64 CHICAGO ACADEMY OF SCIENCES. 



THE GXiACIAli LAKE CHICAGO. 



During' the past ten or twelve years evidence has been gradu- 

 ahy accumulating which points to the existence of several glacial 

 lakes held on the north by the ice sheet and having southward dis- 

 charge over the lowest points in the rims of the basins which they oc- 

 cupied. The area of these lakes would naturally be variable, since a 

 part of the boundary was formed by an oscillating ice sheet. The 

 outlets of the lakes would also be liable to change on account of the 

 covering of certain low outlets in the early stages which were open 

 to discharge in later stages. A change in the extent and the outlets 

 of the lakes, it appears, has also been produced by a warping of the 

 crust or dififerential uplift in the region they occupy. The compli- 

 cations are therefore great, and as yet the full history is not worked 

 out. Enough is known, however, to make certain that the general 

 direction of retreat of the ice sheet was northeastward. The 

 southern and western portions of the Great Lake basins were, 

 therefore, the first to become free from ice and occupied by glacial 

 lakes. 



While the ice sheet was covering the present outlets of Lakes 

 Superior and Michigan, these lakes had no connection with each 

 other, nor with the lakes to the east, and their discharge was south- 

 ward or southwestward into the Mississippi, from the present 

 heads of these lakes. A small district west of Lake Erie was also 

 occupied by a lake that discharged southwestward to the Wabash. 

 Upon the withdrawal of the ice sheet from the southern peninsula' 

 of Michigan and the southern portion of the Lake Huron basin, 

 the lake at the western end of Lake Erie became expanded and a 

 line of discharge was opened eventually from Saginaw Bay across 

 the southern peninsula of Michigan to the Lake Michigan basin, 

 and this being lower than the outlet to the Wabash, that outlet was 

 abandoned. The waters of the Lake Huron basin being held at 

 a somewhat higher level than those of the Lake Michigan basin, 

 the flow of water was from the former to the latter. The glacial 

 lake, which discharged across the southern peninsula of Michigan 

 extended over the district between Lake Huron and Lake Erie, as 

 well as the Lake Erie basin and the low district bordering it on 

 the south and west. It apparently did not extend far into the 

 Ontario basin, as a study of moraines indicates that the ice sheet 

 occupied that basin at the time of this discharge. It thus appears 

 that the Chicago outlet at one time was the line of discharge for an 

 .area much larger than the present Lake Michigan basin. 



