The Niagara Escarpment has origins dating to the Silurian age some 430 to 450 million years ago, a time when the area lay under a shallow warm sea. This sea lay in a depression of the Earth's crust, centered in what is now the lower peninsula of the State of Michigan. Known geologically as the Michigan Basin, the outer rim of this massive saucer-shaped feature governs the location of the Niagara Escarpment, which is shaped like a gigantic horseshoe. The Escarpment can be traced from near Rochester, New York, south of Lake Ontario to Hamilton, north to Tobermory on the Bruce Peninsula. It is covered by the waters of Lake Huron, appearing as Manitoulin Island, then across the Upper Peninsula of Michigan and down the west side of Lake Michigan into the State of Wisconsin.