A Geologic Barrier

Canal engineers faced one of their most perplexing challenges in navigating around the Great Falls of the Potomac. At Widewater, the canal builders used a river channel that the Potomac had abandoned thousands of years earlier when it cut its present gorge. At a bend just above Widewater, the old channel ended at a steep cliff, requiring the engineers to build an immense buttressed wall to support the canal and towpath about 70 feet above the present river level. Before the Ice Age, the Potomac flowed at this level, where the canal now runs, and had not yet cut the deep gorge below. As sea level dropped during the Ice Age, the river cut its channel deeper to reach the lowered sea level. By about 10,000 B.C. the river had abandoned its old channel at Widewater and cut the new one seen today. Just upstream, the Falls remain a place where sand and gravel carried by the river continue to scour and grind away the ancient bedrock.

Potomac, Maryland · United States