o· THE GEOLOGY OF COOK COUNTY, MINNESOTA Wave-built gravel bar damming mouth of small stream west of Deronda Bay on Lake Superior (Sec. 35, T . 63 N ., R. 5 E .), looking west. A series of points beyond the bar are characteristic of the shore line of Keween­ awan Aows which dip gently toward the lake and strike at about 45 ° to the shore line. The spurs are formed by the main part of the Aow and the coves by the amygdaloidal zones. UNIVERSITY OF MINNESOTA MINNESOTA GEOLOGICAL SURVEY GEORGE M. SCHWARTZ, DIRECTOR BULLETIN 39 The Geology of Cook County Minnesota BY FRANK F. GROUT ROBERT P. SHARP GEORGE M. SCHWARTZ MINNEAPOLIS • 1959 THE UNIVERSITY OF MINNESOTA PRESS PRINTED IN THE UNITED STATES OF AMERICA AT THE LUND PRESS, INC., MINNEAPOLIS I~O Library of Congress Catalog Card Number: A59-9565 PUBLISHED IN CANADA BY THOMAS ALLEN, LTD., TORONTO FOREWORD Cook County forms the extreme northeastern part of Minnesota lying between Lake Superior and the Canadian border. Glaciation has scraped off the ridges, leaving bare rock or thin soil at many places. Preglacial erosion has left a rugged topography only moderately changed by ice action and characterized by beautiful long narrow parallel lakes. The position of the county on the north shore of the largest fresh-water lake in the world, which is also deep and cold, results in a relatively cool climate ideal for summer tourists but not favorable to most types of agriculture. As a consequence most of the county is still forested, a con­ siderable proportion belonging to the Superior National Forest, and roads are at a minimum. A part of the National Forest is included in the so­ called roadless area and air space reservation. Access to large areas in the county is mainly by canoe. The relatively extensive area of rock exposure has naturally led to much interest in the mineral possibilities of the county. For this reason the Minnesota Geological Survey has always been actively concerned in working out the geology of the area as a background for investigation of the mineral deposits. The only preceding description of the geology of the county is a chapter by U. S. Grant in the fourth volume of the Final Report, Geology of Minnesota, by N. H. Winchell, published in 1899. It was in this remote and comparatively wild area that Professor Grout started his geological work in 1913 which continued, as conditions permitted, until his retirement in 1948. Literally dozens of assistants have taken part in the difficult task of mapping the nearly 1700 square miles included in the county. :Many of the problems have only been rcvealed and remain to be worked out in detail. The extensive maps compiled by Professor Grout will furnish a basis for future geological work. A debt of gratitude is owed to him by the state of Minnesota for devoted work extending over the major part of a lifetime. It is fitting that after his death on August 1, 1958, his ashes were spread on the waters of the falls at the outlet of Gun­ flint Lake. Robert P. Sharp prepared most of the introductory material and the discussion of glacial geology. George M. Schwartz assisted in prepara­ tion of the remainder of the manuscript. It is hoped that the report will be useful in the development of the county. GEORGE M. SCHWARTZ v ACKNOWLEDGMENTS Work in the county by the senior author of this report began in 1913 and was carried on intermittently to 1956. During this long period Pro­ fessor Grout was assisted by many students acting as field assistants as well as by other members of the staff at the University of Minnesota. The work, often extremely arduous, of these assistants and staff members is gratefully acknowledged. Student assistants included Thomas F. Andrews, Robert Bell, Robert Berg, Ronald Brown, D. M. Danielson, Nathan Davies, Lloyd Dreves­ kracht, E. C. Edwards, M. G. Edwards, Howard Evans, William Fackler, Sigmund Fruehling, Samuel Grantham, Anton Gray, Elliot Griffith, George Gryc, Leroy Hassentab, R. F. Hodgeman, Hugh Kendall, E. H. Lathram, A. I. Levorsen, R. C. Marmaduke, L. M. Miller, John Moga, Clemens Nelson, Edward Nicholson, Lincoln Page, William J. Pettijohn, Warren Pickering, R. A. Ranta, Ronald Sorem, R. P. Sovereign, William Strunk, Stanley Sundeen, Roger Swanson, Donald Taylor, Nelson Tay­ lor, Francis Wells. Members of the staff who worked as field geologists included T. lVI. Broderick, J. W. Gruner, F. B. Hanley, G. M. Schwartz, and R. P. Sharp. Much of the work was under the general direction of William H. Emmons, Director of the Minnesota Geological Survey from 191~ to 1944. Our appreciation and thanks also go out to the many individuals and corporations who have contributed to this report in various ways: lVIr. Otto Munson, Johnson Nickel Mining Co., St. Paul; the lVI. A. Hanna Co., Nashwauk; the