Several times during the ice ages, glaciers flowed out of Canada, sometimes reaching as far south as the Ohio and Missouri rivers. During recent glaciations, southwestern Wisconsin was untouched, because the glaciers were diverted to the east or west by the highlands of north-central Wisconsin and northwestern Michigan.
Here, in the Driftless Area, you see a rugged landscape with deep valleys and rocky crags. Much of the Midwest would look like this today if it had not been glaciated, but the ice sheets smoothed those areas by eroding away the crags and filling in the valleys.
The Driftless Area was never completely surrounded by ice, and the glaciers reached only its eastern edge during the last glacial episode. At that time, about 15,000 years ago, the Wisconsin River was dammed by the glacier, and the sediment deposited in glacial Lake Wisconsin forms the flats between Tomah, Wisconsin Dells and Wisconsin Rapids. Then the Driftless Area was treeless and had a tundra climate.