87 



equal ; the sand is constantly changing- although its level varies but 

 little. Finally an outer crescent, called the deposits (PI. V), sur- 

 rounds the lee slope and is somewhat higher than the g^eneral level. As 

 its name indicates, it is composed of sand removed from the other three 

 parts and carried up by the wind. A long'itudinal section through a 

 typical blow'out is shown in Figure 4. The basin shows a constant 



Fig. 4. Diagrammatic longitudinal section through a t3'pical blowout, i, orig- 

 inal level of sand ; .?, windward slope ; j, basin ; 4, lee slope ; 5, deposits. 



tendency to increase in size or in depth. If this tendency is most pro- 

 nounced toward the rear, i.e., the west, it is showar by a steep wind- 

 ward slope ; if most pronounced toward the front, or east, by 

 g-entle windward and steep lee slopes. If all the sand removed is 

 poured out in one direction, the sides become steep and settle by 

 gravity toward the basin. They may be regarded as a continuation 

 of the windward slope and are occupied by the usual vegetation. On 

 the other hand, if the sand is carried out toward the north and south 

 as w'ell as toward the east, the windward slope is small, and the lee 

 slope and deposits extend around three sides of the blowout. Be- 

 tween the two extremes there is every imaginable gradation. A blow- 

 out of this simple type is occupied by four plant associations which 

 are usually easily recognizable and which are correlated with the 

 four physiographic divisions. 



The maximum observed length of a simple blowout, but without 

 all four parts present, is in this region about 200 yards (200 m.). 

 As their size increases the windward slope may disappear, either by 

 a reduction in its gradient or by stabilization. The continuous cres- 

 cent of deposits may be broken up into several detached segments, 

 separated by patches of sand corresponding to a lee slope or by prom- 

 ontories covered by bunch-grass and with vertical walls. The slope 

 below these steep-sided mounds usually functions as a windw^ard slope, 

 and is occupied by the characteristic vegetation, even though it is at 

 the eastern end of the blowout. There is also a general progressive 

 movement of the blowout from west to east, so that the stabilized re- 

 mains of an old windward slope may be found behind the present 

 active one. 



Blowouts of this simple type are not common ; in fact only one 



