The falls were formed c.10,000 years ago as the retreating glaciers exposed the Niagara escarpment, thus permitting the waters of Lake Erie to flow north, over the scarp, to Lake Ontario. The escarpment has been gradually eroded back toward Lake Erie, a process that has formed the Niagara Gorge (c.7 mi/11 km long); the Whirlpool Rapids and the Whirlpool are there. Horseshoe Falls is eroding upstream at a faster rate than the American Falls because of the greater volume of water passing over it. A great rock slide occurred (1954) at the American Falls and formed a huge talus slope at its base. Water was diverted from the American Falls for several months in 1969 by the U.S. Army Corps of Engineers to study the bedrock and to remove some of the talus.