Published by the American Geophysical Union as part of the Field Trip Guidebooks Series, Volume 202.
The metamorphic rocks of the Potomac Valley, exposed between the Culpeper Mesozoic basin and Laurentian rocks on the west and the Atlantic Coastal Plain overlap on the east, constitute the internides of this part of the Central Appalachian orogen. These rocks occur in a stack of thrust sheets (Drake, 1980, 1983, 1985a, 1985b, 1985c, 1986a, 1986b, 1986c, 1987; Drake and Froelich, 1986; Drake and Lee, in press; Drake and Lyttle, 1981; Drake and Morgan, 1981, 1983; Drake and others, in press; and Drake and others, 1979), that is thought result from an amalgamation of terrane fragments into a Potomac composite terrane (Drake, 1983, 1985b, 1987; Horton and Drake, 1986; Horton and others, 1987, in press). The age of these rocks is uncertain because they are unfossiliferous, but they are almost certainly pre-Arenig (Drake, 1986b; Drake and others, in press; Rankin and others, in press). Those in northern Virginia, and directly along strike in Maryland, are older than the Occoquan Granite which has been dated at 560 Ma by U-Pb techniques on zircons (Seiders and others, 1975) and at 494 ± 14 Ma by Rb-Sr wholerock techniques (Mose and Nagel, 1982). I consider these rocks to range in age from Late Proterozoic to Cambrian.