aac a 
98 NEW YORK STATE MUSEUM 
from the region. To summarize, we may say that the present 
Great Lakes basins are due to a combination of factors, the more 
important of which were: the formation of preglacial valleys by 
stream erosion; a more or less deepening of these valleys by ice 
erosion; the great accumulation of glacial debris along the southern 
side of the Great Lakes region; and the tilting of the land down- 
ward toward the north. 
We are now ready to trace out the principal stages in the history 
of the Great Lakes during the final retreat of the ice sheet. -When 
the ice front had receded far enough northward to uncover the 
western end of Lake Superior, the southern end of Lake Michigan, 
and an area west of the present end of Lake Erie, small lakes 
were formed against the ice walls (see figure 30). One of these 
has been called Lake Duluth which drained southward into the 
Mississippi; the second Lake Chicago which drained past Chicago 
through the Illinois river and into the Mississippi; and the third 
Lake, Maumee which drained southwestward past, Fort Wayne 
through the Wabash river and into the Ohio and Mississippi. 
At a still later stage the conditions shown on the map (figure 31) 
existed. Lake Chicago was then much larger, and Lake Maumee 
Fic. 31 A later stage of Great Lakes history, showing how 
the eastern and western ice margin lakes combined with outlet 
past Chicago. 
After Taylor 
had expanded into the extensive Lake Whittlesey which covered 
nearly all of Lake Erie as well as the immediately surrounding 
country. Lake Whittlesey was at a lower level than the former 
Maumee and the outlet past Fort Wayne ceased, but the drainage 
