MICHIGAN ACADEMY OF SCIENCE. 



27 



tonagoD became conflnent with Lake Diilntli but at a level considerably 

 lower. It consequently vacated much of its bed. 



The St. Croix outlet served as the line of discharge for this glacial 

 lake in the western part of the Superior basin until the ice border had 

 receded to the eastern slope of the Keewenaw Peninsula if not beyond 

 it (Fig. 2). 



It will be observed that the highest shore of Lake Duluth is not at a 

 uniform level but shows a marked rise toward the northeast. Thus at 

 the western end it is 1,0G6 feet above sea level or 4(55 feet above Lake 

 Superior, while at Calumet on the Keewenaw Peninsula it reaches 



Fig. 2. Lake Duluth at greatest extent, and probable contemporary ice border. Numerals 

 and isobases indicate altitude of the highest beach above sea level. 



1,S03 feet, or T(»2 feet above the present lake. This rise of 237 feet 

 seems to be largely attributable to a subsequent differential u])lift, 

 though a small portion is perhaps referable to an actual higher position 

 of the lake at the northeast end due to ice attraction. The effect of ice 

 attraction seems unlikely, however, to amount to 37 of the. 237 feet 

 and would be mostly within a few miles of the ice border. It will be 

 observed that the lines of equal elevation, or isobases, connecting cer- 

 tain points on opposite sides of the lake, trend south of east but they 

 are not uniform in direction even in the small area covered by definite 

 data. It appears from this that the uplift was not regular. There is 

 need for precise levels along the whole shore to clear up the form of 

 warping or uplift this region has experienced. The direction of tilting. 



