48 IOWA ACADEMY OF SCIENCES. 



county, and thence northwestward to Howard county, it 

 will be seen that it is nowhere distant from silt-bearing 

 streams. The Iowa, Cedar, Wapsipinnicon, Maquoketa 

 and Turkey rivers furnished the silt, while the adjacent 

 highlands of the drift border furnished the necessary 

 ecological conditions. 



In the western part of the state vegetation is less abun- 

 dant, the winds are much stronger, and the silt-bearing 

 Missouri furnishes much more material, the result being 

 that thicker and less regular deposits have been formed, 

 and they commonly exhibit distinct wind topography, as 

 for example, along the Missouri bluffs on the Iowa side. 

 A striking illustration[ of this kind of topography may be 

 observed above Missouri Valley, but the finest and most 

 extensive area lies northwest of Sioux City (Plate XIII). 



This area lies directly in the path of the dust-storms 

 which frequently sweep across the broad Missouri bottoms 

 from the southwest. To the ■ observer watching the 

 advance of one of these storms it is evident that the bars 

 of the Missouri furnish a very large part of the dust, and 

 he is not left long in doubt concerning its volume. But 

 even in these western regions the formation of compara- 

 tively stable deposits would not be possible without the 

 presence of plants, for they alone are able to anchor such 

 fine materials. 



We must certainly credit plants with constructive work, 

 not only in the formation of alluvial deposits, but in the 

 accumulation of finer soils and subsoils. It is customaiy to 

 refer to new soils, such as those formed upon lava surfaces, 

 as products of the destructive disintegrating influences of 

 plants. The greater part of the soil so accumulated in 

 most cases, whatever may be the facts concerning lava 

 surfaces, is brought by wind and water from adjacent 

 exposed surfaces, and is retained by the plant covering 

 which is sure to appear sooner or later upon even the most 

 barren surface. 



