The geologic history of the falls is a long and interesting one. Great Falls Park is located along the boundary between the Piedmont Plateau and the Atlantic Coastal Plain, in an area known as the fall line. The Potomac River flows past jagged, rocky surfaces and high-walled cliffs on its journey into the softer rocks of the Coastal Plain. The most dramatic elevation change occurs here at Great Falls Park. In less than a mile, the river drops 76 feet.
The dense, erosion resistant metamorphic rock that makes up the fall line is the reason why the Falls and Mather Gorge exist. The oldest of these rocks formed from sandstones and mudstones approximately 550 million years ago. Several igneous intrusions occurred around 470 million years ago, leaving behind granodiorite formations. Lamprophyre dikes arrived 110 million years later. Since lamprophyre weathers fairly quickly once it is exposed, it is a difficult rock to spot in the park.