64 Physics of the Soft. 



which it falls one or more particles of soil which has been 

 drifting in the higher air from unknown distances. 



The drifting of dust from roads during dry times and 

 from fields in the spring are strong reminders of the po- 

 tency of wind action at times, but it is the less evident but 

 continuous action that counts most in the long run and, 

 were it not for the steady wearing away and rearrangement 

 of the soil surface, wind-formed soils would be much more 

 evident and general than they are. 



On the leeward margins of arid regions and on sandy 

 coasts the building and eroding power of the wind becomes 

 most evident, and the most extensive deposits which have 

 been assigned to this cause are the loess beds of China 

 which have great horizontal extent and- in some places 

 depths reaching even 1,200 and 2,000 feet. These depos- 

 its have been described by Kichthofen as having been 

 formed from dust accumulations drifted by the prevailing 

 winds from the high desert plateaus of Central Asia. 



In Europe, and in this country in the Mississippi val- 

 ley, there are deposits of a similar character. They are 

 distributed along the border of a former ice sheet of the 

 glacial period and from thence they spread down the main 

 streams, along the Mississippi from Minnesota to near the 

 Gulf, along the Missouri from Dakota to its mouth, and 

 along both the Illinois and the Wabash. These deposits 

 are thickest, most typical and coarsest along the bluffs 

 nearest to the streams and they thin out and become finer 

 as the distance back increases. It is thought that the fine 

 silts borne along by the waters of the glacial streams in 

 times of high water were spread out over broad flats and 

 as the waters withdrew they were left to dry in the sun 

 and then picked up by the winds and drifted away. The 

 loess soils -are almost always extremely fertile and very en- 

 during. 



76. The Work of Animals as Soil Producers There are 

 many animals which have contributed largely to the forma- 

 tion of soil through a grinding of pebbles and the coarser 

 sand and soil grains into finer materials. 



