The Niagara River between New York and Ontario, Canada, is shallow upstream of the falls, with many rapids and small islands. It flows over a hard layer of limestone. Beneath the limestone is much softer shale.
When the glaciers retreated from this area about 11,000 years ago, the river flowed over the edge of the limestone layer about seven miles north of where the falls is today. The turbulent water cut away at the soft rock under the limestone layer. Water seeping through the rocks also weakens the soft rocks, and expansion of ice in the winter weakens the shale as well. Eventually the shale is eroded back far enough that the lip of the falls collapses, and the falls retreats. In 11,000 years, the falls have retreated about seven miles.
About 1000 years ago the falls encountered a small island, now called Goat Island. a small amount of water flows east of Goat Island and forms the American Falls. Since the amount of water flowing over American Falls is small, those falls retreat slowly. The Canadian Falls have the largest amount of water and retreat much faster. The fastest retreat of all is in the center of the river, causing the falls to become horseshoe shaped. In roughly another 1000 years, Niagara Falls will have retreated entirely beyond Goat Island and there will only be a single falls.
This view is from an observation tower in Niagara Falls, Ontario.
Created 25 November 2005, Last Update 14 December 2009
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