Valley and Ridge province - The Geology of Virginia

Landform indicative of the Virginia Valley and
Ridge geologic province


he Valley & Ridge province consists of elongate parallel ridges and valleys that are underlain by folded Paleozoic sedimentary rock . The characteristic topography of this region is the result of differential weathering of linear belts of rocks that have been repeated by folding and faulting. Cambrian clastic sediments of the western Blue Ridge are overlain by carbonates that made up the Great American Bank. For at least 70 million years carbonates were deposited in a shallow tropical ocean along the southeast edge of North America. Today these carbonates (up to 3.5 km in thickness) are exposed in the Great Valley (known as the Shenandoah Valley in central and northern Virginia), the easternmost portion of the Valley & Ridge province. Well-developed karst topography is characteristic of the Great Valley and many caverns are located in the subsurface.

Geologic Events in the northern and central Virginia Valley and Ridge

By middle Ordovician time, clastic sediments were shed from highlands marking the onset of the Taconic orogeny. The Taconic orogeny uplifted mountains to the east (in the Piedmont). Late Ordovician and Silurian rocks of the Valley & Ridge record the uplift and subsequent erosion of the Taconic Mountains. By the late Silurian carbonates were again being deposited in this region. Deposition of sediments continued from the Silurian through the Carboniferous in the foreland (i.e. the Valley & Ridge and Appalachian Plateau) and record pulses of uplift & mountain building to the east. Continental collision in the late Paleozoic produced a fold and thrust belt in which the Blue Ridge was imbricated and thrust northwestward over the Paleozoic cover rocks . Paleozoic sedimentary rocks of the Valley & Ridge were also folded and moved westward along thrust faults . The Great Valley is underlain by a two-tier duplex in which the entire Cambro-Ordovician sequence is repeated. Between 50 and 75% shortening occurred in western Virginia during the late Paleozoic deformation event known as the Alleghanian orogeny.