Nearly 12,000 years ago, the Potomac River was formed as a result of the final glacial episode of the Pleistocene Epoch. At that time, the Potomac River was little more than a tributary of the Susquehanna River. A variety of large animals known as megafauna roamed the riverbanks. These animals included the sabertooth tiger, wooly mammoth, and giant ground sloth. Early Native Americans known as Paleo Indians, relied on megafauna as a valuable resource for their survival.
As glaciers receded, melt waters formed the Chesapeake Bay, flooding the Susquehanna River and widening the Potomac River. Today, the Potomac is approximately 383 miles from its headwaters to the Bay and travels through four states: Pennsylvania, West Virginia, Virginia, and Maryland, as well as the District of Columbia.
Five geological formations are exposed in Virginia: the Appalachian Plateau, the Ridge and Valley Region, the Blue Ridge, and the Piedmont Valley, and the Coastal Plain. The Potomac River passes through all of these formations as it journeys from the headwaters to the Chesapeake Bay.