Pool: THE VEGETATION OF THE SANDHILLS OF NEBRASKA 209 



movement either by capillarity or percolation through a soil depend 

 upon the coarseness of the particles. We have already seen that 

 sand movement under the influence of the wind depends upon the 

 same factor. Even after a prolonged period of drought, moist sand 

 is sure to be found within four inches of the surface. 



A relation overlooked by most students of dune phenomena is 

 that, while moisture so near the surface aids considerably in resist- 

 ing wind erosion, yet under prolonged drought conditions this resist- 

 ance is quite readily broken up. At periods with the prevalence of 

 extremely high winds and low saturation deficit, the surface level 

 of exposed situations upon the dunes is rapidly lowered because of 

 the relatively rapid rate of depression of this moist layer. 



The soils of which the dunes of the Nebraska sandhills are com- 

 posed is made up of particles of widely varying dimensions. The 

 accompanying table shows the results of mechanical analyses of 

 these soils. 



MECHANICAL ANALYSES OF SANDHILL SOILSi 



1 Determined by U. S. Dept. Agr., Bureau of Soils, 1913. 



The wind can not move large and small sand grains with the 

 same velocity. The size of the grains moved and also the distance 

 to which they are carried varies according to the force of the wind. 

 As a consequence of these facts the wind exerts a distinctly win- 

 nowing or sorting action which results in the formation of sand 

 ripples. These sand ripples, or small waves resembling in appear- 

 ance those formed in shallow water by the sorting action of water, 

 are characteristic of all dune areas. The mechanism of sand ripple 

 formation has been studied experimentally by Cornish (29). 



The common conclusion appears to be that sand contains very 

 little available plant food materials. Certain writers, Massart (46), 

 have emphasized this deficiency in the production of the sparse vege- 



