100 



dune is held by Paiiicuiii virgatuin, it is usually distributed uniformly 

 around the deposits, and the dune is approximately uniform in 

 height. The bunches of grass are seldom more than a yard or two 

 apart and most of the intervening space is occupied by annuals. 

 Stipa spartea, Andropogon furcatiis, and Care.v Miihlcnbergii are 

 sometimes associated with Pankiim and are probably relics. Carex 

 umhellata, Panicum pseudopubescens, Koeleria cristata, and Viola 

 pedata are less frequent and are undoubtedly relics. Because of the 

 relic nature of the sumach on the deposits, a dune seldom has more 

 than one thicket of it. At that place the dune rises much above the 

 general level and has steep slopes occupied by relatively few other 

 plants. At a little distance from the sumach thicket the bunches of 

 Paniaim and Tephrosia appear. On such blowouts the deposits 

 frequently become irregular or one-sided, or the direction of deposi- 

 tion may be changed, because of the greater efficiency of the sumach 

 as a sand-binder. 



SUCCESSIONS BETW^EEN THE ASSOCIATIONS OE THE 

 BEOWOUT FORMATION 



As the Panicum pscudopubcsccns association becomes more open 

 and more bare sand is exposed in the formation of a young blowout, 

 it is difficult to decide just where the dividing line between the two 

 types of associations should be drawn. For convenience it may be 

 considered that a blowout begins with the first appearance of areas of 

 deposition and erosion, that is, with the differentiation of basin and 

 deposit. These two physiogra]:)hic structures are very soon occupied 

 by their usual vegetation. The windward slope and its attendant 

 plant association appear as soon as the increasing size and depth of 

 the basin begin to disturb the bunches of Paniciiui pscndopiibcscens 

 in the rear. Between the basin and the deposits there must be at 

 least a small space where the movement of sand is about neutral, and 

 this represents an incipient lee slope. The typical vegetation, how- 

 ever, does not appear during the earliest stages of the blowout. The 

 four associations, therefore, appear in the following order : ( i ) the 

 basin and deposit associations, (2) the windward slope association, 

 (3) the blow sand association. 



These four physiographic parts constitute a definite series as to 

 structure and development, but their veg'etation does not fall into a 

 regular successional series. That is, an area now occupied by the 

 windward slope association may not, and probably will not, be occu- 

 pied in turn by the basin, the blowsand, and the deposit associations. 

 The successions are, instead, very complicated and irregular. 



