120 IOWA ACADEMY OF SCIENCES. 



and even tine sand-grains eventually reach the brow of the bluff 

 and are deposited in the reverse order of their fineness*. The 

 position of these deposits is determined, essentially by the 

 surface contours. The wind, crossing the valley, impinging 

 against the hills flanks, is deflected upward, and, coming 

 in contact with the still air above, loses velocity, and, being 

 unable to carry its load further, deposits it over the brow of 

 the hill. In this location its position is reasonably secure, 

 though the entire assemblage of deposits possesses the pro- 

 clivities of the sand dune and may progress bodily inland. 

 This process of wind transport and accumulation of materials 

 may readily be witnessed. During early spring and late 

 autumn, when large tracts of bottom land are unprotected 

 by vegetation, dust storms are common and, often during a 

 single " blow, " a measurable deposit is accumulated. If this 

 be true now, how mucli greater must have been the efficiency 

 of the winds, which blew across the mud flats, before vegeta- 

 tion had time to reclaim the valleys, so recently vacated by 

 the Wisconsin iceV 



The prevailing winds for central Iowa during spring and 

 fall are from the west and hence the greater accumulation of 

 SBolian deposits oq the eastern flanks of the streams. 



These deposits are worthy of more than passing notice, 

 when viewed analytically, on account of their striking similar- 

 ity, in many respects, to the loess. Structurally, texturally 

 and in composition and distribution, there is a remarkable 

 resemblance. Both are essentially devoid of stratification 

 planes, possess a uniform, open texture, are highly siliceous, 

 being composed chiefly of silt and fine sand, and appear to be 

 genetically related to the chief watercourses, along which 

 they attain their maximum development. True, the loess is 

 usually highly calcareous, but this may readily be referred to 

 a difference in the condition of the materials drawn upon, and 



«A most luminous and helpful discussion of wind erosion, transport and deposi- 

 tion, will be found in Professor Udden's memoir, entitled " The Mechanical Composi- 

 tion of Wind Deposit*," published by the Lutheran Aug-ustana Book Concern, of 

 Rock Island, 111., 1898 The subjoined table gives the approximate maximum distances 

 over which quartz fragments of different dimens ons may be lifted by moderately 

 strong winds in single leaps. 



Gravel (diameter from 8-1 mm.) A. few feet. 



Coarse and medium sand (Diam. 1—1-4 mm.) Several rods. 



Fine sand (Uiam. 1-4-1-8 mm.) Less than a mile. 



Very line sand iDlam. 1-8-1-16 mm.) A few miles. 



Coarse dust (l-Ki— 1-32 mm.) 200 miles. 



Medium dust (l-;«-l-04 mm.) 1.000 miles. 



Fine dust (1-64 mm , and less) Around the globe. 



