WORK OF THE WIND 



45 



Wind with Sand. — As soon as the wind picks up pieces of the 

 mantle rock it has tools with which to work, and it becomes a geological 

 agent whose effect in desert 

 regions is not easily over- 

 stated. It is from the man- 

 tle of rock waste formed by 

 the agents of the weather, 

 and from the sediment car- 

 ried to the deserts by the 

 mountain streams that the 

 sheets of sand which cover 

 the deserts are made. The 

 work of wind laden with 

 sand is well illustrated in the 

 artificial sand blast by which 

 granite is polished and glass is etched in desired patterns. Telegraph 

 poles in arid regions are often cut off near the base by wind-blown 

 sand. 



Pebbles worn by the wind (Fig. 22) usually have a character- 

 istic, brazil-nut shape {dreikanter), the faces meeting in ridges. 



Fig. 



22. — Pebbles faceted by the abrasion 

 of wind-blown sand {dreikanter). 



Fig. 23. — Surface eroded by wind-blown sand. The small table is the remnant 

 of a once extensive bed. (De Martonne.) (See Fig 24.) 



This shape is explained as follows : the planing of the exposed surface 

 of the pebble by the wind-blown sand continues until the pebble stands 

 on a narrow base. It is then overturned by a slightly stronger gust 



