Steven Dutch, Professor Emeritus, Natural and Applied Sciences, University of Wisconsin - Green Bay
The panorama scans from SSW at left to north at right. The cluster of buildings at left center near the large green field is the University of Wisconsin-LaCrosse. The city of LaCrosse is built on a broad Pleistocene outwash plain. The Mississippi River is visible in the far distance and the bluffs beyond it are in Minnesota. At right center the distant bluffs are cut by a valley (Dakota Creek) up which Interstate 90 climbs. The bluffs beyond (right of) the valley are the location of Great River Bluffs State Park, where there is a good vista of the valley seen from the other side.
The geomorphically young appearance of the landscape is due to its Pleistocene history. The Mississippi River probably did not occupy this course before the Pleistocene. Not only is the valley young in time, but each Pleistocene ice advance tied up millions of cubic kilometers of water in the form of ice, dropping sea level a hundred meters or more. The mouth of the Mississippi was far offshore on the Gulf of Mexico continental shelf, and valley deepening propagated all the way upriver. Not only was the main valley rejuvenated, but the tributaries as well.
The bedrock floor of the valley here is about 60 meters (200 feet) below river level. During each ice retreat, the valley was scoured with huge discharges of meltwater and occasional catastrophic floods. The huge discharge accounts for the steepness of the bluffs, the great width of the valley, and its smooth curvature. As sea level rose, the valley began filling in, beginning at the mouth and again propagating all the way upriver.
|At left is a view looking southeast from the top of the bluffs. The steep cliff is guarded by a chain link fence, and tilting the camera was necessary to get a picture.
The bluffs are Prairie du Chien dolostone capping Cambrian sandstone. This landform is a classic mesa, as are all the bluffs along the river.
Hearing these landforms called "mesas" may be surprising, because mesas are usually regarded as western landforms. But we have a flat-topped hill with soft rocks protected by a resistant flat layer. What part of the definition doesn't apply. Actually, older geologic literature from a time when Wisconsin was considered part of the West unhesitatingly called these hills mesas.
|Another view of the dolostone cap.|
|Dolomite capping shaly sandstone just below the summit.|
|This view and the two below are from the road to the park. The steepness of the bluffs and the embayments suggest the bluffs were modified by quarrying.|
|The bluff north of Granddad Bluff. This bluff is visible at far right of the panorama above.|
|View of Granddad Bluff from the north|
Created 5 June 2001, Last Update 13 Sep 2018