LANDSCAPE EVOLUTION AND PHOTO GALLERY

The landscape of the area is mostly the result of erosion by the Potomac River during the past 5 million years. As the Potomac River flows to the southeast, it cuts obliquely across several types of north-trending bedrock units. The river follows the trend of joints (cracks) and faults along the straight river course in Mather Gorge. 

Level surfaces called terraces, which have been cut into bedrock at different elevations by the Potomac River, form conspicuous features of the landscape. The terraces are actually remnants of old flood plains of the downcutting Potomac. There are at least six recognizable terrace levels, but they are all shown combined in order to be more readable at the map scale. The highest and oldest terrace is on the crest of Glade Hill in Great Falls Park, where a deposit of river boulders of quartzite and diabase is found (unit QTb).  The Great Falls Park entrance, parking lots, picnic area, visitor center, and Patowmack Canal are on the second highest and next oldest terrace. When the Potomac River was downcutting through this terrace into the bedrock, Glade Hill was an island. There are at least four younger terrace levels within the gorge. Downcutting by the Potomac River into these terraces has created islands, islets, pinnacles, shoestring channels, oxbows, plungepools, and potholes. The erosional features are best seen on Bear Island. The C&O Canal utilizes an abandoned channel of the Potomac River at Widewater that in part creates Bear Island. Alluvial stream deposits are best preserved along parts of Difficult Run, whereas most alluvial deposits along the Potomac River have been removed during floods. The bedrock terraces provide excellent exposures of Piedmont bedrock and the fascinating landforms eroded into them.

Potholes (photo by Dave Usher, USGS)

Boulder deposited by the Potomac River on top of Glade Hill (photo by Dave Usher, USGS)

Map and cross section showing terraces of the Potomac River

Oblique aerial photograph looking north at Great Falls (courtesy NPS)