SURFACE DEPOSITS. 103 



of these terraces, particularly those occupying comparatively 

 high levels in the valleys, have been deposited in stiller waters 

 than those of the rivers as they now flow. The material of 

 these latter terraces is fine and silt-like, and will be more 

 particularly referred to on a subsequent page. The Alluvium 

 proper is largely composed of sand and other coarse mate- 

 rials, but some of the best and most productive soils in the 

 State are upon that deposit. Besides being an inseparable 

 feature of our river valleys, the Alluvium has a certain degree 

 of interest as being the latest of all the deposits, and is even 

 now in process of deposition. 



4. THE BLUFF DEPOSIT. 



The bluffs which border the broad flood-plain or bottom 

 land of the Missouri river, along all that part of its course 

 which forms the western boundary of Iowa, are so peculiar in 

 character and appearance that they cannot fail to attract the 

 attention of every one who sees them for the first time. Their 

 strangely and beautifully rounded summits, occasionally 

 mingled with sharply cut ridges, smooth and abruptly retreat- 

 ing slopes, and the entire absence of rocky ledges, except in 

 rare instances when they appear only at their base, cause 

 them to present a marked contrast with those of the Missis- 

 sippi and other rivers of the eastern part of the State where 

 rocky ledges support and compose the greater part of their 

 bulk. 



From the mouth of Big Sioux river to the southern boundary 

 of the State, these bluffs present a continuous, serrated and 

 buttressed front to the flood-plain of the great river, from 

 which they rise abruptly to a maximum height in different 

 parts of the line, varying from one hundred to little less 

 than three hundred feet. Although the front they present is 

 so definite and continuous, it is nevertheless frequently and 

 deeply cleft, not only by the tributaries of the great river, but 

 also by small creeks and short ravines that drain the surface- 

 water from the uplands beyond, in which the bluffy character 

 is soon lost. 



