Geologic units in Montgomery county, Maryland

Wissahickon Formation (Undivided) (Late Precambrian (?)) at surface, covers 29 % of this area

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

Ijamsville Formation and Marburg Schist (Late Precambrian (?)) at surface, covers 23 % of this area

Ijamsville Formation - Blue, green, or purple phyllite and phyllitic slate, with interbedded metasiltstone and metagraywacke; flattened pumiceous blebs occur locally; and Marburg Schist - Bluish-gray to silvery-green, fine-grained, muscovite-chlorite-albite-quartz schist; intensely cleaved and closely folded; contains interbedded quartzites.

Wissahickon Formation; Lower Pelitic Schist (Late Precambrian (?)) at surface, covers 15 % 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.

New Oxford Formation (Triassic) at surface, covers 14 % 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.

Wissahickon Formation; Boulder Gneiss (Late Precambrian (?)) at surface, covers 10 % 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.

Norbeck Quartz Diorite (Cambrian) at surface, covers 2 % of this area

Ranges from weakly foliated quartz diorite to strongly gneissic and schistose rock with recrystallized textures; more massive rocks have igneous textures; age 510 +/- 10 m.y. * by radiogenic dating.

Kensington Quartz Diorite (Ordovician) at surface, covers 2 % of this area

Moderately to strongly deformed; igneous textures generally destroyed; composition ranges from quartz diorite to granodiorite; comprises thin concordant sheets or wedges localized along plunging crest of Baltimore anticlinorium.

Potomac Group, including Raritan and Patapsco Formations, Arundel Clay, and Patuxent Formation (Cretaceous) at surface, covers 2 % 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.

Georgetown Mafic Complex (Late Precambrian (?) - Early Paleozoic) at surface, covers 1 % of this area

Poorly exposed complex of tonalite, dark quartz diorite, gabbro, amphibolite, and undifferentiated basic rocks.

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.

Urbana Formation (Late Precambrian (?)) at surface, covers 0.8 % of this area

Dark gray to green sericite-chlorite phyllite, metasiltstone, and quartzite; thin lenses of impure marble and calcareous phyllite occur locally.

Diabase Sills and Dikes (Triassic) at surface, covers 0.6 % of this area

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

Marburg Schist (Late Precambrian (?)) at surface, covers 0.4 % of this area

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

Wissahickon Formation; Metagraywacke (Late Precambrian (?)) at surface, covers 0.3 % 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.

Sugarloaf Mountain Quartzite (Late Precambrian (?)) at surface, covers 0.3 % of this area

Massive white quartzite interbedded with softer sericitic quartzite, slate, and phyllite.

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

(Formerly mapped as albite facies of Wissahickon Formation.) Albite-chlorite-muscovite-quartz schist with sporadic thin beds of laminated micaceous quartzite; coarsens from west to east; primary sedimentary structures include normal bedding, graded bedding, and soft-sediment deformational structures; apparent thickness 14,000 feet or more.

Lowland Deposits (Quaternary) at surface, covers < 0.1 % 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.

Newark Supergroup; Sandstone, undifferentiated (Upper Triassic) at surface, covers < 0.1 % of this area

Fine- to coarse-grained, reddish-brown to gray, primary bedding features such as cross-beds, channel lags, and ripple marks , minor conglomerate, siltstone, and shale beds.

Mather Gorge Formation - Metagraywacke (Proterozoic Z-Cambrian) at surface, covers < 0.1 % of this area

Metagraywacke, light- to medium-gray, yellowish- to red dish-brown-weathering, fine- to medium-grained, generally well-bedded, and lesser semi-pelitic schist; contains interbedded quartzose schist and some calc-silicate rock; mineral assemblages as in schist (CZms). Beds range from about 3 cm to 3 m, averaging about 20 cm; graded bedding, sole marks, and slump features are abundant. Mather Gorge is unconformable beneath Popes Head Formation, which is intruded by Occoquan Granite; includes rocks previously mapped in northern Virginia as Peters Creek Schist.

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

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