Geological Survey Bulletin 1309 The Geologic Story of Isle Royale National Park

As we pick up an agate from the beach and admire its color and pattern, we can reflect upon the enormous amount of change that has taken place since it originated as an amygdule in a lava flow a billion or so years ago. The time from the volcanic eruptions to the coming of the glacier that may have brought that agate to its resting place, however, was itself enormous, even in the geologic sense—nearly one-fourth the age of the earth itself. And although things are relatively quiet on Isle Royale as the present time, geologically speaking, changes will continue to take place slowly but certainly. Storm waves will continue to batter the shoreline cliff's, wearing them back. Winter frost will loosen rock fragments on the hillsides for the summer rains to wash downslope. The streams will deposit silt in the landlocked lakes until they are filled and cease to exist as bodies of water.

These continuing changes take place so slowly that they are nearly imperceptible to us. They are quite different from the cataclysmic eruption of a volcano or the steady advance and envelopment of an ice sheet. But perhaps this is just a quiet moment in the continuing geologic history of Isle Royale. While renewed volcanic activity is unlikely, many scientists believe that we are now in an interglacial epoch and that massive ice sheets will again form on the Canadian Shield to move inevitably southward, again modifying the earth's surface as they grind their way over it. Perhaps the future glacier might even bring some new agates to the beach for another visitor to admire.

Alfred C. Lane opened his geologic report on Isle Royale with the following words: "The progress of knowledge is like the growth of a coral reef; each generation builds upon that which has been left behind by those who have gone before." So it is with this report. I have drawn upon so many sources of information in presenting this geologic story of Isle Royale that it is impossible to acknowledge the individual contributions of each. This is especially true of the historical material and of numerous works that have delineated the regional geologic framework within which I have placed Isle Royale. For those who wish to delve further into various aspects of the history and geology of this part of the Lake Superior region, several pertinent references, briefly annotated, are listed in the bibliography; these in turn will lead to additional source materials.

As part of the current work, a new multicolor geologic map of Isle Royale National Park has also been prepared (Huber, 1973c). It has been published separately and is at a scale of 1:62,500 (approximately 1 in. = 1 mi). At that scale it naturally shows much more detail than any of the maps in this report and illustrates more graphically many of the geologic features described herein. It is thus highly recommended as a supplement to this report. Several reports more technical than this one also have been prepared, one of them with Roger G. Wolff (Huber, 1971, 1973a, b; Wolff and Huber, 1973).

The study leading to the preparation of this report and the companion geologic map and other reports was conceived and carried out as a cooperative project between the Geological Survey and the National Park Service with two goals in mind: to take advantage of the geologic research potential of the park and to help interested visitors understand its geologic story. During the four summers that I spent mapping the geology of Isle Royale, the hospitality and enthusiastic support of every member of the Park Service staff contributed immeasurably to the success of the project. In the geologic mapping I was ably assisted by Robert J. Larson in 1966, Harrison T. Southworth in 1967, Charles E. Bartberger in 1968, and David R. Chivington in 1969.

Henry R. Cornwall and Walter S. White aided considerably through numerous discussions that drew upon their broad knowledge of Keweenawan geology. Most of the text and many illustrations for the discussion of the postglacial lake stages are based largely on the work of William R. Farrand (figs. 56, 57, 58, 60) and the unpublished Ph.D. dissertation (Univ. of Michigan, 1932) of George M. Stanley (figs. 55, 61, 64).