The Great Falls of the Potomac are located west of Washington, DC in Great Falls Park. This series of cascading falls has developed on the Potomac River in the Wissahickon Formation. The Wissahickon consists of resistant metamorphic rocks including schists, gneisses, metagraywackes and metaconglomerates. These rocks date from the late Precambrian and are about 750 million years old. The Falls formed when sea level dropped during the last Ice Age, causing the Potomac to downcut its valley.