Tuesday, June 03, 2008
The Michigan Basin is underlain by a thick series of very old rocks, including layers of carbonate rocks deposited in warm seas over 400 million years ago. These sedimentary rocks are basically limestone, but the addition of magnesium has turned the calcite to dolomite and the resulting rock to dolostone. These resistant layers are deepest under Michigan, but like a large saucer, tilt gradually upwards until they break the surface in a large arc around the Great Lakes. These rocks form the Niagara Escarpment, a low hogsback (or cuesta) with a gradual slope on the inner (Michigan) side and a steeper cliff or bluff on the outward facing edge.
The escarpment runs from western New York State north into Canada, around the western end of Lake Ontario, and north across the farmland of southern Ontario. It forms the Bruce Peninsula and Manitoulin Island, between Lake Huron and Georgian Bay, and then sweeps across the upper peninsula of Michigan before heading south to form the Dore Peninsula and Green Bay in Wisconsin (thus the geologic similarity of the Buffalo Bills and the Green Bay Packers?). Streams flowing across the escarpment form cascades, Niagara Falls being the most obvious.