The Great Falls of the Potomac are located west of Washington, DC and about 20 minutes from our habitat. The series of cascading falls has developed on the Potomac River in rocks formerly known as the Wissahickon formation.
The "Wissahickon" consists of resistant metamorphic rocks including schists, gneisses, metagraywackes and metaconglomerates. Indeed ... ! 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.
I want to highlight that the term "Wissahickon Formation" is no longer used to designate rocks of this Precambrian series in Maryland.
The Great Falls are actually a series of cascades and rapids on a two-thirds of a mile stretch of the Potomac. The river drops about 76 feet over this distance.