Niagara Falls was formed about 12,000 years ago, when glaciers retreated north, allowing water from Lake Erie to flow over the Niagara Escarpment, a ridge that extends in an arc across the northern Great Lakes region, from Wisconsin to New York. Erosion slowly pushed the waterfall about 11 km (about 7 mi) upstream, forming the Niagara Gorge. Before large water-diversion projects were built in the 1950s and 1960s, Horseshoe Falls was receding at a rate of more than 1 m (3 ft) per year. With reduced flows slowing erosion, it will take about 30 years for the falls to recede the distance it once did in a year. In 1954 a considerable portion of the American Falls broke off, creating a large talus, or rock slope, at the base of the cataract. In order to study ways of preventing further rockfalls and to remove some of the talus, the American Falls was successfully “shut off” for several months in 1969 by a dam that was constructed between the U.S. mainland and Goat Island.