SUKFACE FEATUEES. 63 



less distinct traces of terraces are not unfrequently seen 

 along its sides. Its borders are not often distinctly defined 

 either in height or width, for they gradually blend with the 

 uplands as they slope away in the distance from the river. 



The Iowa furnishes numerous and very valuable mill-sites. 



Cedar river. Cedar river is usually understood to be a 

 branch of the Iowa, but it ought really to be regarded, as it 

 is in fact, the main stream, and the Iowa as the principal 

 branch. It rises by numerous branches, near the northern 

 boundary of the State, and together with the greater part of 

 these branches, large and small, it flows its entire length 

 through the region occupied by Devonian strata, and along 

 the general trend of the outcrop of that formation. Both 

 it and its branches have more numerous and extensive 

 exposures of rock along their valleys than any of the other 

 rivers hitherto described. This arises in part from the fact 

 that the formation over which they run is largely composed 

 of firm, compact limestone, which is not readily disintegrated 

 and covered by its own debris, as friable rocks often are 

 after they have once been exposed. It is also largely due to 

 the fact that the drift of northern Iowa is comparatively thin 

 in the region drained by the Cedar and its tributaries. There 

 is, however, enough of the drift everywhere to cover the 

 surface with excellent soil, except where it has been removed 

 by the wearing of the water in deepening the valleys.. 



The valley of this river in the upper part of its course, as 

 well as those of its tributaries also, is narrow, and fre- 

 quently without a well-defined flood-plain. The valley-sides 

 slope so gently that it is often impossible to say where the 

 low-lands end and the uplands begin. The exposures of 

 rock are sometimes quite numerous in the valleys, usually 

 appearing as rocky banks to the streams, and sometimes as 

 cliffs from twenty to thirty feet high above them. Below 

 the confluence of the Shellrock with the Cedar, the stream 

 assumes more of the true characteristics of a river. The 

 flood-plain is more distinctly marked, the cliffs upon its 

 banks are less frequent, but occasionally higher, yet they 



