Geological Survey Professional Paper 754-A Glacial and Postglacial Geologic History of Isle Royale National Park, Michigan

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.

FIGURE 10.—Glacial Lake Duluth and contemporary ice border, about 11,500 years ago (after Farrand, 1969).

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).

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.

FIGURE 12.—Glacial Lake Minong and contemporary north shore ice border, about 10,500 years ago (after Farrand, 1969).

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).