344 



HISTORICAL GEOLOGY 



less deepening of these valleys by ice erosion ; (3) the great accumu- 

 lation of glacial debris along the southern side of the Lake district; 

 and (4) the tilting of the land relatively downward toward the 

 north. 



We are now ready to trace out the principal stages in the 

 history of the Great Lakes region during the final retreat of the 

 great ice sheet. When the ice front had receded far enough north- 

 ward to uncover the southern end of Lake Michigan, and an area 



Fig. 216 

 Lake Whittlesey stage of the Great Lakes history, when the eastern 

 and western ice-margin lakes combined with outlet past Chicago. 

 (After Taylor and Leverett, redrawn by W. J. M.) 



west of the present end of Lake Erie, small lakes were formed 

 against the ice walls (see Fig. 215). The first of these has been 

 called Lake Chicago, which drained past Chicago through the 

 Illinois River and into the Mississippi; and the second, Lake 

 Maumee, which drained southwestward past Fort Wa}me through 

 the Wabash River and thence into the Ohio and Mississippi. 



At a later stage the conditions shown on map Fig. 216 existed. 

 Lake Chicago was then larger, and Lake Maumee had expanded 

 into the extensive Lake Whittlesey, which covered nearly all of the 

 area of Lake Erie as well as some of the surrounding country. 

 Lake Whittlesey was at a lower level than the former Maumee, and 

 the outlet past Fort Wayne ceased, but the drainage from Whittle- 



