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USGS Geology in the Parks

Geologic Provinces of the United States: Appalachian Highlands

Great Falls of the Potomac River

Great Falls of the Potomac River.

Photo by Avery A. Drake, Jr, USGS.

Resistant Late Proterozoic metagraywacke interbedded with quartz-muscovite-garnet schist of the Mather Gorge Formation forms the Great Falls of the Potomac River in the Piedmont province of northern Virginia (left) and Maryland (right). Looking north from Great Falls Park, Virginia, the Potomac River drops about 80 feet here at Mather Gorge, the type locality of the formation. These rocks are turbidite deposits of an ancient ocean that was destroyed by orogenies in the Cambrian (Cadomian orogeny?) and Ordovician (Taconic orogeny?). The polydeformed and polymetamorphic rocks were further deformed and transported westward in the Middle (Acadian?) and Late Paleozoic (Alleghanian?) during orogenies related to continental collisions.

The Piedmont province is characterized by a dissected plateau, seen in the tree covered horizon. The boundary of the Piedmont and Blue Ridge province is found at the first ridge to the west. The boundary of the Piedmont and Coastal Plain provinces is to the east at the Tidal Potomac River near the Key Bridge in Washington, D.C. Patches of Cretaceous and Tertiary marine and fluvial sediments of the Coastal Plain occur scattered throughout the region of Great Falls and water falls occur over a distance of 10 miles. Therefore, the classic "Fall Line" that forms the provincial boundary is a broad "Fall zone".

Appalachian Highlands
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