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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.〔Richmond, G.M. and D.S. Fullerton, 1986, ''Summation of Quaternary glaciations in the United States of America'', Quaternary Science Reviews. vol. 5, pp. 183-196.〕〔Gibbard, P.L., S. Boreham, K.M. Cohen and A. Moscariello, 2007, (''Global chronostratigraphical correlation table for the last 2.7 million years v. 2007b'' ), jpg version 844 KB. Subcommission on Quaternary Stratigraphy, Department of Geography, University of Cambridge, Cambridge, England〕 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. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Glacial history of Minnesota」の詳細全文を読む スポンサード リンク
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