Several times during the ice ages, glaciers flowed out of Canada and sometimes reached as far south as the Ohio and Missouri rivers, but during recent glaciations southwestern Wisconsin remained untouched because highlands in north-central Wisconsin and northwestern Michigan diverted the ice to the east or west. The resulting Driftless Area has a rugged landscape of deep valleys and rocky crags, a terrain much of the Midwest would also have if ice sheets had not smoothed other areas by eroding crags and filling valleys. The region was never completely surrounded by ice, and during the last glacial episode glaciers reached only its eastern edge. About 15,000 years ago, a glacier dammed the Wisconsin River, 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.