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

The distribution of individual ice-margin deposits on Isle Royale indicates all were not formed coevally. And yet the system of deposits as a whole must have been formed by a relatively stable ice front during a pause in its general retreat from the Lake Superior basin. The problem, then, is to attempt to relate this system to a specific glacial lake stage and thus to fix the position of the ice margin with respect to that stage.

Several lines of evidence can be brought to bear upon this problem. First, the sharp topographic nature of the ice-margin deposits strongly suggests that they were deposited by ice on land rather than in a lake possibly marginal to the ice front. Thus any lake present at the time of their formation must have been lower than the lowest ice-margin deposit. Second, the deposits show no evidence of wave erosion, further indicating that they were formed subsequent to glacial lakes at higher levels. Third, ice-margin deposits on both the north and south sides of the island extend down to elevations near 800 feet, giving a rough maximum elevation for shore lines to be correlated with this ice-margin. Fourth, the absence of similar deposits below this elevation suggests that a glacial lake was indeed present at about the elevation of the lowest ice-margin deposit and that subsequent rapid retreat of the ice front prevented formation of lower ice-margin deposits associated with progressively lower lake levels. It is perhaps, then, more than coincidence that the highest unequivocal shoreline occurs at about this elevation on western Isle Royale.³

It was noted in the previous section that shorelines somewhat above 800 feet elevation for this part of Isle Royale would correlate with glacial lake stages intermediate to Lake Beaver Bay and Lake Minong, according to the profiles of Farrand (fig. 14). Allowing for subsequent uplift due to rebound, the lake (or lakes) would probably have had an elevation between 500 and 600 feet above sea level. An ice front straddling Isle Royale (fig. 19) during this interval appears compatible with other data presented by Farrand (1960, 1969) for the Lake Superior basin as a whole.

FIGURE 19.—Hypothetical position of glacial ice margin during final stages of deposition of ice-margin deposits on Isle Royale and prior to complete northeastward retreat of ice from the island. Ice-margin deposits shown in solid black.

An ice front during post-Beaver Bay time in the position of the Isle Royale complex of ice-margin deposits poses another problem concerning the position of the ice margin prior to that time. Alternate conclusions depend upon the validity of evidence for higher level shorelines on the island.

If the evidence for higher level shorelines postulated by Stanley (1932) is valid, especially in the vicinity of Mount Desor, the ice margin must have been well northeast of the complex of ice-margin deposits by at least the later stages of Lake Duluth, and the deposits would indicate a readvance of the ice front during Beaver Bay or post-Beaver Bay time. The higher beaches would have to have been preserved on north-facing lee slopes in front of the readvancing ice margin, and any earlier ice-margin deposits that might have existed northeast of the present complex would have been destroyed by the readvance. Farrand (1960, fig. 5, pl. 3; 1969, fig. 8) showed an inferred ice border for Lake Washburn (fig. 11) crossing Isle Royale well northeast of the complex, but this position was based on the data of Stanley (1932). I am not aware of any evidence for a readvance of the ice front during retreat of the Valders ice from the Lake Superior basin.

If it is accepted that the benches near Mount Desor are bedrock-controlled topographic features not formed by wave action, evidence to indicate that any part of Isle Royale was free of ice prior to the Lake Beaver Bay stage would be lacking. According to this interpretation, the ice front would be placed southwest of Isle Royale for all lake stages from Lake Duluth through Lake Beaver Bay, across Isle Royale in the vicinity of the complex of ice-margin deposits sometime during the Beaver Bay—Minong interval, and finally, near the north shore of the Lake Superior basin at the time Lake Minong was formed. While I prefer the interpretation that favors a non-wave-cut origin for the benches, I cannot unequivocally rule out the possibility that they represent higher level shorelines. Nevertheless, further field study of the Mount Desor area is desirable.

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³Shoreline tilt has been neglected in this discussion; for the limited area of western Isle Royale, differences in shoreline elevation at tributable to tilt are probably less than the error of estimate for the elevations given. In addition, during the time of formation of the ice margin deposits, we are most likely dealing with a range of lake levels on the order of several tens of feet, or even more.

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