120 
IOWA ACADEMY OF SCIENCES. 
and even line 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 hill’s Hanks, 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. Daring 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 much greater must have been the efficiency 
of the winds, w^hich blew across the mud fiats, 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 
seolian deposits ou 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 Deposits,” 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 dimensions 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 (Diam. 1-4— 1-8 mm.) Less than a mile. 
Very fine sand {Diam. 1-8—1-16 mm.) A few miles. 
Coarse dust (1-16—1-33 mm.) .300 miles. 
Medium dust (1-33—1-64 mm.) 1,000 miles. 
Fine dust (1-64 mm., and less) Around the globe. 
