The lower Wisconsin River flows west to the Mississippi River from the Baraboo Hills. Has it always done that? Not according to the data being collected by Prof. Eric Carson, a geomorphologist at the Wisconsin Geological and Natural History Survey.
Dr. Carson has been mapping the geology of the Driftless Area of southwestern Wisconsin since 2010. There, he has found evidence that until about a million years ago the upper Mississippi River turned east at Prairie du Chien, flowed east across the Driftless Area, and continued to the northeast to past Door County and ultimately to the Atlantic Ocean.
Carson has named this ancient river, the Wyalusing. He believes that a glacier that advanced from the northeast blocked the river and forced it to flow south, where the Mississippi now runs. When this happened, the eastward-flowing river abandoned its course and the westward-flowing Wisconsin River occupied the valley.
How does he know? Eric has been coring through the terraces on the Wisconsin River, finding an underlying bedrock terrace that slopes toward the east. He says, “This terrace is the ancient bed of the Wyalusing River.”