The Driftless Area, a topographical and cultural region in the American Midwest, comprises southwestern Wisconsin, southeastern Minnesota, northeastern Iowa, and the extreme northwestern corner of Illinois. Never covered by ice during the last ice age, the area lacks the characteristic glacial deposits known as drift. Its landscape is characterized by steep hills, forested ridges, deeply carved river valleys, and karst geology with spring-fed waterfalls and cold-water trout streams. Ecologically, the Driftless Area's flora and fauna are more closely related to those of the Great Lakes region and New England than those of the broader Midwest and central Plains regions. The steep riverine landscape of both the Driftless Area proper and the surrounding Driftless-like region are the result of early glacial advances that forced preglacial rivers that flowed into the Great Lakes southward, causing them to carve a gorge across bedrock cuestas, thereby forming the modern incised upper Mississippi River valley. The region has elevations ranging from 603 to 1,719 feet (184 to 524 m) at Blue Mound State Park, and together with the Driftless-like region, covers . Geologic origin Retreating glaciers leave behind material called drift composed of silt, clay, sand, gravel, and boulders. Glacial drift includes unsorted material called till and layers deposited by meltwater streams called outwash. While drift from early (pre-Illinoian) glaciations has been found in some parts of the region, much of the incised Paleozoic Plateau of Wisconsin and northwestern Illinois has no evidence of glaciation. Numerous glacial advances throughout the world occurred during the most recent Quaternary glaciation (also known as the Pleistocene glaciation). The Upper Midwest and Great Lakes region of North America was repeatedly covered by advancing and retreating glaciers throughout this period. The Driftless Area escaped much of the scouring and depositional action by the continental glaciers that occurred during the last ice age, which created significant differences in the topography and drainage patterns within the unglaciated area compared to adjacent glaciated regions. The region has been subjected to large floods from the melting Laurentide Ice Sheet and subsequent catastrophic discharges from its proglacial lakes, such as Glacial Lake Wisconsin, Glacial Lake Agassiz, Glacial Lake Grantsburg, and Glacial Lake Duluth. The last phases of the Wisconsin Glaciation involved several major lobes of the Laurentide Ice Sheet: the Des Moines lobe, which flowed down toward Des Moines on the west; the Superior lobe and its sublobes on the north; and the Green Bay