44 PHYSICAL GEOGBAPHY. 



they afford comparatively few secure mill-sites, although their 

 fall per mile is very considerable, as shown by the table on a 

 preceding page. Wherever any practicable mill-site does 

 occur upon these streams, they are occasioned by the exposure 

 to a greater or less degree of some of the strata that underlie 

 the very heavy surface deposits. 



Another cause of the great uniformity in the character of 

 the river valleys of western Iowa, is the simplicity of the 

 stratigraphical geology of the region. Almost the whole of 

 the region occupied by the streams of the western drainage 

 system is underlaid by only two formations; namely, the 

 Upper coal-measures and Cretaceous. The strata of the last 

 named rocks are usually so friable and easily disintegrated 

 that they have offered very little impediment to the erosion 

 of the valleys; and, for the same reason, they do not often 

 appear as exposures along the valley sides, even when they 

 are known to exist beneath the surface. It is for this reason, 

 in addition to the fact that the surface deposits are upon the 

 whole thicker in western Iowa, that exposures of strata are 

 so much more rare there than they are along the rivers of the 

 eastern system. 



In view of the intended limit of this chapter, comparatively 

 little discussion can be devoted to each individual stream, 

 which fact must serve as a sufficient explanation of the brief 

 manner in which they are treated. 



5. RIVERS OF THE WESTERN SYSTEM. 



All of the rivers of the western system of drainage, except 

 the Missouri itself, are quite incomplete as rivers, in conse- 

 quence of their being really only branches of other larger 

 tributaries of that great river; or, if they empty into the 

 Missouri direct, they have yet all the usual characters of 

 upper branches of Iowa rivers, from their sources to their 

 mouths. 



Chariton and Grand rivers (or rather the upper branches 

 of rivers which bear those names in, and drain a part of the 



