Geologic units in Falls Church City, Virginia

Sykesville Formation (Cambrian) at surface, covers 72 % of this area

Sykesville Formation (Hopson, 1964; Drake, 1985). Light- to medium-gray, medium-grained metasedimentary melange consisting of a quartzofeldspathic matrix that contains quartz "eyes"and a heterogeneous suite of pebble- to boulder and larger-size olistoliths. These include: Mather Gorge Formation migmatite, phyllonite, and metagraywacke; also, ultramafic, metagabbroic, and felsic and mafic metavolcanic rocks, plagiogranite, and quartzite. The Sykesville is intruded by Occoquan Granite.

Falls Church Intrusive Suite (Cambrian-Ordovician) at surface, covers 22 % of this area

Falls Church Intrusive Suite (Drake and Froelich, 1986; in press). Tonalite, granodiorite, monzogranite, and trondhjemite. Tonalite, medium-dark-gray, medium- to coarse grained biotite-hornblende tonalite and biotite tonalite with abundant inclusions of mafic and ultramafic rock. Tonalite is typically well-foliated; in many places it has a strong quartz-rod lineation. Granodiorite, medium-gray, medium- to coarse grained, poorly- to moderately-well-foliated, biotite- and biotite-muscovite-bearing. Monzogranite, medium-grained, muscovite-biotite and garnet-muscovite-biotite-bearing. Trondhjemite, light-pink to light-gray, medium-grained, muscovite-bearing; strongly deformed and recrystallized. Tonalite from this suite has been dated at 481±11 Ma (U-Pb single-crystal zircon; J.N. Aleinikoff, written communication, 1993).

Miocene Sand and Gravel (Tertiary) at surface, covers 6 % of this area

Fine- to coarse-grained sand, sandy gravel, silt, and clay, gray to light-yellowish-gray, commonly oxidized to yellowish-orange and yellowish-brown; pebbles and cobbles are deeply etched. Commonly caps interfluves at northwestern edge of Coastal Plain and constitutes thin Coastal Plain outliers in easternmost Piedmont where deposits directly overlie weathered crystalline rocks. In part, may represent a fluvial to marginal-marine facies of the Choptank Formation. Thickness is 0 to 30 feet.