Thickness of overburden map, Annandale Quadrangle, Virginia

Volcanic and sedimentary rocks of late Precambrian age include four formations of volcanic rocks (three newly recognized) and four formations of sedimentary rocks, with a total apparent thickness of more than 60,000 feet. The Siemens Creek Formation, composed of: thin basalt flows, and the overlying Kallander Creek Formation, composed predominantly of intermediate flows, are both of Lower Keweenawan age. They were folded before the Portage Lake Lava Volcanics of: middle Keweenawan age were deposited on them. An unnamed Formation of volcanic rocks conformably overlies the Portage Lake Volcanics and is overlain by reddish sandstones and conglomerates of the Copper Harbor Conglomerate. The Copper Harbor Conglomerate is conformably overlain by the dark gray siltstones and fine sandstones of the Nonesuch Formation of Upper Keweenawan age. These are overlain by the reddish, cross-bedded sandstones of the Freda Formation. Folding occurred after the Freda was deposited. Significant copper mineralization occurs near the base of the Nonesuch Formation.