62 



SOIL PHYSICS AND MANAGEMENT 



taken from the flood plains of streams that carried the waters from 

 the melting glaciers, depositing the rock flour over the flooded 

 plains of these streams (Fig. 60). During the cold part of the 

 year the flood plains were bare and dry and this fine material was 

 carried over the upland by the wind. The depth of the deposit 

 varies with the width of the flood plain from which the material 

 was derived and the distance from the stream. The coarser material 

 was deposited on the upland adjoining the flood plain, while the finer 

 was carried to much greater distances. Near the flood plains from 

 which it was derived it was occasionally deposited upon the uplands 



Fia. 60. Alluviation by glacial stream below Hidden Glacier, Alaska. This occurred 

 to a large extent during the Glacial Period. The upland loess was derived from these allu- 

 vial deposits. (Chamberlain and Salisbury, Courtesy Henry Holt <fe Co.) 



in the form of dunes, either as hillocks or ridges. These frequently 

 show the typical dune topography. In Illinois and other states we 

 find that along the larger streams the loess deposit is deeper on the 

 upland adjoining the wide bottom lands. Where no bottom land 

 exists, the deposit on the adjacent upland is very thin. This indi- 

 cates a very close relation existing between the loess and the botfom 

 land. The deposit is deeper on the east side of the flood plains than 

 on the west, indicating prevailing westerly winds at the time of 

 deposition. Very much of the loess of North America was deposited 

 at the close of the lowan glaciation. The melting of this glacier 

 seems to have been accompanied, as Leverett says, by heavy periodic 



