Prince William Forest Park - Geology

The fall line between the Piedmont and Coastal Plains physiographic provinces, which bisects the park, provides outstanding opportunities for the in-depth study of geology. Approximately two-thirds of the park is in the Piedmont and one-third in the Coastal Plain. The topography is undulating, with narrow ridgetops and relatively steep-sided valleys. The park is underlain by late Precambrian to early Paleozoic rocks, which are overlain in the eastern part of the park by unconsolidated Cretaceous period deposits. The soils of the park are sandy, relatively infertile, and easily disturbed. The steep terrain and poor quality soils combine to create severe erosion problems. Relief is moderately high, and the elevation ranges from about ten feet to nearly 400 feet above sea level. Ridgetops are narrow to moderately wide and nearly level to gently sloping. Side slopes are moderately wide to narrow and sloping to very steep. In the piedmont, the geology consists largely of granite gneiss, hornblende gneiss, and mica schist rock types. The ridges of the piedmont are capped with thin mantels of coastal plain or other alluvial sediments in many places. Fairly broad floodplains have developed along the larger streams. The coastal plain is underlain by stratified marine sediments of sand, silt, clay and gravel. The lowland soils are strongly acidic and of low natural fertility. The soils have low permeability, making them subject to at least seasonal wetness. The slopes and gently sloping ridges are occupied by more porous soils that are more easily eroded. They also are strongly acidic and of low fertility. Unconsolidated soil types are generally located in the coastal plain, coastal plain caps, floodplains, and floodplain and stream terraces. The erosion potential in these areas ranges from moderate to high. Outcrops of folded and faulted rock are scattered throughout the park, and they dip nearly vertically in some areas, especially along streambeds. Some of these outcrops expose some formations such as the Wissahikon, which was formed as the ocean floor was driven beneath the eastern edge of the North American continent. Many of the faulted rocks may represent the fall line, a unique geological feature where streams form falls or rapids as they leave the harder rocks of the piedmont and enter the softer rocks of the coastal plain. In many places the ridges of piedmont areas are capped with thin mantels of coastal plain or other alluvial sediments, and fairly broad floodplains have developed along larger streams. In addition to its geological diversity and observable geological processes, the park has large mineral deposits, in particular pyrite and associated minerals. The largest concentration of pyrite is at the confluence of the main branches of Quantico Creek, and the water"s interaction with exposed mineral formations has formed unusual compounds and crystalline formations.