Becker, George C. / Fishes of Wisconsin
Wisconsin waters, pp. 3-17 PDF (5.6 MB)
Wisconsin Waters Glacial History Available evidence indicates that the north central states were affected by at least four major glacial periods, which were separated by intervals during which the climate was warmer than at present. The last major glacial advance, known as the Wisconsin glaciation, moved south as far as mid-America. It is com- monly accepted that fish in northern waters moved southward ahead of the lip of the advancing ice sheet, and northward as the ice sheet retreated. Students of fish distribution have noted that many Great Lakes fishes are related to spe- cies of the Mississippi River basin. An explanation for this relationship is found when the drainage patterns of the glacial Great Lakes are examined. As the ice cap retreated, bodies of meltwater formed in the lake basins carved out by the glacier and drained in part into the Mississippi basin. Early in the Wisconsin glaciation, the first meltwater lake, known as Lake Chicago, ap- peared where southern Lake Michigan is located today; it drained by way of the Illinois River southwestward into the Mississippi River. Approximately 8 to 9 thousand years ago, when most of the Great Lakes were freed of glacial ice, an emerging Lake Superior, known as glacial Lake Duluth, drained southward by way of the St. Croix River into the Mississippi River system (see map, Main Algonquin stage). During the same period, Lake Ontario (glacial Lake Iroquois) drained eastward into the Hudson River system, providing an early entryway for Atlantic drainage fishes into the Great Lakes. The present lakes and streams of northern and eastern Wisconsin have been largely shaped by past glaciations. The southwestern quarter of the state, how- ever, lacks the features of erosion and deposition brought about by the conti- nental glacier (see map of glacial lobes and the driftless area). This is the drift- less area of Wisconsin, famous the world over because it is completely surrounded 3
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