Background Information: Virginia’s Physiographic Provinces
Introduction
The Appalachian Mountains extend from Nova Scotia to Alabama, and include the state of Virginia. Geographers and Geomorphologists recognize more than 20 physiographic provinces in North America. Virginia extends westward from the Atlantic Ocean to include parts of five provinces. The topography and geology of the five provinces are unique unto themselves and are described below.
The Coastal Plain
The Coastal Plain is the youngest of Virginia’s physiographic provinces, its rocks having been deposited after the Atlantic Ocean began to form early in the Mesozoic Era. It covers the eastern one-fourth of the state and was once referred to as “Tidewater.” Today “Tidewater” designates the Norfolk area.
The western boundary of the Coastal Plain is the Fall Line; this topographic break extends southward from Washington, D.C. through Fredericksburg, Richmond, Petersburg, Emporia, and into North Carolina. It was called the Fall Line because the zone of rapids or falls marks the transition from the more resistant rocks to the west to the less resistant rocks and sediments of the Coastal Plain to the east.
From the Piedmont where the elevation is about 200 feet above sea level, the Coastal Plain slopes seaward continuing beneath the Atlantic Ocean to the edge of the continental shelf, about 600 feet below sea level. The shoreline has been moving up (transgressing) for about 18,000 years at a rate of 2 to 3 feet every 100 years.
During the ice ages of the Pleistocene, sea level was lowered, exposing the shelf. Streams flowed across the shelf and carved their valleys into it. One such large valley became the Chesapeake Bay when sea levels rose again and flooded the shelf.
In the sediments of the Coastal Plain are reefs and fossilized shells, bones, and teeth. A fossil clam, Chesapecten jeffersonius, was named the Virginia state fossil in 1992 because of its relative abundance within the Pliocene-age Yorktown Formation and its name honors Thomas Jefferson’s interest in fossils.
Piedmont
The largest of the provinces, the Piedmont, is directly west of the Coastal Plain and literally means “foot of the hill” because the Blue Ridge, the highest mountains, is directly west. The Piedmont covers the middle one-third of the state. The Piedmont is an area of rolling landscape – gentle hills and valleys with a few isolated mountains made of more resistant rocks. The Piedmont is characterized by deep weathered, very old erosion surface. It widens from the northwest to the southwest, from 30 miles on the Maryland border to about 160 miles at the North Carolina line. Elevation is from 1000-1300 feet to 200 feet at the Fall Line.
Regionally, the Piedmont extends from Alabama to New Jersey. It is underlain by Precambrian and Paleozoic metamorphic and igneous rocks and is more resistant to erosion than the Coastal Plain, so it is a higher area. Within the Piedmont, there are found low-lying areas (basins) covered by Triassic and Jurassic sediments intruded by igneous dikes and sills (also Mesozoic). These basins contain unmetamorphosed sandstone and shale where dinosaur tracks have been preserved.
The Blue Ridge
The Blue Ridge is a long narrow area extending from southwest Virginia through the west-central portion through the north-central part of the state. It has the highest peaks in the state.
The Blue Ridge is divided into two distinct parts: 1. Northeast of Roanoke – Very narrow (less than 2 miles across in some places like Afton Mountain on I-64), fairly rugged and reaches 4100 feet near Luray. 2. Southwest of Roanoke – Here the Blue Ridge broadens into very high mature erosion surfaces up to 50 miles wide. On the surface are peaks such as Mt. Rogers (5719 feet and the highest), Whitetop Mountain, and Peaks of Otter.
Regionally, the Blue Ridge extends from Northwest Georgia to South-central Pennsylvania with the widest part in eastern Tennessee to western North Carolina, and is called the Great Smokies. The Blue Ridge contains mainly granite, gneiss, and metamorphosed igneous and sedimentary rocks. They are very hard and resistant rocks and create the highest area in Virginia.
The Valley and Ridge
The Valley and Ridge covers the western part of Virginia, from the Tennessee border to Maryland – West Virginia. It consists of long, linear ridges (some greater than 4000 feet above sea level) with well-defined long linear valleys in between. Trending northwest to southwest, the Valley and Ridge is bounded by the Blue Ridge and the Plateau. Its long valley runs the length of the province on the eastern border adjacent to the Blue Ridge and is known as the Shenandoah Valley in northern Virginia, but it is generally called the Great Valley.
This province is composed of folded and faulted 550 to 300 million-year-old sedimentary rocks (Paleozoic-age). Most ridges are held up by resistant sandstone, and most valleys are underlain by less resistant shale, limestone, and dolostone. Underlain by mostly carbonate rocks, the valley of this province is a region of Karst. Solution of the carbonate bedrock has created sinkholes on the surface and many caves and large caverns beneath the surface.
Regionally, the Valley and Ridge extends from Alabama to Canada.
The Appalachian Plateau
In Virginia, the Appalachian Plateau province encompasses only the southwestern counties and includes the Southwest Virginia coal field. This province contains a portion of the Cumberland Plateau and is characterized by deep narrow valleys and steep, rugged mountainsides.
This province generally contains 320 to 280 million-year-old sandstone and shale formations with coal beds. The rocks in this province have not been deformed as in the Valley and Ridge and still occur today in horizontal beds.
Regionally, the Plateau extends from Alabama to New York.