SURFACE FEATURES. 47 



far as now known. The country around and between these 

 streams is almost entirely prairie. 



The Bast, West, and Middle Nodaway rivers and their val- 

 leys are very fine examples of the small rivers and valleys of 

 southern Iowa. They have the general characters of drift val- 

 leys with beautifully sloping* and undulating sides. These 

 characters are very slightly modified at several distant points, 

 by the presence in their beds and immediate banks of limited 

 exposures of Upper coal-measure strata. As the southerly 

 dip of these strata, however, coincides almost exactly with 

 the slope of the southerly flowing streams, they offer com- 

 paratively little obstruction to their flow, yet they are suffi- 

 cient to cause a few good mill-sites on each of the streams. 



The Nodaways drain one of the finest agricultural regions 

 in the State, the soil of which is tillable almost to their very 

 banks. These banks and the adjacent narrow flood-plains 

 are almost everywhere composed of a rich, deep, dark loam. 

 The average depth of these valleys, from the surrounding 

 prairie level, is not far from one hundred and fifty feet, the 

 southerly slope of the country keeping the depth of the val- 

 leys nearly uniform along a great part of their length. Their 

 width varies from an eighth to a quarter of a mile, but their 

 margins blend gradually with the prairie surfaces of the high 

 lands. 



The East and West JYishnabotany. Both these rivers, from 

 their sources to their confluence, and also the main stream 

 from thence to the point where it enters the great flood-plain 

 of the Missouri, run through a region the surface of which is 

 occupied by the Bluff Deposit described in a following chap- 

 ter. They have so eroded their valleys that the underlying 

 Drift Deposit is exposed in their immediate beds along almost 

 their entire length; but the valley sides, flood-plains, and 

 even the banks, are everywhere composed of the Bluff De- 

 posit. This is owing to the fact that near their sources the 

 Bluff Deposit is thin and gradually increases in thickness 

 towards the Missouri river, at a rate that coincides very nearly 

 with the slope of the streams. 



