At Great Falls, the Potomac River crosses the Fall Zone

At Great Falls, the Potomac River crosses the "Fall Zone"

Geology of the Piedmont Province

The Piedmont Province’s metamorphic, sedimentary, and igneous rocks were created through the crushing blows of mountain-building continental collisions, the erosion of mountain chains, and the pulling apart of a supercontinent.

The gently rolling landscape of the eastern Piedmont looks very different from the relatively flat landscape of the western Piedmont. This difference is caused by a change in the rock underlying the surface, or bedrock, from harder, mostly metamorphic rock in the east to softer, mostly sedimentary rock in the west.

The eastern edge of the Piedmont contains the "Fall Zone," a transitional zone between the metamorphic rocks of the Piedmont and the more easily eroded sediments of the Coastal Plain. The "Fall Zone" is manifested in a series of waterfalls and rapids as water moves from higher elevation in the Piedmont to lower elevation in the Coastal Plain. You can see these waterfalls in the Potomac Gorge at Great Falls Park (Virginia) or C&O Canal National Historic Park (Maryland).

The geologic history of the Piedmont began in the late Proterozoic (~ 600 million years ago) when rifting broke apart an ancient supercontinent and formed a new ocean called the Iapetus. This ocean, named after the father of Atlas in Greek mythology, preceded the Atlantic.

Three mountain building episodes, named by geologists the Taconic, Acadian, and Alleghenian Orogenies, later closed the Iapetus Ocean when large masses of land collided with the ancient North American continent. These collisions happened between 440 and 265 million years ago and formed the supercontinent Pangaea.

The collisions attached Iapetus Ocean basin sediment, part of a volcanic island chain, and a chunk of continental crust to the continent. Within the eastern Piedmont, these rocks, along with eroded fragments of the mountain chains, were heavily metamorphosed (changed due to temperature and pressure) and faulted (fractured) during the collisions.

An additional geologic process took place in the western Piedmont that caused its rocks to be different. About 220 million years ago, a rift began to tear apart Pangaea, forming depressions in the land surface. These depressions, or basins, frequently filled with large lakes and became choked with sediment from the adjacent Appalachian Mountains, then at heights rivaling the Himalayas. Eventually this rift failed. Manassas National Battlefield Park preserves sediment from one of these basins.

Farther to the east, a second rift successfully tore apart Pangaea and began forming the Atlantic Ocean, which continues to widen today. The volcanic rocks of the Piedmont were created when magma rose to the surface through cracks caused by the two rifts.

Great Falls

Just 15 miles from Washington D.C., the Great Falls pour over ancient metamorphic rock, dropping 76 feet over 2/3 of a mile where the Potomac River crosses the “Fall Zone”. Click the link below for more information on the Great Falls.

Manassas National Battlefield Park

At Manassas National Battlefield Park, sedimentary deposits associated with a failed Triassic rift are exposed. The red shale visible in this photo is interbedded with thin layers of gray siltstone. Rocks from this formation may contain fossils.


RELATED STORIES
Great Falls of the Potomac (NPS)
Prince William Forest Park Geology E-Walk (NPS)

LEARN MORE
Geologic Time Scale (USGS)
Geologic Glossary (USGS/NPS)
Geologic Map of the National Parks in the National Capital Region
Geologic Map of C & O Canal National Historical Park
NPS Geologic Resource Reports for Parks

 

 
   

Contact:


National Park Service
Center for Urban Ecology
4598 MacArthur Blvd NW
Washington, DC 20007

(202) 342-1443