46 IOWA ACADEMY OF SCIENCES. 



buried in a layer of stratified sand not less than six feet 

 deep, and which had certainly been deposited within thirty 

 years, probably considerably less. It was certainly a wind 

 formation. One need but observe the dust-storms of west- 

 ern Iowa and of Nebraska to be convinced of the motive 

 power of wind. 



The anchorage for material so carried is usually fur- 

 nished by vegetation. During dry seasons, when vegeta- 

 tion is checked, sandy areas are likely to be extended over 

 formerly fertile tracts. More favorable seasons or condi- 

 tions which bring about a greater growth of plants cause 

 an encroachment of vegetation upon the sandy areas, and 

 as soon as this has taken place the shifting ceases, and new 

 soil is soon formed. This has been especially well illus- 

 trated along the borders of the sand hills in Nebraska and 

 there are to-day large tracts serving as cattle ranges 

 which, within the memory of settlers now living, were 

 deserts of shifting sand. 



That plants do hold loose material which would other- 

 wise be swept away, is well illustrated in almost every 

 field in Iowa in winter. If a field is plowed it is much 

 more likely to be swept bare of snow than the adjoining 

 stubble-field or weed patch, and in deep woods snow sel- 

 dom drifts, but usually settles down in an almost uniform 

 blanket-like layer. The effect upon dust is the same. 



The best evidence now available indicates that the 

 loess, so widely distributed in the Missouri valley, was 

 deposited upon plant-covered land surfaces by wind. That 

 plant life was abundant during the deposition of the loess 

 is shown by the presence of terrestrial herbivorous mol- 

 luscs, which were quite as abundant as the same species 

 are to-day in the fauna of the plant-covered surface of the 

 state. The same fact is indicated by the comparatively 

 uniform thickness of the loess, which would not have been 

 maintained if there had been no plant anchorage. In this 

 connection it is well to note that the comparative uni- 

 formity and fineness of the loess materials, and the fre- 

 quent distribution of terrestrial shells vertically through 

 the deposit, give evidence that it was not laid down in 



