GEOLOGICAL CLASSIFICATION OF SOILS 63 



little eroded and is supposed to be still accumulating. 

 Some valleys are known to be iSlled to a depth of 3000 

 feet with this material. Its characteristics are its fine 

 texture, its great depth, its wide distribution, and its 

 great fertility when moisture conditions are suitable for 

 crop growth, 



49. Sand dunes. — Sand dunes are the outgrowth 

 of two conditions — a large quantity of sand and a 

 wind that blows in a more or less prevailing direction. 

 Under such conditions the sand and other fine material 

 not only is blown into heaps, but also tends to move in 

 the direction of the prevailing wind. Such heaps or 

 mounds of sand may travel several feet a day by the 

 continual movement of the sand grains up the windward 

 side of the dune, only to be deposited again on the lee- 

 ward side. Sand dunes may often assume gigantic 

 proportions, being sometimes ' several hundred feet high 

 and twenty or thirty miles long. In such proportions 

 they become a grave menace to agriculture, not only 

 because they are an absolutely valueless medium for plant 

 growth, but also because they cover fertile lands and 

 entirely blot out all plant growth. The particles of this 

 wind-blown sand are usually round, from the continual 

 abrasion that they receive. A great many minerals 

 may be represented, but quartz is the commonest, es- 

 pecially if the dune originally had its origin on a lake or 

 a seashore. 



50. Volcanic dust. — From early geologic times de- 

 posits of the very fine material that is continually being 

 ejected from volcanoes have been distributed over the 

 earth's surface. These deposits are usually flour-like, 

 and while at one time they probably covered many 

 square miles of territory, they have succumbed very 



