THE QUATERNARY PERIOD 



343 



to show that by ice erosion portions, at least, of all the lake basins 

 were appreciably deepened. Even so, however, we have not yet 

 accounted for the present closed basins. Probably the two most 

 important phenomena which have contributed to the formation of 

 the closed basins of the Great Lakes are (1) the great drift accumu- 

 lations along the south side and (2) the tilting of the land down- 

 ward on the north side of the region. The deep drift deposits 

 must certainly have been very effective in damming up the south 

 or southwest-flowing pre-Glacial streams of the region. A great 



Fig. 215 

 First stage in the history of the Great Lakes, when all the 

 rest of the lake basins were still buried under the ice. 

 Shaded portions show Lake Chicago on the left and 

 Lake Maumee on the right. (After Taylor and Leverett, 

 redrawn by W. J. M.) 



dumping ground of ice-transported materials from the north was 

 in general along the southern side of the Great Lakes and south- 

 ward. Late in the Ice age the land on the northern side of the 

 Great Lakes region was lower than it is today, as proved by the 

 tilted character of certain well-known beaches of extinct lakes 

 (see below) . Such a differential tilting or warping of the land must 

 have helped to form the closed basins by tending to stop the south- 

 ward or south west ward drainage from the region. To summarize, 

 we may say that the present Great Lakes basins are due to a com- 

 bination of factors, the more important of which were: (1) the 

 formation of pre-Glacial valleys by stream erosion; (2) a more or 



