Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 241 



gon, for instance, which are seldom killed except by the most severe 

 fires send up a number of green stems and leaves whose presence is 

 made much more emphatic because the old stems and leaves have 

 been consumed by the fire. However, when the high desiccating 

 winds of August reach such areas the unprotected growth often 

 suflfers very seriously and is often unable to withstand the fury of 

 the wind and the cutting sand-blast. 



Grazing animals often greatly reduce the plant cover of the 

 uplands and the soil may be tramped bare of plants for wide inter- 

 vals so that the wind readily strikes at the open sand and thereafter 

 range destruction may be rapid. It must not be supposed that the 

 bunch-grasses will soon be followed by some more valuable species, 

 such as blue grass, as Gleason {Z7) reports for the inland dunes of 

 Illinois. 



When the young blow-out is no more than one foot in depth 

 the sand begins to slide into the depression from the sides. This 

 sand which rolls into portions of the shallow blowout is caught up 

 by the wind and blown away. Sand continues to slide in with the 

 increasing depth of the blow-out and in this way the depression 

 increases in area at the same time that the wind is scooping it out to 

 a greater depth. These two processes continue for a number of 

 years with varying rate for different exposures until, as in many 

 cases, the highly 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 as it strikes 

 upon the prominent inner face of the blow-out and is partially 

 deflected downward. As the wind impinges upon the farther inner 

 slope of the blow-out, which soon assumes a gradient of about 30°, 

 a prominent reverse current is developed which strikes beneath the 

 rim and dips more or less into the bottom of the deepening crater. 

 In this manner wind action reaches to the very bottom of the blow- 

 out which at this time may be fifty or more feet below the highest 

 point of the rim. The grinding action of these downwardly deflected 

 currents heavily laden with finely divided quartz particles can best 

 be appreciated by placing oneself in such an intensely dynamic 

 habitat on an especially windy day. These winds continue to loosen 

 sand at the sides of the blow-out, causing it to slide into lower depths 

 only to be caught by the wind and hurled over the rim and deposited 

 upon the advancing lee slope. Because of the presence of grinding 

 spiral winds in deep blow-outs during this rather late period of 



