590 THE ICE AGE IN NORTH AMERICA. 



The controlling influence of the preglacial topography may 

 be observed in the neighborhood of Galesburg, Illinois, where 

 all along the divide between the Illinois and Mississippi 

 rivers the surface presents an extensive general level, covered 

 with glacial drift of Illinoisan age to a considerable depth. 

 But as one proceeds on either side towards the rivers men- 

 tioned the size of the preglacial valleys increases so rapidly 

 that the glacial blanket is not sufficient to disguise them, 

 while near the water-shed on both sides appear the original 

 extensive amphitheaters characteristic of valleys of extreme 

 age. 



Evidently the post-glacial erosion is not by any means so 

 great as would at first appear to be the case. But what seem 

 to be valleys of post-glacial erosion are simply adjustments of 

 the glacial blanket to the precedent valleys of erosion. 



Again, the broad valleys bordering the south-flowing 

 streams of gentle gradient in the Illinoisan and Kansan 

 regions differ only in moderate degree from similar valleys 

 in the Wisconsin area. As instances I would note the valleys 

 of the Nishnabotna, the Tarkio, the Nodaway, and the 

 Platte rivers of northwestern Missouri, all of which rise in 

 southwestern Iowa, and, after flowing long distances, enter 

 Missouri. One can but be impressed in crossing these valleys 

 with their great width, and with the signs that they were 

 occupied by immensely larger streams of water than it is 

 possible to provide under present conditions. On comparing 

 these valleys with that of the river Styx, a small stream in 

 Wisconsin drift just south of the water-shed in Medina County, 

 Ohio, we find a very small stream occupying a level flat- 

 bottomed valley, a third of a mile wide, which is evidently 

 the product of the lingering ice and the floods pouring through 

 the valley during the melting of the late Wisconsin period. 

 The contrast between this valley and the valleys in north- 

 western Missouri is by no means great, certainly not so great 

 as to imply an enormous lapse of time between their formation. 



