CHAPTER XXIII 



GLACIAL LAKES WHICH MARKED THE DECLINE OF 

 THE LAST ICE AGE 



Interference of glaciers with drainage. Every advance and 

 every retreat of a continental glacier has been marked by a com- 

 plex series of episodes in the history of every river whose territory 

 it has invaded. Whenever the valley was entered from the direc- 



.,- -*- -^-- ^ on ^ ^ s Divide, ^ ne 



effect of the advanc- 

 ing ice front has gen- 

 erally been to swell 

 the waters of the river 

 into floods to which 

 the present streams 

 bear little resemblance 

 (Fig. 346). Because 

 of the excessive melt- 

 ing, this has been even 

 more true of the ice 

 retreat, but here when 

 the ice front retired up the valley toward the divide. A sufficiently 

 striking example is furnished by the Wabash, Kaskaskia, Illinois, 

 and other streams to the southward of the divide which surrounds 

 the basin of the Great Lakes (Fig. 347). 



Wherever the relief was small there occurred in the immediate 

 vicinity of the ice front a temporary diversion of the* streams by the 

 parallel moraines, so that the currents tended to parallel the ice 

 front. This temporary diversion known as " border drainage " 

 was brought to a close when the partially impounded waters had, 

 by cutting their way through the moraines, established more perma- 

 nent valleys (Fig. 348). 



320 



FIG. 346. The Illinois River where it passes through 

 the outer moraine at Peoria, Illinois, showing the 

 flood plain of the ancient stream as an elevated 

 terrace into which the modern stream has cut its 

 gorge (after Goldthwait). 



