page_title: Driftless Area
section_title: Geologic origin
hierarchical_section_title: Driftless Area / Geologic origin
caption_attribution_description: English: Glacial map of the great lakes region
context_page_description: The Driftless Area is a region in southwestern Wisconsin, southeastern Minnesota, northeastern Iowa, and the extreme northwestern corner of Illinois, of the American Midwest. The region escaped the flattening effects of glaciation during the last ice age and is consequently characterized by steep, forested ridges, deeply carved river valleys, and karst geology characterized by 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. Colloquially, the term includes the incised Paleozoic Plateau of southeastern Minnesota and northeastern Iowa. The region includes elevations ranging from 603 to 1,719 feet at Blue Mound State Park and covers 24,000 square miles. The rugged terrain is due both to the lack of glacial deposits, or drift, and to the incision of the upper Mississippi River and its tributaries into bedrock.
context_section_description: 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 lobe and Lake Michigan lobes on the east. The northern and eastern lobes were in part diverted around the area by the Watersmeet Dome, an ancient uplifted area of Cambrian rock underlain by basalt in northern Wisconsin and western upper Michigan. The southward movement of the continental glacier was also hindered by the great depths of the Lake Superior basin and the adjacent highlands of the Bayfield Peninsula, Gogebic Range, Porcupine Mountains, Keweenaw Peninsula, and the Huron Mountains along the north rim of the Superior Upland bordering Lake Superior. The Green Bay and Lake Michigan lobes were also partially blocked by the bedrock of the Door Peninsula, which presently separates Green Bay from Lake Michigan. In earlier phases of the Wisconsinan, the Driftless Area was totally surrounded by ice, with eastern and western lobes joining together to the south of it.
  Another factor that may have contributed to the lack of glaciation of the Driftless area is the fractured, permeable bedrock within the paleozoic plateau underlying it, which would have promoted below-ground drainage of subglacial water that would otherwise have lubricated the underside of the glacial ice sheet. The dewatering of the underside of the ice sheet would have inhibited forward movement of the glacier into the Driftless Area, especially from the west.
  The latest concept explaining the origin of the Driftless Area is the pre-Illinoian continental glacial ice flowing over the Driftless Area and depositing on it pre-Illinoian till, which is more than 790,000 years old. When the ice retreated and uncovered the area, intensive periglacial erosion removed it. Anticyclonic snow-bearing winds episodically dropped large amounts of snow, which then gradually removed superficial sediment from slopes by solifluction and snowmelt overland flow (sheetwash), washing the deposits down to stream valleys that ultimately flowed into the Mississippi River.
  In the adjacent glaciated regions, the glacial retreat left behind drift, which buried all former topographical features. Surface water was forced to carve out new stream beds. This process was absent in the Driftless Area, where the original Drainage systems persisted during and after the ice age. Water erosion continued carving the existing gullies, ravines, stream beds, and river valleys ever deeper into the paleozoic plateau, following the original drainage patterns.
caption: Glacial map of the great lakes region. Areas with diagonal hatching were glaciated previously.
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