The most arresting geologic feature of the Potomac Gorge is the Great Falls, where the Piedmont meets the tidal waters of the Coastal Plain. Geologists call this transition from higher elevations to tidal waters the “fall zone.”
Most of us know geology has something to do with rocks and minerals. Geology is actually a way to interpret the story of the Earth. Just looking at the riverbed above and below the Gorge, we can learn about the history of waterfalls and the formation of the Gorge.
When you look at a map, you can see the boundaries of states. Geologists also divide the world into sections, called provinces, based on the observed rocks, minerals and drainages found in each. The Potomac Gorge is at the eastern end of the Piedmont, or foothills, province.
Above the Potomac Gorge, the Potomac River passes over the hard bedrock of the Piedmont. This rock is erosion-resistant, meaning that even after many tens of thousands of years the river bottom changes only slowly.
Below the Gorge, the river enters the Coastal Plain province. The river bottom is composed of softer rock, which erodes more easily. As the river bottom has eroded over time, the drop of the falls has become larger. The river bottom in the Coastal Plain is more like the sand and mud along the bay and beaches. That’s because the “alluvial” sediments from upriver have been slowly deposited into the tidal portion of the river.
The varied walls of the Gorge tell its natural history. In some places, they are sharp and vertical. In other places the walls slope gradually, opening ever wider on the Coastal Plain.
As it passes through the Gorge, the elevation of the Potomac drops from 140 feet above sea level to only 10 feet. It is one of the steepest and longest drops of any major American river flowing to the Atlantic. And it’s certainly one of the most dramatic.