634 GEOLOGY OF CENTRAL WISCONSIN. 



shore of Lake Superior, we find two great promontories, Keweenavr 

 Point, and the Bayfield Peninsula. Both of these projections have a 

 course somewhat transverse to the general trend of the lake, bearing 

 some 30 south of west. Both have high central ridges or backbones, 

 which rise 1,000 to 1,500 feet above the adjacent lake, and are made 

 up of bedded igneous rocks, sandstones, and conglomerates of theCop- 

 per Series. Both of these ridges continue far westward on the main- 

 land, having between them a valley, partly occupied by the lake, 

 which is a true synclinal trough, the rocks of the two ridges dipping 

 towards one another, North of the Bayfield Peninsula, and again 

 south of Keweenaw Point, we find two other valleys running in from 

 the lake shore in the same direction. In all probability each one of 

 these valleys has given direction to a glacier tongue. An inspection 

 of a good map of the northern part of Wisconsin, Minnesota and 

 Michigan will serve to show that the almost innumerable small lakes 1 

 of these regions are concentrated into three main groups, each group 

 corresponding to a great development of morainic drift, and lying in 

 the line of one of the three valleys just indicated. I suppose that 

 each of the lake groups is a moraine of the glacier which occupied 

 the valley in whose line it lies. The main ice sheet coming from 

 the north met, in the great trough of Lake Superior, over 2,000 feet in 

 depth, an obstacle which it was never able to entirely overcome, and 

 so reached further southward in small tongues composed perhaps of 

 only the upper portions of the ice. These tongues being deflected 

 westward by the rock structure of the country, and having their force 

 mainly spent on climbing over the watershed, left the region further 

 south untouched. The eastern part of the Lake Superior trough is 

 not nearly so deep as the western, and the divide between Lake Su- 

 perior and the two lakes south of it never attains any great altitude, 

 so that here the ice mass, having at the same time perhaps a greater 

 force on account of its nearness to the head of the ice movement on 

 the Laurentian highlands of Canada, was able to extend southward 

 on a large scale, producing the glaciers of the Green Bay valley and 

 of Lake Michigan. 



Although quite crude in its details, I am convinced that the main 

 points of the explanation thus offered for the -existence of the drift- 

 less region in the northwest will prove to be correct. To obtain a full 

 elucidation of the subject, much must yet be done in the way of in- 

 vestigation, not only in Wisconsin, but over all of Minnesota and the 

 states south, in order that the details of the ice movement for the 

 whole northwest may be fully understood. 



1 Far more numerous in reality than shown on the best maps. 



