102 MAN AND THE GLACIAL PERIOD. 



of the region. The land is not higher than that to the 

 north of it, nor is there any manifest protection to it by 

 the highlands south of Lake Superior. Kor yet is there 

 any reason to suppose that any extensive changes of level 

 in former times seriously affected its relations to the sur- 

 rounding country. Professor Dana, however, has called 

 attention to the fact that even now it is in a region of 

 comparatively light precipitation, suggesting that the 

 snow-fall over it may always have been insignificant in 

 amount. But this could scarcely account for the failure 

 of the great ice- wave of the north to overrun it. We are 

 indebted again to the sagacity of President Chamberlin in 

 suggesting the true explanation. 



By referring to the map it will be noticed that this 

 area sustains a peculiar relation to the troughs of Lake 

 Michigan and Lake Superior, while from the arrange- 

 ments of the moraines in front of these lakes it will be seen 

 that these lake basins were prominent factors in determin- 

 ing the direction of the movement of the surplus ice from 

 the north. It is the more natural that they should do so 

 because of their great depth, their bottoms being in both 

 cases several hundred feet below the present water-level, 

 reaching even below the level of the sea. 



These broad, deep channels seem to have furnished the 

 readiest outlet for the surplus ice of the North, and so to 

 have carried both currents of ice beyond this drif tless area 

 before they became again confluent. The slight elevation 

 south of Lake Superior served to protect the area on ac- 

 count of the feebleness of direct movement made possible 

 by the strength of these diverging lateral ice-currents. 

 The phenomenon is almost exactly what occurs where a 

 slight obstruction in a river causes an eddy and preserves 

 a low portion of land below it from submergence. A 

 glance at the map will make it easily credible that an ice- 

 movement south of Manitoba, becoming confluent with 

 one from Lake Superior, pushed far down into the Mis- 



