CHAPTER XVI 



THE FEATURES IN DESERT LANDSCAPES 



The wandering dunes. Over the broad expanse of the desert, 

 sand and dust, and occasionally gypsum from the saline deposits, 

 are ever migrating with the wind ; on quiet days in the eddying 

 " sand devils," but especially during the terrifying sand storms 

 such as in the windy season darken the air of northern China and 

 southern Manchuria. This drift of the sand is halted only when 

 an obstruction is encountered a projecting rock, a bush, or a 

 bunch of grass, or again the buildings of a city or a town. The 

 manner in which the sand is ar- 

 rested by obstacles of different 

 kinds is of great interest and im- 

 portance, and is utilized in raising 

 defenses against its encroachments. 

 If the obstacle is unyielding but 

 allows some of the wind to pass 

 through it, no eddies are produced 

 and the sand is deposited both to 

 windward and to leeward of the 

 obstruction to form a fairly sym- 

 metrical mound (Fig. 218 a). An 

 obstruction which yields to the 

 wind causes the sand to deposit 

 in a mound which is largely to 

 leeward of the obstruction (Fig. 

 218 6). A solid wall, on the other 

 hand, by inducing eddies, is at 

 first protected from the sand and mounds deposit both to wind- 

 ward and to leeward (Fig. 218 c and Fig. 219). 



Except when held up by an obstruction, the drifting sand travels 

 to leeward in slowly migrating mounds or ridges which are known 

 as dunes. Their motion is due to the wind lifting the sand from 

 p 209 



FIG. 218. Diagrams to illustrate the 

 effects of obstructions of different 

 types in arresting wind-driven sand. 



a, An unyielding obstruction which 

 permits the wind to pass through it ; 



b, a flexible and perforated obstruc- 

 tion ; c, an unyielding closed barrier 

 (after Schulze). 



