The glacial history of Minnesota is most defined since the onset of the last glacial period, which ended some 10,000 years ago. Within the last million years, most of the Midwestern United States and much of Canada were covered at one time or another with an ice sheet. This continental glacier had a profound effect on the surface features of the area over which it moved. Vast quantities of rock and soil were scraped from the glacial centers to its margins by slowly moving ice and redeposited as drift or till. Much of this drift was dumped into old preglacial river valleys, while some of it was heaped into belts of hills (terminal moraines) at the margin of the glacier. The chief result of glaciation has been the modification of the preglacial topography by the deposition of drift over the countryside. However, continental glaciers possess great power of erosion and may actually modify the preglacial land surface by scouring and abrading rather than by the deposition of the drift. The marks of glaciation vastly altered Minnesota's topography. Probably the most significant change was in the character and extent of the drainage. In preglacial times, there is reason to believe that most of the rainwater or meltwater from snow was quickly carried back to the ocean. Today, much of the precipitation is retained temporarily on the surface in the lakes. Streams meander from lake to lake, and only part of the total precipitation is carried away by the rivers. Such topography could be described as immature because the streams have not yet been able to establish themselves into a network that quickly and efficiently drains the land. The Mississippi River has cut a deep valley below St. Anthony Falls, but even the waters of this large river do not flow freely to the ocean because of Lake Pepin, which acts as a storage basin for some of the water. Streams have been actively engaged in their erosive work only for the last 10,000 years, the estimated length of time since the last glacier began its final retreat. This time span is relatively insignificant compared to the long history of the Earth.
Sequence of glacial events
Minnesota has been covered, at least in part, by a glacier numerous times during the Quaternary ice age. In order of increasing age, these advances took place during the Wisconsin Episode and Illinoian stages; prior to this continental ice sheets advanced into and retreated from Minnesota multiple times during the Pre-Illinoian Stage. The ice moved into Minnesota at different times from three glacial centers, the Labradorian center in northern Quebec and Labrador; the Patrician center, just southwest of Hudson Bay; and the Keewatin center, northwest of Hudson Bay. Deposits left by the continental ice sheets advancing from these three centers reflect the characteristics of the rocks over which they passed. The Keewatin ice encountered the Cretaceous limestones and shales of Manitoba and the Red River Valley, whereas the Patrician and Labradorian ice moved over iron-rich Pre-Cambrian crystalline rocks of the Canadian Shield.
Pre-Wisconsin Episode glaciation There are few areas in which the earlier drifts from the glacial deposits of the Pre-Ilionian or Illinoian stages are exposed at the surface. The extreme southeastern and southwestern portions of Minnesota (Driftless Area) have extensive areas of pre-Wisconsin drifts, but they are masked almost everywhere by surficial covering of loess (wind-blown silt). Furthermore, these regions of older drift are maturely drained, because the streams have had a longer time to evolve into an efficient drainage system compared with the streams flowing in areas covered by younger glacial deposits. Howard Hobbs has proposed that the Pre-Illinoian glacial deposits in southeastern Minnesota are actually younger Illinoian glacial deposits.
Changes in the course of continental rivers As the ice sheets moved into the central portion of North America, the rivers that used to flow from the Rocky Mountains to the northeast into the Arctic Ocean found their valleys choked with ice. The rivers had to divert around the farthest extensions of the ice. When the ice retreated, the new valleys eroded into the landscape kept the rivers from moving back to their old positions.
Wisconsin Episode Glaciation
The Wisconsin glacial episode, the most recent glacial period, has been subdivided into four substages, each representing an advance and retreat of the ice. The substages, named from the oldest to the youngest, are the Iowan, Tazewell, Cary and Mankato. Only the Iowan, Cary and Mankato are recognized in Minnesota, but studies indicate that the Tazewell drift may be present in southwestern Minnesota. The Iowan drift occurs extensively at the surface only in southwestern and southeastern Minnesota, and contains few, if any, lakes because of the relatively mature surface drainage. The Tazewell drift in the southwestern Minnesota is devoid of lakes; in fact, the criterion of drainage was used by Robert Ruhe to distinguish Tazewell from Cary drifts. Nearly all of the lakes in Minnesota are found within the borders of the Cary and Mankato drifts. For this reason, it is necessary to consider in some detail the nature and distribution of these two drift sheets.
Cary substage The glaciers that advanced out of the northeastern portion of Canada were of sufficient thickness to produce significant erosion in northeastern Minnesota. Because the affected area reached somewhat south of Minneapolis, it is called the "Minneapolis lobe". The Minneapolis lobe is characteristically red and sandy because of red sandstone and shale source rocks to the north and northeast; it may be recognized as well by pebbles of basalt, gabbro, red syenite, felsite and iron formation from northeastern Minnesota. Ground moraine with uncharacteristic reddish iron-rich sediments extended from St. Cloud, Minnesota, back northeastward. The glaciers produced a set of terminal moraines which extend from northwest of St. Cloud into the Twin Cities and up into central Wisconsin. They deposited reddish sands and gravels westward and southward in outwash plains.
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