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GLACIAL AND POSTGLACIAL GEOLOGIC HISTORY OF ISLE ROYALE NATIONAL' PARK, MICHIG.AN Prepared in cooperation with the National Park Service / GEOLOGICAL SURVEY PROFESSIONAL PAPER 754-A Glacial and Postglacial Geologic History of Isle Royale National Park, Michigan By N. KING HUBER THE GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN GEOLOGICAL SURVEY PROFESSIONAL PAPER 754-A Prepared in cooperation with the National Park Service UNITED STATES GOVERNMENT PRINTING OFFICE, WASHINGTON 1973 UNITED STATES DEPARTMENT OF THE INTERIOR ROGERS C. B. MORTON, Secretary GEOLOGICAL SURVEY V. E. McKelvey, Director Library of Congress catalog-card No. 72-600299 For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C. 20402 - Price $1 (paper cover) Stock Number 2401-02209 CONTENTS Page Page Abstract __________________________________________ _ A1 Modification of topography by glacial deposition-Con. Drumlins _____________________________________ _ Introduction ______________________________________ _ 1 AS Historical background and acknowledgments __________ _ 1 Ice-margin deposits ____________________________ _ 8 Direction of ice movement_ _________________________ _ 4 Post-Valders lake stages in the Lake Superior basin ____ _ 8 Bedrock geology and its control of preglacial topography_ 4 Abandoned shorelines on Isle Royale __ .. ______________ _ 10 Modification of topography by glacial erosion _________ _ 6 Modification of topography by glacial deposition _______ _ 7 Significance of ice-margin deposits on Isle Royale ______ _ 12 General distribution of glacial deposits ___________ _ 7 References cited _______ ~ ___________________________ - 14 ILLUSTRATIONS Page PLATE 1. Map showing glacial (eatures and abandoned shorelines, Isle Royale National Park, Keweenaw County, Michigan __________________________________________________________________________________ lnpocket FIGURE 1. Map of the Lake Superior region______________________________________________________________________ A2 2. Map of Wisconsin ice retreat from central North America________________________________________________ 3 3. Shaded-relief map of Isle Royale National Park_________________________________________________________ 5 4. Sketch of the relation between ridge-and-valley topography and tilted lava flows and sedimentary rocks_______ 6 5-8. Photographs of: 5. Glacial striations ________________________________________________________________ -- _____ ------ 6 6. Glacially rounded and striated outcrop ______________________________________________________ ---_ 6 7. Spheroidal-weathering zone in a basalt lava flow__________________________________________________ 7 8. Granitic erratic near sum;mit of Feldtmann Ridge________________________________________________ 8 9. Stereoscopic pair showing drumlins, ice-margin deposits, and abandoned shorelines west of Siskiwit Bay_______ 9 10. Map of glacial Lake Duluth and contemporary ice border________________________________________________ 10 11. Diagram of water-level change and suggested intervals of ice retreat for the Lake Superior basin______________ 10 12. Map of glacial Lake Minong and contemporary North Shore ice border____________________________________ 10 13. Diagram of chronology_of lake-level changes in the Lake Superior basin____________________________________ 10 14. Diagram of profiles showing former water planes in northwestern Lake Superior basin_______________________ 11 15. Diagram of profile across barrier-bar beaches between Rainbow Cove and Feldtmann Lake___________________ 13 16. Sketch of wave-cut shoreline illustrating development of stacks and arches _________________________ -------- 13 17. Photograph of Monument Rock, a stack associated with the shoreline of glacial Lake Minong________________ 13 18. Photograph of wave-cut arch on Amygdaloid Island_____________________________________________________ 14 19. Map of hypothetical position of glacial ice margin prior to complete retreat of ice from Isle Royale____________ 14 III THE GEOLOGY OF ISLE ROYALE NATIONAL PARK, MICHIGAN GLACIAL AND POSTGLACIAL GEOLOGIC HISTORY OF ISLE ROYALE NATIONAL PARK, MICHIGAN By N. KING HUBER ABSTRACT INTRODUCTION Isle Royale was overridden by glacial ice during each of the Isle Royale (fig. 1) lies within an area that was gla­ four major glaciations of the Pleistocene Epoch, and each suc­ cessive glaciation essentially obliterated all direct evidence of ciated repeatedly during the Pleistocene Epoch; how­ preceding glaciations on the island. In the waning phase of the ever, direct evidence remains for only the latest glacia­ last major glaciation, the Wisconsin Glaciation, the frontal ice tion, the Valders Stade of the \iVisconsin Glaciation. margin retreated northward from at least the greater part of This stade represents a partial readvance, during the the Lake Superior basin, then readvanced into the liasin during general retreat of the Wisconsin ice sheet, which re­ Valders time, about 11,000 years ago. 