THE WORK OF THE ATMOSPHERE 21 



material is known as loess, the particles of which are smaller than 

 sand grains, but larger than particles of clay. In China, the loess 

 is said to be several hundred feet thick in some places, but in the 

 United States it is rarely more than 30 to 50 feet thick. It makes a 

 most fertile soil in regions where the climate is not too dry. Steep 

 slopes or cliffs are sometimes formed in the loess after its deposition. 

 Such slopes often stand for a long time, even when they are nearly 

 vertical (Fig. 11). In parts of China, the people have made dwell- 

 ings in successive tiers in these steep slopes (Fig. 12). 



Wind-blown dust has, in the course of ages, buried former cities. 

 Nineveh is supposed to have been buried in this way. 



Since dust is being blown from the land to the sea all the time, 

 the wind tends to lower the land and to build up the sea bottom. 



Sand 



Sources. Strong winds pick up and carry grains of sand, and 

 even very small pebbles. Like dust, sand is blown about only when 

 it is dry. Abundant sand is found along many shores of seas and 

 lakes, on the bottoms of some river valleys, in desert regions, and 

 in certain other situations. In most of these places, the sand is dry 

 at times, and in some of them it is dry most of the time. 



Lodgment of wind-blown sand. Sand is not often carried up 

 so high as dust, nor do the grains stay so long in the air. Because 

 they are larger, they fall more rapidly when the wind is checked. 

 Because they are carried chiefly in the lower part of the air, they 

 are likely to be stopped by obstacles of all sorts on the surface. 

 Thus logs, stumps, buildings, and fences, against which sand is 

 blown, are likely to cause some of it to lodge, just as they cause 

 drifting snow to lodge. It follows that sand, instead of being scat- 

 tered somewhat evenly over the surface, as dust is, is often left in 

 mounds and ridges. 



Dunes. Mounds and ridges of wind-blown sand are dunes. 

 They may begin their growth about almost anything on the 

 surface which blocks the way (Fig. 13). After being started, 

 a dune becomes an obstacle to blowing sand, and as more sand 

 lodges against it, the dune grows. Dunes sometimes reach a 

 height of several hundred feet, but small dunes are much more 





