104 ^ THE MICHIGAN ACADEMY OF SCIENCE. 



preglacial valleys 1,000 feet or more of glacial material will be found. 



The high tract constituting the Thumb reaches about 1,300 feet in 

 northern Oakland county and 1,284 feet at Bunday Hill, a gravel knoll 

 west of Somerset in northern Hillsdale county. The rock in the vicinity 

 of Bunday Hill rises nearly to an altitude of 1,200 feet, and the drift 

 on the hills of part of Hillsdale and Jackson county is so Ihin as to 

 scarcely form a continuous coating, though the preglacial valleys are 

 filled to great depths. The effect of glaciation on the Thumb has been 

 to lessen the reliefs of that region by filling the valleys and bordering 

 plains more deeply than it has coated the hills. But in the elevated 

 tract in the northern part of the ])eninsula the drift accuumlation on 

 its highest parts has been so excessive as to give a greater relief above 

 bordering lowlands than that region ]>resented in preglacial times, and 

 the present surface is 350 to 500 feet or more above the crests of the 

 rock ridges. 



The plains bordering the Great Lakes are generally covered to a 

 depth of 100 to 200 feet by glacial deposits, but there are ])laces between 

 Detroit and Monroe where the drift coating is very thin. It is thin also 

 on each side of the mouth of Saginaw Bay, and along the shore of Lake 

 Huron from Alpena to the Straits of Mackinac, except Avhere the rock 

 formations are interrupted, as at Cheboygan, by deep preglacial valleys. 

 Eock outcrops appear also along the shore of Lake Michigan from the 

 mouth of Grand Traverse Bay to Little Traverse Bay, except where 

 interrupted by buried valleys. But the headlands of drift in that part 

 of the shore rise to heights of 200 to 400 feet above the rock outcrops. 



Lakes have filled some of the deepest depressions on the borders of 

 the State, and have also tended to reduce the reliefs of the interior. 

 The bottom of Grand Traverse Bay, an arm of Lake Michigan, extends 

 below sea level near its southern end opposite Elk Kapids. but the north- 

 ern end, where it opens out into Lake jNIichigan has only a shallow 

 depth of water, about one-tenth that near the southern end. This fea- 

 ture suggests that the southern portion may have been deepened by the 

 ice, though there is a ]>ossibility that it marks a preglacial valley, which 

 opened oiit to Lake Michigan along some other line than that through 

 the mouth of the bav. The other narrow finger like lakes which inter- 

 rupt the headlands along the shore of Lake IMichigan in the vicinity 

 of Grand Traverse Bay ap])ear either to mark the line of preglacial val- 

 leys or to be places in which glacial erosion was exceptionally intense. 



POSSIBLE EXTENSION OF THE KEEWATIN ICE FIELD OVER MICHIGAN. 



The several glacialists Avho have had opportunity to compare the Kan- 

 san drift sheet with the Illinoian, recognize that the Kansaii has been 

 much more deeply affected by weathering and erosion than the Illinoian, 

 and it appears to be more than twice as old. From this has sprung the 

 idea that the Labrador ice field mav have been relativelv small at 

 the time of the culmination of the KecAvatin ice field. Studies in Michi- 

 gan and Ohio apparently bear out this idea, and indicate that the Kee- 

 watin ice field may have spread over this peninsula and reached into 

 Ohio before the Labrador field had extended into Michigan. Copper and 

 certain rocks peculiar to the Lake Superior region, which have been 

 found in the southern peninsula and even in western Ohio, support this 

 view. It has also been noted by A. P. Low of the Canadian Geological 



