THE RIVER AND THE ROCKS The Geologic Story of Great Falls and the Potomac River Gorge

The carving of the falls continues

As the sea finally withdrew from the Atlantic Coastal Plain in the Washington area between 10 and 20 million years ago, streams draining eastward from the Appalachian Highlands spread a blanket of sand and gravel over the newly exposed sea floor and nearby parts of the Piedmont Plateau. This deposit was not laid down by a single major river, but by numerous streams that constantly shifted their courses back and forth to form a complex series of fan-shaped deposits that coalesced into the blanket of sand and gravel. Remnants of this blanket are still preserved capping some of the highest hills in the Piedmont near Tysons Corner, Va., 5 miles south of Great Falls. The deposit is a source of sand and gravel used for construction purposes in the metropolitan area.

Continued slow uplift of the Piedmont Plateau and the Appalachian Highlands to the west increased the slope of the land surface, causing the streams to deepen their valleys and eventually to coalesce into a river which was to become the Potomac. As this river deepened its valley, scattered remnants of its former flood plains were left at various levels as gravel-covered terraces. About 2 million years ago the river had succeeded in carving a broad, open valley in approximately its present position.

With the beginning of continental glaciation in the Pleistocene Epoch—about 2 to 3 million years ago—sea level was lowered, and the Potomac River began deepening this early valley. As water was withdrawn from the oceans to form the great ice sheets on the land, sea level around the world fell by as much as 500 feet. Most of the continental shelf off the eastern United States was exposed, and the shoreline lay as much as 75 miles east of its present position. Actually, continental glaciation occurred not just once, but at least four times in the last 2 to 3 million years. The last glacial episode ended only about 15 thousand years ago.

STAGES IN THE CARVING OF GREAT FALLS. The area of the block diagrams covers about the some area as that shown on the map. Refer to the map for scale and names of geographic features. A. The Great Falls area as it probably appeared before the Ice Age. The river occupies a broad valley, and the future site of the falls is marked by rapids and ledges of resistant rock. Gravel-covered benches (T) are remnants of an older, higher valley floor. Difficult Run (D—D') flows across a broad, flat flood plain to empty into the Potomac River. B. The same landscape during a period late in the Ice Age. The lowered sea level has caused the river to cut deeply into the floor at its former valley. Floods at this time may have been even higher and more frequent than they are today because of greatly increased snawfall. The downcutting has been relatively rapid in the slightly softer rocks below Great Falls and especially rapid where the rocks are broken along a fault (F—F'). At F the river encounters a series of closely spaced fractures or joints. This zone of weakness has caused the river to cut laterally (J—J'). Diversion of water into the deeply cut channels along the fault and fracture zone has caused the river to abandon several channels on the Maryland side, including the one now occupied by Widewater (W—W'). The channel around Glade Hill (G—G') still carries water, but it is rapidly being cut off. Deepening of the river valley has caused Difficult Run to correspondingly deepen the lower part of its valley (D—D'), destroying its old flood plain and building a new lower one. C. Great Falls today. Continued erosion along the fracture zone (J—J') has diverted all the water from the channel around Glade Hill (G—G'). Channelways are now being cut upstream from the fracture zone, leaving Olmsted (O) and Falls (Fl) Islands above water level. Construction of the dam (X—X') to divert water into the Washington aqueduct has further modified the details of the landscape above the falls. Most of Cone Island (C), for example, has been built since the construction of the dam. The abandoned channel north of Bear Island (B) has been flooded by building dams at W and W' so that it could be used as part of the C. and O. Canal. Difficult Run (D—D') continues to deepen its valley so that its old flood plain survives only above the first rapids (D). (click on image for an enlargement in a new window)

As sea level fell, the river cut correspondingly deeper into the floor of its former valley. The valley was rapidly deepened in the soft, easily eroded materials in the Coastal Plain, but in the hard rocks of the Piedmont Plateau the down-cutting was much slower. It was this downcutting into the hard bedrock floor of the older wider valley that produced the spectacular rocky gorge of the Potomac between Little Falls and Great Falls. At Great Falls the river encounters a series of thick layers of metamorphosed sandstone that are particularly resistant to erosion, and these hard ledges have slowed the progress of valley cutting. The river valley above Great Falls thus remains essentially the unmodified, original pre-Pleistocene valley, but below the falls the river flows in a gorge excavated within the last 2 million years. Along the gorge the original valley floor can be recognized as a flat gravel-covered bench 50 to 60 feet above the present river level. MacArthur Boulevard follows this bench from Cabin John to Anglers Inn. Some of the details of the cutting of the gorge and the sculpturing of Great Falls are illustrated in the block diagrams.