Geological Formation
Retreating glaciers leave behind silt, clay, sand, gravel, and boulders — called drift. Glacial drift includes till (unsorted material) and outwash (layers deposited by meltwater streams). While some glacial drift has been discovered, this is said to be of Pre-Illinoian age about 500,000 years old.
What is clear is that 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 (see Jökulhlaup).
The earlier local phases of the Wisconsinan glaciation are poorly understood, but the last involved several major lobes, the Des Moines lobe, which flowed down to Iowa's capital city 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 exceedingly 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.
Regarding the origin of the Driftless Area, a particular new concept or train of thought involves the pre-Illinoian continental glacial ice originally flowing over the Driftless Area while depositing pre-Illinoian till (only that till which is older than 790,000 years old) upon it but then periglacial erosion later removed it when the ice retreated and uncovered the area. This was done through anticyclonic snow-bearing winds episodically dropping a large amount of snow which consequently gradually removed superficial sediment by solifluction and snowmelt sheetwash processes.
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.
Read more about this topic: Driftless Area
Famous quotes containing the words geological and/or formation:
“The crystal sphere of thought is as concentrical as the geological structure of the globe. As our soils and rocks lie in strata, concentric strata, so do all men’s thinkings run laterally, never vertically.”
—Ralph Waldo Emerson (1803–1882)
“Out of my discomforts, which were small enough, grew one thing for which I have all my life been grateful—the formation of fixed habits of work.”
—Elizabeth Stuart Phelps (1844–1911)