Driftless area
The Driftless Area or Paleozoic Plateau is a region in the American Midwest noted mainly for its deeply carved river valleys. While primarily in southwestern Wisconsin, it includes areas of southeastern Minnesota, northeastern Iowa and extreme northwestern Illinois. The region includes elevations ranging from 603 to 1,719 feet (184 to 524 m) at Blue Mound State Park and covers an area of 16,203 square miles (42,000 km2). The region's peculiar terrain is the result of its having escaped glaciation in the last glacial period.
Retreating glaciers leave behind silt, clay, sand, gravel, and boulders called drift. Glacial drift includes unsorted material called till and layers deposited by meltwater streams called outwash While some glacial drift has been discovered, this is said to be of Pre-Illinoian age, about 500,000 years old.
The region has been subject to the regular catastrophic effects of glacial lake outburst floods involving the cataclysmic collapse of ice dams holding in such bodies as Glacial Lake Agassiz, Glacial Lake Grantsburg, and Glacial Lake Duluth.
The last phases involved several major lobes: 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. 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.
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