452 



HISTORICAL GEOLOGY 



Disturbance of river courses. Before the ice covered the 

 northern region the many rivers had become in large meas- 



FIG. 469. Preglacial hills and valleys obliterated by the deposition of 

 glacial till. (After Tarr.) 



ure adjusted to the hard and soft rocks in which they were 

 excavating their valleys. As the ice overspread their basins 



many such valleys, with 

 their rivers, were wholly 

 destroyed, and the new 

 streams which arose after 

 the ice melted pursue 

 courses quite unrelated to 

 those of their predeces- 

 sors. The Rock River in 

 Illinois and Wisconsin ex- 

 emplifies this (Fig. 470). 

 Other streams, espe- 

 cially those located near 

 the margin of the ice 

 sheet, were merely 

 crowded to one side and 

 forced to make new val- 

 leys. Thus the Missouri 

 River appears to have 

 been displaced by one of 

 the earlier ice sheets. It 

 cut a new channel along 



FIG. 470. A portion of northwestern 

 Illinois, showing the course of the Rock 

 River before and after glaciation. (After 

 Leverett.) 



the front of the glacier 



