THE GREAT AMERICAN DESERT 



221 



away and more continues to slide in, and in this manner the blow-out 

 increases in area as well as in depth. These two processes continue for 

 a number of years until, in many cases, the well-developed crater-form 

 depression is blown out of the hill. Naturally with the increasing depth 

 of the blow-out the direct force of the wind becomes considerably 

 checked by the prominent rim of the crater. But peculiarly enough, as 

 the wind strikes the farther slope of the blow-out a reverse current is 

 developed which strikes beneath the rim and dips into the bottom of the 

 crater. In this way a spiral wind movement is frequently developed 

 and the wind reaches to the very bottom of the blow-out, which may 

 now be fifty or more feet below the rim. This grinding action of the 

 wind continues to loosen more sand at the sides, causing it to slip more 

 and more into the bottom, where the wind catches it and hurls it up 

 over the sloping interior surface of the blow-out and out over the rim. 

 This action is quite appropriately called the " sand mill." The action 

 of these spiral currents are conspicuous during rather low winds as 

 well as on very windy days. Such activity is a very important factor in 

 hollowing the blow-outs to the greater depths. 



After many years of this sort of growth, blow-outs at the end of 

 their maximum activity become enormous depressions with a rim some- 

 times 300 to 900 feet in circumference with sides of bare sand sloping 

 inward at an angle of about 30 degrees to the bottom, which may be 

 from 20 to 75 feet or more beneath the rim. In the western portion of 

 the region where blow-outs are formed in rather low hills among the 

 lakes the sand is removed from the interior until the water table is 

 reached. 



During the years of greatest blow-out activity plants fail abso- 

 lutely to gain a foothold and establish themselves in the blow-out be- 



FiG. 8. A Square Meteh Quadrat on the Slope shown in Fig. 7. 



