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 rocks formerly known as 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.
Please note that the term "Wissahickon Formation" is no longer used to designate rocks of this Precambrian series in Maryland. More information can be found at "Summary of Nomenclature and Subdivision of the Wissahickon".
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. None of the individual falls exceeds a 20 foot drop.
Because the Falls presented a major obstacle to 18th century boat traffic, George Washington proposed the construction of the Patowmack Canal at Great Falls in 1784. The project required 16 years to complete.
Gold has long been associated with this region of Maryland. Follow this link for more information on Gold in Maryland. For more information on waterfalls look at FactSheet 9: Maryland's Five Highest Waterfalls and Mountains.