Geologic units in Cecil county, Maryland

Upland Deposits (Eastern Shore) (Quaternary) at surface, covers 27 % of this area

Gravel, sand, silt, and clay. Mostly cross-bedded, poorly sorted, medium- to coarse-grained white to red sand and gravel; boulders near base; minor pink and yellow silts and clays; (Wicomico Formation of earlier reports); thickness 0 to 90 feet, locally thicker in paleochannels.

Potomac Group, including Raritan and Patapsco Formations, Arundel Clay, and Patuxent Formation (Cretaceous) at surface, covers 14 % 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.

Port Deposit Gneiss (Paleozoic) at surface, covers 12 % 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 (Undivided) (Late Precambrian (?)) at surface, covers 11 % of this area

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

Metagabbro and Amphibolite (Early Paleozoic (?)) at surface, covers 9 % of this area

Weakly to strongly lineated metagabbro and epidote amphibolite.

Lowland Deposits (Quaternary) at surface, covers 8 % 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 Gabbro Complex (Late Precambrian (?) - Early Paleozoic) at surface, covers 6 % 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.

Monmouth Formation (Cretaceous) at surface, covers 3 % of this area

Dark gray to reddish-brown, micaceous, glauconitic, argillaceous, fine- to coarse-grained sand; basal gravel in Prince Georges County; thickness 0 to 100 feet.

Volcanic Complex of Cecil County (Late Precambrian (?)) at surface, covers 2 % of this area

Metamorphosed andesitic and dacitic volcanic rocks (greenstone, greenschist, quartz amphibolite, and schistose felsite); amygdules and volcano-clastic textures locally preserved; thickness unknown.

Upland Deposits (Western Shore) (Quaternary) at surface, covers 2 % 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.

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.

Matawan Formation (Cretaceous) at surface, covers 1 % of this area

Dark gray micaceous, glauconitic, argillaceous, fine-grained sand and silt; absent in outcrop southwest of Patuxent River; thickness 0 to 70 feet.

Pamunkey Group; Aquia Formation (Paleocene) at surface, covers 0.8 % of this area

Dark green to gray-green, argillaceous, highly glauconitic, well-sorted fine- to medium-grained sand; locally indurated shell beds; thickness 0 to 100 feet.

Magothy Formation (Cretaceous) at surface, covers 0.5 % of this area

Loose, white, cross-bedded, "sugary", lignitic sands and dark gray, laminated silty clays; white to orange-brown, iron-stained, subrounded quartzose gravels in western Anne Arundel County; absent in outcrop southwest of Patuxent River; thickness 0 to 60 feet.

Wissahickon Formation; Boulder Gneiss (Late Precambrian (?)) at surface, covers 0.4 % 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.

Wissahickon Formation; Metagraywacke (Late Precambrian (?)) at surface, covers 0.2 % 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.

Glenarm Wissahickon"" formation (Probably lower Paleozoic) at surface, covers 0.1 % of this area

Lithologically similar to oligoclase-mica schist of the Wissahickon Formation (PZw), but also includes lenticular amphibolite bodies having ocean-floor basalt chemistry.

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.

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

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

Potomac Formation (Cretaceous) at surface, covers < 0.1 % of this area

Variegated silts and clays with beds of quartz sand.

Rancocas Formation (Tertiary) at surface, covers < 0.1 % of this area

Grayish-green and green, silty, glauconitic sand.

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

Chlorite-sericite schist containing interbedded quartzite.

Wissahickon Schist (Paleozoic) at surface, covers < 0.1 % of this area

Dense micaceous schist, gneiss and migmatite.

Port Deposit Granodiorite (Paleozoic) at surface, covers < 0.1 % of this area

Granodiorite

Mt. Laurel-Navesink Formation (Cretaceous) at surface, covers < 0.1 % of this area

Dark greenish brown and dark gray, highly glauconitic sandy silt and silty sand.

Merchantville Formation (Cretaceous) at surface, covers < 0.1 % of this area

Dark gray to bluish-gray slightly glauconitic, micaceous silty, very fine sand.

Unit C (Tertiary) at surface, covers < 0.1 % of this area

Grayish-green, clayey glauconitic silt and sand.

Magothy Formation (Cretaceous) at surface, covers < 0.1 % of this area

White and buff quartz sand with beds of gray and black clayey silt.

Unit B (Cretaceous-Tertiary) at surface, covers < 0.1 % of this area

Greenish-gray sandy and clayey glauconitic silt.

Amphibolite (Paleozoic) at surface, covers < 0.1 % of this area

Amphibolite

Wenonah Formation (Cretaceous) at surface, covers < 0.1 % of this area

Gray and rust-brown fine to medium, micaceous, sparingly glauconitic quartz sand.

Redbank Formation (Cretaceous) at surface, covers < 0.1 % of this area

Reddish-brown, slightly micaceous and glauconitic, fine to medium sand.