THE GREAT AMERICAN DESERT 



2 19 



green patches from ten to seventy feet across contrast very greatly with 

 the surrounding bunch-grass vegetation. Bessey's sand cherry is one of 

 the most ubiquitous plants of the whole region. It is found in almost 

 every site of the uplands and with its low, short twigs with tufts of 

 glossy green leaves is seen springing from the sand on practically every 

 side of every hill. Very frequently it forms extensive communities. 

 The prairie clovers seldom form well-defined communities, but they 

 occur as more or less scattered individuals, especially on the lower slopes 

 of the hills adjacent to the larger valleys. 



The most striking habitats of the uplands are the " blow-outs." 

 Blow-outs are conical or rounded depressions of varying depth and 

 diameter formed by the blowing of the sand and vegetation from cer- 

 tain spots on the upper slopes and crests of the hills. The rim of the 

 more or less conical depression is sometimes almost circular but it is 

 usually irregular with a general circular outline. Since the prevailing 

 winds of the region are from the west, and since " blow-outs " are the 

 direct products of wind action, these peculiar structures are mostly con- 

 fined to the west sides of the hills. The greatest number occur on the 

 northwest-facing slope, but they range in position from northwest to 

 southwest, depending somewhat upon the shape of the hill concerned 

 and its relation to the adjacent hills. Blow-outs do not occur on all 

 hills, nor does a single hill show more than a single blow-out, as a rule. 



On an exposed upper slope when the vegetation becomes broken or 

 seriously depleted from any cause, the wind as it sweeps up the slope 

 catches the sand and carries it over the crest of the hill a few yards 

 farther away and dejDosits it upon the lee face of the hill. In this way 



Fig. G. a Blow-out from the East. The sand has been blown out of the crater on 

 the other side. Bunch-grass in the foreground. 



