50 PHYSICAL GEOGRAPHY. 



tlie bed and immediate banks are everywhere muddy; and 

 the soil of the whole valley differs little if any from that of 

 the npland. 



The only rocks exposed along the entire course of Boyer 

 river are those of the Upper coal-measures, and these occnr 

 only at a single locality near Reel's mill in Harrison county. 

 The exposures here are slight, and unimportant, except that 

 they are the only exposures of underlying strata in the 

 county, and the most northerly ones of the Upper coal-mea- 

 sures now known in Iowa. 



The valley of Boyer river .has usually gently sloping sides, 

 and an indistinctly defined flood-plain. Its depth from the 

 adjacent uplands varies from one hundred to one hundred 

 and fifty feet; but the upland borders of the valleys of the 

 stream which traverse the Bluff Deposit, are not usually so 

 well defined as those of the streams which traverse the Drift 

 Deposit alone frequently are. Along the lower half of its 

 course, especially, the adjacent upland loses the originally 

 well defined surface level, and presents a surface of the bil- 

 lowy character, so common near the valley of the Missouri 

 river and peculiar to the Bluff Deposit. An unusually well 

 marked example of this billowy surface of the same deposit, 

 may be seen in the sketch accompanying the chapter on 

 northwestern Iowa. It will thus be seen that the whole valley 

 receives its characters from the surface deposits alone, without 

 any modification from the underlying strata. 



Soldier river presents very little that is peculiar to it, as 

 compared with the other rivers which traverse the Bluff De- 

 posit thus far described. However, near the point where its 

 valley joins the great flood-plain of the Missouri river, it 

 presents some examples of more or less distinct terraces, 

 which are comparatively more rare in the Bluff Deposit than 

 in the drift, owing to the peculiar composition of the former. 

 The mode of formation of these terraces will be discussed in 

 the chapter on Surface Deposits, accompanying which, will 

 be found a sketch of the locality where the valley of Soldier 

 river joins that of the Missouri. 



