90 PHYSICAL GEOLOGY 



ermost few feet of air than in the more swiftly moving cur- 

 rents above. As a result, such elevations are often undercut, 

 and have steep and even overhanging slopes. Small, isolated 

 elevations may take on the form of great mushrooms. If the 

 pedestal is worn through, the larger upper part falls, and may 

 be worn away in turn. Wind-worn slopes are frequently 

 characterized also by the absence of accumulations of angular 

 fragments (talus) at their base (Why?). Hard, compact stones 

 on wind-swept surfaces are sometimes given flat and highly 

 polished faces. 



Erosion in deserts. The final result of erosion in an in- 

 terior desert basin is the formation of a nearly level rock- 

 floored plain, covered more or less generally but thinly with 

 hard, gravelly waste, and surmounted here and there by eleva- 

 tions representing the most resistant portions of the beds that 

 have been worn away. In the earlier stages of the production 

 of such a plain, when the slopes of the basin are still steep, the 

 work of intermittent desert streams and occasional floods is 

 more important than that of the wind. It is to be noted that 

 the driest regions of the world now and then receive sufficient 

 rains to cause floods. In the later stages, however, streams be- 

 come less important as the surface becomes more even, and 

 the wind comes to play the leading role. Whirlwinds lift dust 

 high into the air, where it is taken up by the upper currents 

 and carried outside the desert area. Strong winds sweep 

 sand across the basin floor, and, if the inclosing slopes are not 

 too steep, carry it over the rim to outside regions. By the re- 

 moval of its waste in these ways, the basin is slowly lowered. 

 During the process the wind may scour out depressions where 

 the rocks of the desert floor are weak, but such depressions 

 cannot become deep, because of the inwash of waste from the 

 surrounding higher ground by temporary streams born of 

 occasional showers. Thus the streams serve as a check upon 

 the winds, and, as indicated above, an old-age desert plain is 

 nearly level. Wind-degraded plains of interior basins differ 

 from normal old-age plains developed by running water 



