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Geological Survey Professional Paper 754-A
Glacial and Postglacial
Geologic History of Isle Royale National Park, Michigan
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POST-VALDERS LAKE STAGES IN THE LAKE SUPERIOR BASIN
Some uncertainty exists as to whether Valders ice
completely filled the Lake Superior basin (Black, 1969, 1970; Farrand,
1970; Wright, 1970; Wright and Watts, 1969; Zoltai, 1965). However far
the Valders ice advanced, when it retreated it left a series of lakes
that progressively filled more and more of the Lake Superior basin. The
elevation of water in the lakes was controlled by periodic changes in
their outlets, which were at times dammed by glacial ice. Lake levels
were further influenced by isostatic rebound, or uplift of the earth's
crust as the weight of the ice was removed. The sequential history of
the series of lakes has been largely deduced from correlations of their
abandoned shorelines around the Lake Superior basin. The most recent
comprehensive study of these shorelines is that of Farrand (1960;
summarized 1969). This study, together with observations of Stanley (1932)
for Isle Royale, provides the basic framework for much of the following
discussion. Hough (1958; 1963) presented regional syntheses for the
entire Great Lakes system; Kelley and Farrand (1967) gave a nontechnical
account of the glacial lakes around Michigan, and Dorr and Eschman
(1970) included similar material in their volume on the geology of
Michigan.
The first glacial lake (and the highest at 1,085 feet
above sea level) to fill a major portion of the Lake Superior basin
following the Valders Stade was Lake Duluth (fig. 10). The history of
Lake Duluth and subsequent lakes has been concisely summarized by
Farrand (1969, p. 194) as follows:
Apparently the ice sheet was in a period of rather rapid retreat when
Glacial Lake Duluth formed. The uppermost Duluth beach
can be followed some 90 miles northeastward from
Duluth along the Minnesota shore and as far east as the base of the
Keweenaw Peninsula. Subsequent shorelines of Lake Duluth extend to the
international border at the Pigeon River and to Isle Royale. Then the
ice front seems to have paused in its retreat; the shorelines of the
subsequent Post-Duluth lakes do not extend
much farther north and east than those of Lake Duluth
itself, although lake level fell some 500 feet during this time [fig.
11]. * * * A number of individual lake stages (named High
bridge, Moquah, Washburn, Beaver Bay, etc.) have been recognized in this
interval, but they are mostly weakly developed and apparently mark
minor halts in a period of rapidly falling lake level. Another period of
relatively rapid retreat of the ice front opened up the entire lake as
we now know it, bringing into existence Glacial Lake Minong [fig. 12].
The original altitude of Lake Minong before rebound, was around 450 feet
above sea level, or some 140 feet lower than the present Lake.
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FIGURE 10.Glacial Lake Duluth and contemporary
ice border, about 11,500 years ago (after Farrand, 1969).
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FIGURE 11.Water-level change and suggested intervals of ice
retreat for the Lake Superior basin. Curvature is due to post glacial
rebound (uplift), progressively greater to the north ast (after
Farrand, 1969).
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An even lower lake, Lake Houghton (375 ft above sea
level), followed Lake Minong. Then, during a long period, slowly
rising lake levels controlled by glacial rebound resulted about 5,000
years ago in the Nipissing lake stage at 605 feet (fig. 13). As a result
of minor changes related to decreasing but continuing rebound, the
present Lake Superior, at 602 feet, evolved. Farrand (1969) provided
details on changes in outlets that were the immediate causes for the
establishment of various lake levels.
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FIGURE 12.Glacial Lake Minong and contemporary
north shore ice border, about 10,500 years ago (after Farrand,
1969).
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FIGURE 13.Chronology of lake-level changes in
the Lake Superior basin. Note the levels of various outlets for
different lake stages and the role played by postglacial rebound
(uplift) in controlling lake level since about 10,000 years ago (after
Farrand, 1969).
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pp/754a/sec6.htm
Last Updated: 01-Mar-2005
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