ORTONVILLE REGION

Of all the geological agents which have played a part in shaping the face of Minnesota, the most overwhelming and powerful one is glacial ice. At least four times during the last million years, continental glaciers have spread over the state during long periods of cold climate, each advance being followed by widespread retreat.

The last glacier that invaded Minnesota from Canada came by the low valley of the Red River of the North, pushed its way southward across Minnesota and advanced as far as Des Moines. During its slow retreat, 11,000 years ago, the largest glacial river of them all, the Glacial River Warren, formed the valley in which the Minnesota River now flows. This valley, as seen from here, is an impressive reminder of the volume of water it once carried.

Big Stone Lake, now the headwaters of the Minnesota River, visible to the northwest, was formed be-hind a delta-like barrier of sand and silt deposited across this ancient drainage channel by the Whetstone River of South Dakota.

Erected by the Geological Society of Minnesota in cooperation with the Department of Highways, State of Minnesota. 1960