The Niagara Gorge preserves more than 40 million years of natural history. Its layers of limestone, sandstone, and shale were formed over millions of years when this land lay beneath a tropical sea. The Niagara River and the Great Lakes Basin are legacies of the last Ice Age about 23,000 years ago, when a sheet of ice covered the region. As the glacier melted, depressions carved into the earth filled with meltwater and created the Great Lakes. The Niagara River gradually formed as the remaining outlet for glacial meltwaters flowing over the Niagara Escarpment, and that relentless flow created Niagara Falls. Over the last 12,500 years, the river's erosive force has continued to carve its path to the Falls' present location.