, covers 0.2 % of this area

Brown to dark bluish-gray banded shale, to light bluish-gray, finely laminated phyllite; distinctively pale purple in basal part; bedding obscured by cleavage; increasingly metamorphosed toward east from shale to slate and phyllite; estimated thickness 2,000 feet.

Ultramafic Rocks (Early Paleozoic - Late Precambrian (?)) at surface, covers 0.2 % 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.

Libertytown Metarhyolite (Late Precambrian (?)) at surface, covers < 0.1 % of this area

Purple, bluish-black, and red, dense, fine-grained metarhyolite with feldspar phenocrysts; interbedded with blue and purple amygdaloidal metaandesite; both rhyolite and andesite interbedded with blue, purple, and green phyllitic slates.

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

Biotite-quartz-feldspar gneiss and biotite-hornblende gneiss; amphibolite widespread but subordinate; texturally varied; granitic gneiss, veined gneiss, augen gneiss, banded gneiss, and migmatite, in places complexly intermingled; age 1,1000 m.y. * by radiogenic dating. Layered paragneiss in Baltimore City southeast of Relay Quartz Diorite.

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

Metadolomite, calc-schist, and calcite marble are predominant; calc-gneiss and calc-silicate marble widespread but minor; thickness about 750 feet.

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

Upper member: Feldspathic mica schist and mica gneiss; total thickness 200 to 500 feet. Middle member: Impure quartzite interstratified with thin beds of mica schist; total thickness 200 to 500 feet. And Lower member: Medium-grained, feldspathic mica schist; locally granitized; total thickness 200 to 500 feet.

Chilhowee Group; Antietam Formation (Cambrian) at surface, covers < 0.1 % of this area

White to dark gray and brown, thick-bedded, fine- to coarse-grained quartzite with thin argillaceous partings; first occurrence of Lower Cambrian fossils; cleavage generally obscures bedding; increasingly metamorphosed and phyllitic toward east; estimated thickness 300 to 800 feet.

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

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

Diabase Sills and Dikes (Triassic) at surface, covers < 0.1 % of this area

Sills: Greenish-gray to black, medium-grained; dikes: Greenish-gray to black, medium- to dine-grained; local contact metamorphic aureoles.

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.