GEOLOuY OF SOUTHWESTERN IOWA. 355 



of from one hundred and fifty to nearly three hundred feet 

 from the level of the flood-plain. Although they are in most 

 places composed almost entirely of that fine homogeneous 

 material, designated as Bluff Deposit, they are nearly as 

 steep and abrupt as the rocky bluffs of the Mississippi valley. 

 In many cases, these bluffs of the Missouri river valley are so 

 steep that they appear almost perpendicular, but this is far 

 from beings the case, appearances of this kind always being 

 deceptive. The steepest fronts will average less than an 

 angle of forty-five degrees with the horizon, and the boldest 

 and most precipitous one observed, upon which the grass was 

 growing, when tested by the clinometer, proved to have an 

 angle of only fifty degrees. Some of the ravines which open 

 into the flood-plain through these bluffs, are deep, wild 

 gorges, such as one would hardly suspect the existence of, 

 but they are usually short and end abruptly by slopes so 

 steep that one can climb them only with difficulty. 



Proceeding inland from the border of the valley the surface 

 quickly becomes much less broken, although the material 

 that constitutes the ground for a great depth is the same, 

 in the higher parts of the county near the great river reach- 

 ing a depth of two hundred feet. The more level inland 

 surfaces are prairie, which present much the same general 

 appearance that prairie surfaces elsewhere do, the peculiari- 

 ties of the deposit being more especially shown where the 

 surface is considerably broken. 



Coming to the vallej^s of the East and West ISTishnabota- 

 ny's, which have their confluence into a common stream near 

 the center of the county, although they have eroded their 

 valleys out of this deposit almost alone, we find none of 

 those bold, precipitous bluffs, such as border the valley 

 of the Missouri river. On the contrary, they are bordered by 

 valley-sides of moderate height which gently slope from 

 the high general surface to the lowlands that more imme- 

 diately border the streams. The lowlands of these two 

 important tributaries of the Missouri river, although resulting 

 as such from the same natural causes as the flood-plain of 



