Swift Run Formation (Jonas and Stose, 1939; King, 1950; Gathright, 1976). Heterogeneous assemblage includes: pebbly to cobbly quartzite and feldspathic metaconglomerate; gray, grayish-pink, or grayish-green, feldspathic quartzite and metasandstone, locally crossbedded; greenish-gray, silvery quartz-sericite-chlorite sandy schist; and, greenish-gray to grayish-red-purple chlorite-sericite tuffaceous phyllite and slate. In Loudoun County, contains pinkish-gray and yellowish-gray to light brownish-gray, fine-grained dolomitic marble (Southworth, 1991). Individual lithologies are laterally discontinuous; formation ranges up to 350 feet in total thickness, but is locally very thin or absent (Gathright, 1976). The Swift Run was originally defined on the northwest limb of the Blue Ridge anticlinorium (Stose and Stose, 1946), where the unit rests unconformably on Grenville-age rocks, and is overlain conformably by the Catoctin Formation; the upper contact is mapped at the bottom of the lowest massive metabasalt. In places Swift Run lithologies are interbedded with Catoctin metabasalts, and the contact between the two units is gradational (Gathright, 1976). Swift Run metasedimentary rocks on the northwest limb have been interpreted as deposited in alluvial fan, floodplain, and lacustrine environments (Schwab, 1986); these are interbedded with metamorphosed tuffaceous and volcanoclastic units (Gathright, 1976; Bartholomew, 1977). Although the Swift Run has been interpreted as a thin western equivalent of the Lynchburg Group in the southeastern Blue Ridge (Stose and Stose, 1946; Brown, 1970), some workers have correlated the Swift Run with discontinuous lenses of feldspathic sandstone interbedded with felsic metatuff that occur immediately below the Catoctin on the southeast limb of the anticlinorium (Nelson, 1962; Conley, 1978; 1989; Wehr, 1985). On the Geologic Map of Virginia (1993), the Swift Run is terminated along an east-west-trending normal fault just west of Leesburg, and is not mapped farther southwest on the southeast limb of the Blue Ridge anticlinorium.
Virginia Division of Mineral Resources, 1993, Geologic Map of Virginia: Virginia Division of Mineral Resources, scale 1:500,000.
Virginia Division of Mineral Resources, 2003, Digital Representation of the 1993 Geologic Map of Virginia, Publication 174, CD ROM (ISO-9660) contains image file, expanded explanation in pdf, and ESRI shapefiles, scale 1:500,000.