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. In the Driftless Area, a rugged landscape of deep valleys and rocky crags remains, while much of the Midwest was smoothed by ice sheets that eroded away crags and filled in valleys. The Driftless Area was never completely surrounded by ice, and during the last glacial episode the glaciers reached only its eastern edge. About 15,000 years ago, the Wisconsin River was dammed by a glacier, and sediment deposited in glacial Lake Wisconsin formed the flats between Tomah, Wisconsin Dells and Wisconsin Rapids. At that time, the Driftless Area was treeless and had a tundra climate.