120 N. H. WINCHELL GLACIAL LAKES OF MINNESOTA 



trations of the action of the ice-margin on the surface drainage in those 

 regions where the natural slope was toward the glacier. 



12. Lake Issati 



In the same manner that the Saint Croix river was ohstructed and 

 caused to form a lake (lake Shafer) above the point of obstruction and 

 to pass round the ice toward the east, so the Mississippi and the Rum 

 rivers, both flowing southward, were obstructed, with similar results. 

 In this case, however, the oV)struction was b\' the northeastern ice-lobe 

 and at an earlier date. Lake Issati was an enlarged lake, which covered 

 the region of lake Mille Lacs. It has not been studied, and it can only 

 be said that it stood at a level of about 1,266 feet above the sea and 

 probably had an outlet through Crow Wing county b}^ way of the 

 Nokasippi river into the Mississippi. The ice-tongue from the northeast 

 lay with its northern edge encroaching on tlie southern part of the present 

 area of lake Mille Lacs, shutting up the passage southward by way of 

 Rum river and depositing a coarse marginal moraine. When this ob- 

 struction was removed the level of lake Issati was lowered to about that 

 of Mille Lacs, amounting to about 12 feet. 



13. Lake Aitkin 



This lake was named and described by Mr Upham.* It stood at 1,205 

 feet above the sea and formed distinct beaches and deposited a lacustrine 

 clay. According to Mr Upham\s interpretation, the obstruction of the 

 Mississippi river below this lake was due to the northwestern ice-lobe, and 

 the region of Willow river was covered by the northwestern glacier, lim- 

 iting the lake in that direction. It is more in accord with the manner 

 of retreat of these ice-lobes from the state, as already intimated in the 

 description of lake Sliakopee, to suppose that lake Aitkin was due to the 

 Lake Superior ice-lobe, whose northern margin can be traced by a nearly 

 continuous moraine from Crow Wing county to Carlton and Saint Louis 

 counties, thrusting the Mississippi river toward the west and southwest 

 from Sandy lake to the mouth of the Crow Wing river. That would 

 leave a large area toward the north, through which the Willow river 

 flows, and which in its ph3'sical features is like the region about Aitkin, 

 subject to overflow by lake Aitkin ; hence lake Aitkin is allowed a wide 

 extension toward the north on the accompanying map. 



The outlet of this lake must have been toward the southwest around 

 or over the western edge of the obstructing glacier, and it may have 



* Final Report of the Geological Survey of Minnesota, vol. iv, p. 46. 



