20 



depending upon the wind and the rapidity of erosion. The 

 surface sand on this slope is exposed to the full sun and 

 keeps loose and dry. The wind, which in Illinois is pre- 

 vailingly from a westerly direction, carries a part of the loose 

 sand up the slope and piles it up in a long dune parallel with 

 the river and higher than the general level of the sand plain. 

 The whole dune consists therefore of two divisions, the lower 

 of sand now being uncovered by the wind and erosion and 

 removed by wind, and the upper of sand deposited by the ac- 

 tion of the wind. 



In spite of general current opinion regarding the relation 

 of wind to the formation of dunes, the general effect of wind 

 alone on sand is to reduce the elevations, fill up the depres- 

 sions, and make the general surface more nearly level. While 

 wind furnishes the actual force in piling up sand into dunes, 

 the sand does not remain so indefinitely unless it is held by 

 some efificient means, usually by the action of plants. So, in 

 the region under discussion, the river dune is initiated by the 

 wind, but perpetuated by sand-binding plants upon its crest. 

 Most notable among these are the redroot, Ceanothus ovatus 

 Desf., switch grass, Paniciim virgatum L., and, most important 

 of all, sumach, Rhus canadensis Marsh, var. illinoensis (Greene) 

 Fernald. The latter grows in dense thickets, and possesses 

 the ability to grow up indefinitely, keeping the tips of its twigs 

 always at least six inches or a foot above the surface of the 

 sand. The sand beneath these thickets is effectually pro- 

 tected from wind action, while more may be added with every 

 wind storm. Aided by these three species and some others of 

 less importance the wind builds up the river dune to a height 

 which may reach eighty feet above the general level, or one 

 hundred feet above the river. The amount of sand thus ex- 

 poserl offers a considerable surface for colonization by plants, 

 and is occupied in turn by several distinct associations. 



Contemporaneous with the sand-binders there appears on 

 the riverward side of the ridge a group of herbaceous plants 

 which may be called the blowsand association. It consists 

 primarily of annuals, such as the partridge-pea, Cassia 



