The collapse of ice dams holding back Glacial Lake Duluth and Glacial Lake Grantsburg carved out the Dalles of the Saint Croix River. "The Upper Mississippi River valley likely originated as an ice-marginal stream during what had been referred to as the "Nebraskan" glaciation. Current ... ( +view full text )
Mississippi River Landscape -- Description:
The collapse of ice dams holding back Glacial Lake Duluth and Glacial Lake Grantsburg carved out the Dalles of the Saint Croix River. "The Upper Mississippi River valley likely originated as an ice-marginal stream during what had been referred to as the "Nebraskan" glaciation. Current terminology would place this as Pre-Illinoian Stage."