Geologic units in Baltimore county, Maryland

Potomac Group, including Raritan and Patapsco Formations, Arundel Clay, and Patuxent Formation (Cretaceous) at surface, covers 37 % of this area

Potomac Group - Interbedded quartzose gravels; protoquartzitic to orthoquartzitic argillaceous sands; and white, dark gray, and multicolored silts and clays; thickness 0 to 800 feet. Includes Raritan and Patapsco Formations - Gray, brown, and red variegated silts and clays; lenticular, cross-bedded, argillaceous, subrounded sands; minor gravels; thickness 0 to 400 feet; Arundel Clay - Dark gray and maroon lignitic clays; abundant siderite concretions; present only in Baltimore-Washington area; thickness 0 to 100 feet; and Patuxent Formation - White or light gray to orange-brown, moderately sorted, cross-bedded, argillaceous, angular sands and subrounded quartz gravels; silts and clays subordinate, predominantly pale gray; thickness 0 to 250 feet.

Baltimore Gabbro Complex (Late Precambrian (?) - Early Paleozoic) at surface, covers 29 % of this area

Hypersthene gabbro with subordinate amounts of olivine gabbro, norite, anorthositic gabbro, and pyroxenite; igneous minerals and textures well preserved in some rocks, other rocks exhibit varying degrees of alteration and recrystallization with a new metamorphic mineral assemblage.

Lowland Deposits (Quaternary) at surface, covers 13 % of this area

Gravel, sand, silt, and clay. Medium- to coarse-grained sand and gravel; cobbles and boulders near base; commonly contains reworked Eocene glauconite; varicolored silts and clays; brown to dark gray lignitic silty clay; contains estuarine to marine fauna in some areas (includes in part Pamlico, Talbot, Wicomico and Sunderland Formations of earlier reports); thickness 0 to 150 feet.

Baltimore Gneiss (Precambrian) at surface, covers 6 % 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.

Setters Formation (Late Precambrian (?)) at surface, covers 3 % 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.

Upland Deposits (Western Shore) (Quaternary) at surface, covers 3 % of this area

Gravel and sand, commonly orange-brown, locally limonite-cemented; minor silt and red, white, or gray clay; (includes Brandywine, Bryn Mawr, and Sunderland Formations of earlier reports); lower gravel member and upper loam member in Southern Maryland; thickness 0 to 50 feet.

Gunpowder Granite (Paleozoic) at surface, covers 3 % of this area

Remobilized Baltimore Gneiss. Fine- to medium-grained quartz monzonite with biotitic schlieren; grades into Baltimore Gneiss in core of Towson dome; age 550 +/- 50 m.y. * by radiogenic dating.

Relay Quartz Diorite (Early Paleozoic - Late Precambrian (?)) at surface, covers 2 % of this area

Intensely foliated, fine-grained, light colored; ranges from quartz diorite to albite granite; age 550 +/- 50 m.y. * by radiogenic dating.

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

Port Deposit Gneiss (Paleozoic) at surface, covers 1 % of this area

Moderately to strongly deformed intrusive complex composed of gneissic biotite quartz diorite, hornblende-biotite quartz diorite, and biotite granodiorite; all rocks foliated and some strongly sheared; age 550 +/- 50 m.y. * by radiogenic dating.

Wissahickon Formation; Metagraywacke (Late Precambrian (?)) at surface, covers 0.5 % 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.

Wissahickon Formation; Lower Pelitic Schist (Late Precambrian (?)) at surface, covers 0.3 % 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.