During the retreat of a glacier the North American system of Great Lakes was produced. Our geologists have very accurately deduced the various stages of this development and have correctly surmised that these bodies of water did, at different times, empty first into the Mississippi valley, then eastward into the Hudson valley, and finally by a northern route into the St. Lawrence. It is thirty-seven thousand years since the connected Great Lakes system began to empty out over the present Niagara route. [1]