he Piedmont Physiographic Province encompasses approximately 16,300 mi² along the eastern part of the study area and consists of a gently rolling plain underlain by polydeformed and metamorphosed Proterozoic and Paleozoic rocks (Fisher, 1970, p. 295). A thick mantle of soil and weathered rock (saprolite, regolith, or residuum) that overlies the  fractured crystalline bedrock is a characteristic feature of the province (Meyer and others, 1965; Conley, 1985, p. 1; Swain and others, 1991, p. 12). Generally, regolith developed on the crystalline rocks is thick under the hilltops and thin to absent in the stream valleys (Richardson, 1982, p. 6; Pavich and others, 1989, p. 43). Elevations range from about 200 ft above sea level along the eastern border to 1,000 ft above sea level along the western border of the province. Down-faulted Mesozoic sedimentary basins, which encompass approximately 1,340 mi², are located within the Piedmont Physiographic Province. These basins are underlain by shale, siltstone, mudstone, sandstone, and basalt and are characterized by generally lower relief than the surrounding Piedmont (Hack, 1989, p. 461). Thin soils and shallow weathering profiles are characteristic features of the rocks in the Mesozoic Basins (Froelich, 1985; Conley, 1985, p. 2). Hack (1982, p. 3) noted that the Piedmont Physiographic Province of Virginia consists of the Foothill zone on the west, the Piedmont Lowlands on the east, and the Northeastern Highlands on the north. The Foothill zone also encompasses the southeastern part of the Blue Ridge Physiographic Province. The part of the Foothill zone north of the Roanoke River is underlain by resistant volcanic and metamorphic rocks that form chains of isolated hills and ridges. The part of the Foothill zone south of the Roanoke River is underlain by resistant rocks of the Smith River allochthon that form an upland (Hack, 1982, p. 24). The Piedmont Lowlands are underlain by feldspathic gneiss and schist intruded by granitic plutons with lesser amounts of metasedimentary and metavolcanic rocks that form low ridges or hills and ravines.

Ground-Water Flow

Ground-water flow in the crystalline terranes occurs (1) in the regolith (predominantly saprolite) and (2) along fractures and joints in the bedrock.  Most of the ground-water storage in the crystalline terranes is in the saprolite, which overlies the crystalline bedrock.

Ground-water flow in the Mesozoic Basins predominantly occurs along fractures, joints, and bedding plane partings in the bedrock.  Ground-water storage in the rocks of the Mesozoic Basins is predominantly in the fractures, joints, and bedding plane partings in the bedrock because of the thin mantle of saprolite.

Apparent ground-water ages can be relatively young because flow velocities and gradients can be high in the vicinity of pumping wells and ground-water storage is limited in the fractured rock aquifers in the Piedmont.

References

Conley, J.F., 1985, Geology of the southwestern Virginia Piedmont:  Virginia Division of Mineral Resources Publication 59, 33 p.

Focazio, M.J., Plummer, L.N., Böhlke, J.K., Busenberg, Eurybiades, Bachman, L.J., and Powars, D.S., 1998, Preliminary estimates of residence times and apparent ages of ground water in the Chesapeake Bay Watershed, and water-quality data from a survey of springs: U.S. Geological Survey Water-Resources Investigations Report 97-4225, 75 p.

Froelich, A.J., 1985, Map and geotechnical properties of surface materials of the Culpeper Basin and vicinity, Virginia and Maryland:  U.S. Geological Survey Miscellaneous Investigations Series Map I- 1313-E, 1 sheet, scale 1:125,000.

Fisher, G.W., 1970, The Piedmont:  Introduction, in Fisher, G.W., Pettijohn, F.J., Reed, J.C., Jr., and Weaver, K.N., eds., Studies of Appalachian geology:  Central and southern:  New York, Wiley- Interscience, p. 295-298.

Hack, J.T., 1982, Physiographic divisions and differential uplift of the Piedmont and Blue Ridge: U.S. Geological Survey Professional Paper 1265, 49 p.

_____1989, Geomorphology of the Appalachian Highlands, in Hatcher, R.D., Jr., Thomas, W.A., and Viele, G.W., eds., The Appalachian-Ouachita orogen in the United States, v. F-2 of The geology of North America:  Boulder, Colo., Geological Society of America, p. 459-470.

Meyer, Gerald, Wilmoth, B.M., and LeGrand, H.E., 1965, Availability of ground water in the Appalachian region, in Schneider, W.J., and others, Water resources of the Appalachian region, Pennsylvania to Alabama: U.S. geological Survey Hydrologic Investigations Atlas HA-198, 11 sheets, scale 1:2,500,000.

Nelms, D.L., Harlow, G.E., Jr., and Hayes, D.C., 1997, Base-flow characteristics of streams in the Valley and Ridge, the Blue Ridge, and the Piedmont Physiographic Provinces of Virginia: U.S. Geological Survey Water-Supply Paper 2457, 48 p.

Pavich, M.J., Leo, G.W., Obermeier, S.F., and Estabrook, J.R., 1989, Investigations of the characteristics, origin, and residence time of the upland residual mantle of the Piedmont of Fairfax County, Virginia:  U.S. Geological Survey Professional Paper 1352, 58 p.

Richardson, C.A., 1982, Ground water in the Piedmont Upland of Central Maryland:  U.S. Geological Survey Water-Supply Paper 2077, 42 p.

Swain, L.A., Hollyday, E.F., Daniel, C.C., III, and Zapecza, O.S., 1991, Plan of study for the Regional Aquifer-System Analysis of the Appalachian Valley and Ridge, Piedmont, and Blue Ridge Physiographic Provinces of the Eastern and Southeastern United States, with a description of study- area geology and geohydrology:  U.S. Geological Survey Water-Resources Investigations Report 91- 4066, 44 p.