'Ve can attribute to the Valders ice the final aspect of glaciation on Isle Royale, includ­ turned ice to the greater part, if not all, of the Lake ing both erosional and depositional features. Superior basin about 11,000 years ago (fig. 2). Because It is impossible to estimate the quantity of glacial debris or this readvance obliterated nearly all evidence of previ­ other surficial materials that might have been present on Isle ous glaciations, the striations, deposits, and most other Royale prior to the Valders readv·ance, but the readvancing ice features of glacial origin on the island can be attributed appears to have removed most of what might have been present, to Valders ice. Ice-margin deposits developed locally as judged by the thin surficial cover on the eastern two-thirds during pauses in the retreat of the Valders ice. In addi­ of the island today. During the Valders retreat, a series of lakes formed in the Lake Superior basin in front of the retreating ice tion, glacial and postglacial lakes, many of which filled margin. the Superior basin to higher elevations than the present The retreating ice opened successively lower outlets, and thus Lake Superior, left their marks on the island, chiefly in the general trend of lake elevations is downward. Distinct lake the reworking of glacial materials to form wave-cut stages reflect periods of relative stability during which well­ benches, cliffs, and beach deposits. defined shoreline features developed. The ice front forming the north margin of the earlier lakes probably remained south of HISTORICAL BACKGROUND AND Isle Royale until about the time of glacial Lake Beaver Bay, ACKNOWLEDGMENTS when it retreated to a position straddling Isle Royale west of Lake Desor. Aliundant deposits of glacial debris were left upon Charles T. ,Jackson, noted physician, chemist, and the newly deglaciated west end of the island, and the ice front geologist, \Yas in charge of the first significant geologi­ remainf'd stable long enough to build a complex of ice-margin cal exploration of the Lake Superior region; his chief deposits across the island. Shorelines fol'med by the glacial lake associat1~d with this ice front are found on the western part of assistants 'Xere ,John \V. Foster and Josiah D. Whitney. the island about 200 feet above present Lake Superior. ::\'Iount \Yhitney in California was named for the latter. Subsequent renewed and complete retreat of the ice margin The surn•,v, sponsored by the Federal Govemment, was from Isle Royale was rapid enough that only a minor amount of completed in 1850 under the supervision of Foster and glacial debris was deposited on the central and eastern parts 'Vhitm•y, after the resignation of ,Jackson. of the island. When the ice margin reached the north edge of the Lake Superior IYasin, Lake Minong was formed, and the en­ Jackson and Foster first visited Isle Royale together tire basin was filled for the first time since the Valders advance. in 1847. Jackson (18-±9, p. 609) later "characterized Lake ~iinong marked a relatively stable episode in the history [the island] as a succession of ridges and swamps, of the liasin, and its beaches are among the liest developed of densely covered with trees and shrubs." He (1849, p. the aliandoned shoreline features on Isle Royale. Lake ~iinong ±21) also commented upon the presence of "drift lieaches and later lower beaches are liest developed on the southwest end of Isle Royale, where abundant glacial debris boulders" on the island and noted that as many were of provided easily worked materials for beach construction. a kind of rock not native to the island, they "must have Al A2 GEOLOGY OF I·SLE ROYALE NATIONAL PARK, MICHIGAN 49" 45" 89" 85" FIGURE 1.-Lake Superior region. Dotted lines from Isle Royale :-lational Park to mainland indicate passenger ferry service. been transported from a distance and deposited here." but had observed in Europe much of the evidence upon Jackson ( 1849, p. 388-389) believed these "erratic which the theory w.as based. Although Desor owes a stones" to have been transported at least in part by ice great debt to Agassiz in his interpretations, he was and did discuss seasonal freezing of lakes and rivers, nevertheless the first to provide detailed descriptions but he did not elaborate upon "the glacial theory" then of the gl~tcial geology of Isle Royale- striations, fur­ recently expounded by Louis Agassiz. rows, erra,tics, direction of transport, nature of the drift, Agassiz himself visited the Lake Superior region in and other features.

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