SURFACE FEATURES. 69 



the bluffs approach the stream upon both sides. Its waters, 

 although always having a stained appearance, are quite 

 clear, especially as compared with those of the Missouri. 



7. RELATIVE AGES OF THE RIVER VALLEYS. 



There is evidently much difference in the relative ages of 

 the river valleys of Iowa, if we take into consideration the 

 full history of their formation; but if we consider them only 

 in their relation to the present general surface of the State, 

 they all evidently date back only to the close of the Glacial 

 epoch. At the close of that epoch, as will be explained more 

 at length in the next chapter, the surface of the whole State, 

 as well as that of the surrounding region, was evidently 

 so covered by the Drift Deposit as to present a surface 

 nearly as level as that which has been assumed in the 

 description of the Surface Features of the State on a pre- 

 vious page. That is, upon the recedence of the glaciers, the 

 surface waters gathered into rivers, and had all their valleys 

 to cut out anew, for the deep, continuous mantle of drift had 

 filled, and covered all the valleys of the rivers that had 

 drained the land before the Glacial epoch. 



To what extent our rivers of to-day occupy pre-glacial 

 valleys it is difficult, if not impossible to determine; but that 

 the Mississippi, together with all the rivers of northeastern 

 Iowa, if no others, had at least a large part of the rocky 

 portions of their valleys eroded by pre-glacial, or perhaps 

 even by palaeozoic rivers, can scarcely be doubted. 



When, at the close of the Glacial epoch, the new-formed 

 rivers selected their courses by the law of gravitation, some 

 of them doubtless found their way along the valleys of ancient 

 rivers, and gradually swept out the incoherent drift that had 

 rilled them, leaving the rocky cliffs up each side as the ancient 

 rivers had left them. Others seem to have followed those 

 ancient valleys only a part of their course, while still others, 

 especially those of the western system of drainage, date the 

 earliest period of the history of their valleys back only to the 



