True to its name, the Valley and Ridge physiographic province consists of a series of parallel ridges and valleys.
The ridges are made of rocks that are more resistant to erosion than those of the valleys. Therefore, the ridges remained as high points on the landscape, while the valleys were eroded into gently sloping lows.
The origin of the rocks in the Valley and Ridge province is traced back to the Paleozoic Era (~540-250 million years ago) and is tied to the continental collisions that took place during the buildup of the Appalachian Mountains a short distance east.
Valley and Ridge rocks record marine and near shore environments when ocean level was higher and a warm shallow sea covered the area. These environments teemed with life, now registered as fossils throughout the province. Carbonates, composed mostly of the shells of microscopic photosynthetic organisms, were deposited during these intervals of high biologic productivity.
During the Taconic and Acadian Orogenies, sediments were shed over the Valley and Ridge from the highlands rising to the east. These are the land–derived sedimentary rocks.
Each type of sediment, whether it was land–derived or carbonate, forms a layer in the Valley and Ridge. These layers are stacked on top of one another, and were later folded and faulted into their characteristic valleys and ridges when Africa crashed into North America between 325 and 265 million years ago during the final period of Appalachian mountain-building.
The modern landscape is a result of erosion of the province’s folded and faulted sedimentary rocks, leaving behind the resistant rocks (sandstones) as ridges and the more easily eroded rocks (carbonates) as valleys. Crumpled rocks of the Valley and Ridge are exposed along the western reaches of Chesapeake and Ohio National Historical Park.