89 



the currents of the wind are changed and the bottom of the 

 blowout ceases to be eroded. The basin is thus converted into an ex- 

 tension of the lee slope, where the sand is merely in motion, without 

 an essential change in le\el. Very little information is available con- 

 cerning the rate of movement of the sand. That the motion is con- 

 tinuous through the summer is shown by a rough experiment in the 

 Hanover area. A hole about eight inches (2 dm.) deep was dug 

 June 2, 1908, on a flat expanse of sand, with no vegetation except 

 a few plants of Hiidsonia. On June 13 it was found filled to a 

 depth of two inches (5 cm.). This does not indicate erosion or dep- 

 osition, but merely the amount of sand which traveled across the 

 area in the given time and was caught in the hole. 



In spite of the physiographic diversity, the vegetation, if any, 

 can be referred to four different associations. One of these, which 

 from its position may be called the windward slope association, is 

 primarily relict in its nature, being derived from the bunch-grass or 

 the Pa)iicuin pscudopuhcsccns association. The basin association 

 consists of a very sparse growth of perennials, analogous to those 

 described by Pound and Clements (i8p8: 392; ipoo: 365) in sim- 

 ilar situations in Nebraska. The lee slope is occupied by the blow- 

 sand association, limited in duration and consisting almost entirely 

 of annuals. Lastly, the deposit association is composed chiefly of 

 sand-binding perennials, which serve to build up the deposits into 

 dunes. Each of these associations is well correlated with the dynam- 

 ic conditions of its environment; so well correlated in fact that 

 the vegetation is one of the chief means of recognizing the nature or 

 rate of movement of the sand. \Mien the different physiographic 

 parts are obscured or obliterated in a complex of blowsand, the vege- 

 tation is still correlated with the dynamic conditions, and the nature 

 of the movement of sand may be compared with accuracv to the 

 appropriate portion of a nonnal blowout where the same vegetation 

 is developed. Care must of course be exercised to avoid confusing 

 relics with the typical plants of the station. 



The specific composition of each of these associations varies 

 greatly from area to area and from blowout to blowout. The varia- 

 tion is frequently peq^lexing, and becomes especially so when the 

 number of species is small and the individual plants few or scattered. 

 Relics of a preceding vegetation are also frequently found and add 

 considerably to the difticulty of distingiiishing the associations. 



The development of a blowout in the prairie is first evidenced by 

 an exposed area of bare sand surrounded by the Panicum pseiido- 

 pubcsccns association. The young blowout may in fact be regarded 



