Geology of Minnesota

Geology of Minnesota. Stillwater Region. , The site of this tablet marks the northern limit of Lake St. Croix, impounded by the natural dam of sand and gravel, made by the Mississippi where it is joined by the St. Croix River, twenty miles below Stillwater. The valley, with its steep banks, is typical of youthful topography - of a young stream - and its size, compared with the river, indicates that a much larger volume of water flowed here when the St. Croix was an outlet of glacial Lake Duluth, the ancestor of Lake Superior. The highway and picnic grounds occupy a river terrace on which the river flowed at an earlier stage. The rock walls of the valley are chiefly sandstones formed in the sea when it covered Minnesota during the Cambrian period 500 million years ago. Because of the thickness of the beds and the excellence of the exposures along the river, these formations, wherever they appear in North America, are known as the St. Croixian series. , Erected by the Geological Society of Minnesota , in cooperation with the Department of Highways , State of Minnesota.