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

Charles T. Jackson, noted physician, chemist, and geologist, was in charge of the first significant geological exploration of the Lake Superior region; his chief assistants were John W. Foster and Josiah D. Whitney. Mount Whitney in California was named for the latter. The survey, sponsored by the Federal Government, was completed in 1850 under the supervision of Foster and Whitney, after the resignation of Jackson.

Jackson and Foster first visited Isle Royale together in 1847. Jackson (1849, p. 609) later "characterized [the island] as a succession of ridges and swamps, densely covered with trees and shrubs." He (1849, p. 421) also commented upon the presence of "drift boulders" on the island and noted that as many were of a kind of rock not native to the island, they "must have been transported from a distance and deposited here." Jackson (1849, p. 388—389) believed these "erratic stones" to have been transported at least in part by ice and did discuss seasonal freezing of lakes and rivers, but he did not elaborate upon "the glacial theory" then recently expounded by Louis Agassiz.

Agassiz himself visited the Lake Superior region in 1848 and later published descriptions of glacial phenomena observed during that excursion (Agassiz, 1850). He traversed the north shore of Lake Superior as far west as the Hudson Bay Co. post at Fort William, opposite Isle Royale on the Canadian shore, but was un able to visit the island because of stormy weather.

During the final field season of the survey, in 1849, Foster and Whitney were fortunate in securing as an assistant geologist, Edouard Desor, who for the previous 10 years had served as secretary to Louis Agassiz, both in Switzerland and in America, until disagreements forced their separation (Marcou, 1896). Desor was not only intimately familiar with Agassiz's theory of continental glaciation, at that time new to the world, but had observed in Europe much of the evidence upon which the theory was based. Although Desor owes a great debt to Agassiz in his interpretations, he was nevertheless the first to provide detailed descriptions of the glacial geology of Isle Royale—striations, furrows, erratics, direction of transport, nature of the drift, and other features. Moreover, his summary of the glacial geology of the Lake Superior region was a significant. contribution to the knowledge of that remote wilderness (in Foster and Whitney, 1850, p. 186—218; in Foster and Whitney, 1851 p. 232—273). Without specific reference to Isle Royale, Desor also described beaches and terraces related to higher lake levels, noting that they postdated the drift. The highest point on Isle Royale and the second largest lake on the island bear his name, Mount Desor and Lake Desor.

Since the days of Desor, numerous studies have increased the understanding of the glacial and postglacial record and geologic history of the Lake Superior region. Suprisingly few studies have been carried out on Isle Royale itself, however. Perhaps the following quotation from Foster and Whitney (1850, p. 81) helps explain why.

The physical obstructions to a successful exploration of the island are greater than we encountered in any other portion of the mineral district. The shores are lined with dense but dwarfed forests of cedar and spruce, with their branches interlocking and wreathed with long and drooping festoons of moss. While the tops of the trees flourish luxuriantly, the lower branches die off and stand out as so many spikes, to oppose the progress of the explorer. So dense is the interwoven mass of foliage that the noonday sunlight hardly penetrates it. The air is stiffled; and at every step the explorer starts up swarms of musquitoes, which, the very instant he pauses, assail him.

Alfred C. Lane's (1898) classic report on the geology of Isle Royale placed greatest emphasis upon the bedrock formations, because of native copper mineralization within them on the island, and the glacial geology received only very brief treatment. George M. Stanley (1932; 1941) investigated shorelines of various lakes ancestral to Lake Superior and from evidence on Isle Royale was able to recognize a major postglacial lake stage, which he named Lake Minong. Minong is an old Indian name for Isle Royale, and the name is perpetuated as Minong Ridge on the Island. James H. Zumberge (1955) briefly discussed the nature of glacial erosion on Isle Royale in comparison with an area in northern Minnesota.

The present report is an outgrowth of a general study of the geology of Isle Royale National Park that is being carried out in cooperation with the National Park Service, in part to help provide geologic information for interpretive programs. The generous support by the Park Service staff that made this study possible is greatly appreciated. Discussions and a visit in the field with George Stanley have greatly furthered my understanding of the evidence for various glacial and postglacial lakes in the Superior basin.

During summers on the island, I was ably assisted, successively, by Robert J. Larson, Harrison T. Southworth, Charles E. Bartherger, and David R. Chivington.