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Becker, George C. / Fishes of Wisconsin

Wisconsin waters,   pp. 3-17 PDF (5.6 MB)

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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
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
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
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
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,
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,
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

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