E. J. Longyear Co., Minneapolis; Butler Brothers, St. Paul; the Mines Experiment Station, University of Minnesota; the Office of the Commissioner of the Iron Range Resources and Rehabilitation, St. Paul. Local residents were uniformly helpful in the work and so many were involved that a list would resemble a directory of Cook County. Their cooperation was invaluable, and sincere thanks are extended to all who helped in making the geology of their county better known. Special thanks are due to the regional office of the U.S. Forest Service at Milwaukee for a set of township maps of the 1948 Timber Survey. vii CONTENTS FOREWORD............................................... v ACKNOWLEDGMENTS ..................................... VII ABSTRACT ................................................. xv 1. INTRODUCTION ........................................ 1 General Features, 1. Physiographic Divisions, ~. Coastal Hills, ~. In­ terior Upland, 4. Northern Ridges and Valleys, 5. Rivers, 7. Lakes and Swamps, 8. Culture, 9. Climate, 10. 2. GENERAL GEOLOGY ................................... 1~ Ely Greenstone, 1~. Saganaga Granite, 15. Lamprophyres, 18. Knife Lake Group, 18. Animikie Group, ~O. Pokegama Quartzite, ~O. Gun­ flint Formation, ~1. Rove Formation, ~5. Keweenawan Group, ~9. Puckwunge Formation, ~9. North Shore Volcanics. 30. Basalt, 31. Ophite, 31. Porphyrite, 31. Rhyolite, 3~. Flow Units, 33. Interflow Sediments, 34. Crevice Fillings, 35. Sequence of Lava Flows, 35. Re­ gional Relations of the Flows, 39. Duluth Gabbro Complex, 40. Gab­ bro Facies, 41. Mineral Descriptions, 43. Intermediate Rocks, 44. Granophyre, 45. Anorthosite in Gabbro, 47. Contact Metamorphism, 47. Inclusions in Gabbro, 49. Logan Intrusions, 50. Topographic Ex­ pression, 51. Petrography, 5~. Reservation River Diabase, 54. Pigeon Point Sill, 54. Anorthosite Inclusions, 55. Granite and Granodiorite in Western Cook County, 56. The Wine Lake Intrusion, 57. The Dent Lake Intrusion, 57. The Smoke Lake Intrusion, 57. Basalt Dikes, 58. 3. GLACIAL GEOLOGY. .. .. .. .. .. .. .. .... 59 Features of Glacial Erosion, 59. The Glacial Tills, 60. Rainy Lobe Till, 60. Superior Lobe Till, 63. Glacial Lake Deposits, 65. Knob and Kettle Drift, 66. Eskers, 67. Drumloid Ridges, 69. Shorelines of the Glacial Great Lakes, 70. 4. SUMMARY OF THE GEOLOGICAL HISTORY OF COOK COUNTY ......................................... , 75 5. ECONOMIC GEOLOGY .................................. 78 Principal Deposits, 78. The Gunflint Iron-Bearing District, 78. Titani­ ferous Magnetite, 80. Petrographic Character, 85. Minor Deposits, 86. Copper-Nickel Sulfides, 86. Cobalt, 88. Anorthosite, 89. Gravel ix x GEOLOGY OF COOK COUNTY and Sand, 90. Soils, 92. Building Stone, 93. Other Resources, 93. Water Suppl~', 93. Water Power, 94. Forest Resources, 94. 6. DESCRIPTIONS OF TOWNSHIPS ........................ 95 APPENDIX. .. 153 REFERENCES ............................................. 158 INDEX ..................................................... 161 LIST OF ILLUSTRATIONS 'Wave-built gravel bar, west of Deronda Bay on Lake Superior ..................................... Frontispiece 1. Location of Cook County, Minnesota ............. 2 2. Drainage basins of the principal streams of Cook County ..... 3 3. Sections across Cook County showing physiographic divisions. 3 4. Elongate lakes in Duluth gabbro area . 5 5. Intersecting ridges formed by diabase dikes and sills ......... 6 6. High Falls on the Pigeon River, Minnesota and Ontario ...... 7 7. Grand Marais harbor, sawtooth hills in the background 9 8. lVlap showing complex relations on the west side of the Saganaga batholith. .. 14 9. Sketch map of the structure of the Saganaga batholith ........ 17 10. Sawtooth topography of diabase sills in Rove formation 24 11. Northward-facing diabase bluff with talus slope beneath 24 12. Cross-sections of Rove formation area showing sills and dikes of diabase ................... .. 26 13. Wauswaugoning Bay, Lake Superior ...................... " 27 14. Cross-bedding in sandstone of the Puckwunge formation ...... 29 15. Five flow units at Sugarloaf Point, Lake Superior .......... " 33 16. Field sketch of drift section exposed in west wall of the Brule (Arrowhead) River gorge ........................... 61 CONTENTS Xl 17. Field sketch of drift section exposed on Kimball Creek 18. Interpretative sketch of relations on the Poplar River suggesting a thick till filling of a preglacial rock gorge .................. 64 19. Map of Twin Lake esker ................................ " 68 '20. Map of Devils Track esker . .. 69 '21. Profile plots of shoreline features in Cook County . .. 71 '2'2. Possible angles of tilt for Lake Duluth, highest Lake Algonquin, and Lake Nipissing shorelines .............................