THE GEOLOGY OF NORTH DAKOTA 25 



ice, so that all these streams were forced to seek new channels. 

 Lakes were formed in the valleys of the Yellowstone and Little 

 Missouri rivers, the water rising until it overflowed at its lowest 

 point the divide between the latter and the Knife River. The com- 

 bined waters of the three rivers flowed east and southeast to the 

 mouth of the Cannonball, where they entered the Missouri Valley. 

 The length of this Pleistocene valley from the head of the Knife to 

 the mouth of the Cannonball is 155 miles. Upon the withdrawal of 

 the ice sheet the Missouri and Yellowstone rivers returned to their 

 former valleys, but the lower valley of the Little Missouri was per- 

 manently abandoned and that river took an easterly course from the 

 point where its preglacial course was blocked by the front of the 

 ice sheet. 



After the first invasion the climate grew warmer and the glacier 

 retreated northward, so that conditions were probably favorable 

 for the return of animal and plant life. Upon the recurrence of the 

 cold climate the ice sheet again advanced over the region and prob- 

 ably reached about to the Missouri River. Evidence that this 

 second advance crossed the latter stream is lacking, and it is known 

 to have stopped far short of the limits reached by the first invasion. 

 Then after a relatively short interglacial interval, during which 

 the glacier withdrew from the region, there was a third invasion 

 of the ice sheet, coming as before from the center west of Hudson 

 Bay. The limit reached by this Late Wisconsin ice sheet is marked 

 by the Altamont moraine. This remarkably well-developed mo- 

 raine forms a very rough belt of massive hills and ridges which 

 extends without interruption for hundreds of miles. In places 

 it is fully 20 miles wide and throughout much of its extent in North 

 Dakota its width probably averages 12 to 15 miles. While form- 

 ing the moraine the ice front doubtless fluctuated back and forth 

 across the belt for a long period. 



During its recession the ice sheet halted again and again and 

 thus built a series of moraines. Some of these halts were brief and 

 the resulting moraines poorly defined; others were of much longer 

 duration, as shown by the great amount of material deposited and 

 the large size of the hills and ridges. 



