42 PHYSICAL GEOGRAPHY. 



The Drift and Bluff Deposits are both so thick in Iowa that 

 its streams not only rise npon their surfaces, but they also 

 frequently reach considerable size upon, and have eroded 

 their valleys to a considerable depth into these deposits 

 alone. In a few cases, as for example near the dividing 

 ridge between the two great rivers, where the Drift Deposit is 

 deepest, the valleys, small as the streams are there, have 

 been eroded by those streams to a depth of nearly or quite 

 two hundred feet from the general prairie level, showing 

 nothing but drift anywhere in their bottoms or sides. It is 

 known that the Bluff Deposit also reaches fully as great a 

 thickness in southwestern Iowa, where some of the valley- 

 sides and bottoms are composed entirely of that homogeneous 

 material. 



In consequence of the great thickness of the surface deposits, 

 many of the streams, more especially those of western Iowa, 

 are very largely and some of them wholly without exposures 

 of rocky strata along their whole course. In other words, 

 such streams rest wholly or in large part upon the incoherent 

 surface deposits alone, and into which they have eroded their 

 valleys. 



When the material out of which a valley is eroded, 

 especially in Iowa, consists of alternating hard and soft 

 strata, the valley is usually wide and the soft beds fre- 

 quently give rise to broad, fiat, flood-plains; but if the 

 valley is cut out of material of uniform character it is usually 

 deep, narrow, and gorge-like. When the valley sides are 

 wholly composed of the incoherent surface deposits, they are 

 of course never precipitous like those which contain rock in 

 their composition. They are sometimes, however, quite 

 steep, but always slope more or less directly from the 

 surrounding highland to the flood-plain or the stream. 

 The upper branches are, almost without exception, prairie 

 streams; their valleys not being well defined as such, but 

 form a part of the general undulatory character of the 

 surface. When the valley is cut out of the stiff, clayey 

 drift, like some of that of southern Iowa, the sides are 



