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

Abandoned shorelines for a number of the lake stages between Lake Duluth and Lake Superior are evident on Isle Royale, although direct correlations are difficult except for some of the best-developed ones, such as those of the Minong and Nipissing stages. The highest well-developed shoreline features appear to correlate with lake levels between those of Lake Beaver Bay and Lake Minong, according to the profiles of Farrand (fig. 14). Examples include a mile-long barrier beach closing off a swamp slightly arbove 800 feet elevation¹ near the west end of Feldtmann Ridge (pl. 1) and a wave-cut cliff with a sea cave at about 810 feet elevation (Stanley, 1932, p. 39) south of Washington Harbor.

FIGURE 14.—Profiles of former water planes in northwestern Lake Superior basin. The profiles are essentially parallel to the Minnesota shoreline (after Farrand, 1960). (click on image for an enlargement in a new window)

The western part of Feldtmann Ridge, which is mantled with easily reworked glacial debris, was well suited for beach development. Study of aerial photographs discloses a series of beach bars, many of which close off swampy areas behind them, between the present lake level at Cumberland Point and the beach previously noted, which is slightly more than 200 feet above the present lake level. No higher beach features can be discerned from the photographs even though glacial debris readily susceptible to beach formation continues to mantle the ridge at higher elevations. If Farrand's profiles are correct with respect to Isle Royale, beaches that might be correlated with Lake Beaver Bay would exist close to 300 feet above the present lake level at this location. One cliff of conglomerate at about 300 feet elevation at the west end of Feldtmann Ridge has a distinct overhang, but it is not clear whether this might be due to undercutting by former wave action or to simple slumping of the rubbly conglomerate making up the cliff face.

On the north slope of Greenstone Ridge near Mount Desor, a series of benches extends nearly to the summit (800 ft above Lake Superior). Unfortunately, the slope is covered by glacial debris, talus, and other surficial materials; no bedrock outcrops or possible beach deposits were noted in several traverses down and along the slope. Stanley (1932, p. 41—42) first interpreted these benches as wave-cut features, but he did not find definitive exposures of beach materials and is not now confident of his earlier interpretations (G. M. Stanley, written commun., 1971). Because Stanley's (1932, p. 80) correlation of these benches with glacial lake stages immediately subsequent to Lake Duluth has been utilized by more recent workers (Farrand, 1960, p. 23; 1969, p. 194), their validity as wave-cut features becomes important.

Relatively level benches are characteristic of the north-facing slopes of all ridges On Isle Royale underlain by more than a single lava flow, including Greenstone Ridge. Near Ishpeming Point, about 5 miles east of Mount Desor, the north slope of Greenstone Ridge has benches very similar to those at Mount Desor. Near Ishpeming Point, however, the mantle of glacial debris is thinner than near Mount Desor, and scattered out crops can be found along many of the benches. The amygdaloidal texture of the rock of many of these out crops is typical of that near flow tops, and the benches appear to have been formed by differential erosion of the highly amygdaloidal, less-resistant flow tops within the sequence of flows making up the ridge. While wave-cut features and bedrock-controlled topographic features are not mutually exclusive, there is no need to attribute the formation of the benches near Mount Desor to wave action, and in the absence of supporting evidence such as beach deposits, a non-wave-cut origin appears more probable for these benches. Such an origin is supported by the fact that benches are not cut into ice-margin deposits angling down the slope on the south side of Greenstone Ridge near Mount Desor, which would be readily susceptible to wave erosion. Limited observations from the present study are not sufficient to resolve the problem of the nature and origin of the benches; if wave cut, they would indicate that at least the west end of Isle Royale was free of ice during glacial lake stages in the Lake Duluth—Lake Washburn interval, stages with which such shorelines would correlate (fig. 14).

A few other possible shoreline features higher than 800 feet have been noted by Stanley (1932, p. 43, 71) on the west end of the island east of Washington Harbor and on Feldtmann Ridge. These features are not well developed, however, and as they also consist of benches on north-facing ridge slopes with possible structural control, their significance is uncertain.

Lake Minong is the earliest of the lake stages for which abandoned beaches and other shoreline evidence can be found along the full length of Isle Royale (Stanley, 1932, p. 82 85).² Such evidence is most strikingly developed on the west end of the island where the abundance of glacial drift permitted more pronounced development, of shoreline features. Beaches from the Nipissing stage also are well developed on Isle Royale, as shown by Nipissing (and Minong) beaches discernible on the aerial photographs of glacial features west of Siskiwit Lake (fig. 9) and by a profile of barrier-bar beaches of the two lake stages between Rainbow Cove and Lake Feldtmann (fig. 15).

FIGURE 15.—Profile across barrier-bar beaches between Rainbow Cove and Feldtmann Lake (after Stanley, 1932). (click on image for an enlargement in a new window)

On the east end of Isle Royale, where glacial debris is limited and abandoned beaches are less evident, wave-cut features in the bedrock mark old shorelines (fig. 16). Prominent examples are Monument Rock, a stack associated with the Lake Minong shoreline north of Tobin Harbor (fig. 17) and an arch cut through a narrow ridge crest on Amygdaloid Island (fig. 18), probably associated with the shoreline of the Nipissing stage.

FIGURE 16.—Wave-cut shoreline illustrating development of stacks and arches.

FIGURE 17.—Monument Rock, a stack associated with the shore line of glacial Lake Minong. Adjacent wave-cut cliff is just out of photograph to right. National Park Service photograph by Tom Haas.

FIGURE 18.—Wave-cut arch on Amygdaloid Island. A short distance west is a collapsed arch.

In summary, shoreline evidence in itself indicates that the west end of Isle Royale may possibly have been free of glacial ice some time prior to the Lake Beaver Bay stage (see fig. 11) and was definitely free of ice after Lake Beaver Bay. Lake Minong was formed by the retreat of the ice front all the way to the north shore of the Lake Superior basin, and the entire island was, of course, free of ice at that time.

---

¹All elevations originating with this report have been obtained by interpolation from a topographic map (20-ft contour interval) after plotting location from aerial photographs and should be considered only approximate.

²In the reference cited, the Minong stage was initially correlated with the Fort Brady stage of Leverett and Taylor (1915) and is so labeled there. See Stanley (1936, p. 1955—1956; 1941, p. 1955) for initial Minong usage.

---