58 IOWA ACADEMY OF SCIENCES. 



in the protecting shelter of forests, the forest preceding the 

 deposition of loess. Subsequent observations have only 

 strengthened this belief, with some modifications. 



It must be borne in mind that whatever work of this kind 

 can be done by forests may also be done by smaller vegetation, 

 but perhaps in different degree. If forests could collect and 

 retain dust, prairie grasses and shrubs could do the same, and 

 if the supply of material was greater, might even accomplish 

 more. It is therefore not necessary to assume that all the 

 region now covered by the loess was at one time a forest area, 

 though evidences suggesting this for one locality at least, may 

 be found in the vicinity of Council Bluffs.* 



The thickets of Symphoricarpos and other shrubs, now com- 

 mon in many prairie sections, and, indeed grasses and other 

 herbaceous plants, could to some extent operate in the same 

 manner, though the deposit would probably be subject to 

 greater variation in amount and distribution. The difference 

 between the loess of eastern and western Iowa suggest some- 

 thing of the kind. It is a well-known fact that the loess of 

 eastern Iowa is finer and more nearly uniform in thickness, 

 which is less, than in the west. The eastern part of the state 

 was (and is) farther removed from the source of dust supply, f 

 its greater forest area insured a more uniform deposition of 

 only the finest material, and its shorter dry seasons and less 

 violent winds resulted in a deposit of lesser thickness. That 

 the differences in conditions during the deposition of the loess 

 as indicated by the fossils were essentially the same as those 

 which exist between the two sections of the state to-day, has 

 already been emphasizad by the author. J 



The greatest amount of the material carried by winds would 

 be deposited where there are obstructions, such as ridges or 

 hills, § and would generally be expected on the leeward side of 

 the hills, II though the shifting of winds during different seasons, 



•Discussed by the author in Proc. Iowa Acad. Scl., Vol. VI, pp. 98-113; also In the 

 Jour, of Gaol., Vol. VII, pp. 122-140. 



tThe western part of the state Is drier, and moreover lies in the path of the 

 stronger southwest winds which sweep over the dry prairies in summer while there is 

 still a considerable amount of vegetation which may serve as an anchorage for the 

 dust. Incidentally it may be noted that in summer the exposure of bare tracts by the 

 partial or complete drying up of ponds and streams, the work of burrowing worms, 

 insects and mammals and scratching birds, and the decay of vegetable matter all tend 

 to increase the supply of material which may be transported or deposited. 



*Proc. Iowa Acad. Sci., Vol. Ill, p. 8t; Vol. V, p. 15; Vol. VI, p. 110. 



SOompare with drifting snow. 



llOn the drift nills of western Lyon county the fine soils do commonly appear only 

 on the leeward sides, i. e., north and east, as has been noted. 



