139 



Aquilegia canadensis Asclepias phytolaccoidcs 



Ranunculus abortivus Apocymun androsaemifolium 



Hcuchcra hispida Monarda mollis 



Fragaria virginiana, Bupatoriuni serotinum 



var. illinoensis Bupatorium urticaefolmm 



Geuni canadense Antennaria plantaginifolia 



The River Dunes and their Plant Associations 



In the preceding pages those various associations have been de- 

 scribed which comprise most of the vegetation of the sand areas. 

 The chief physical factors concerned in molding their topography or 

 differentiating their associations have been wind and soil moisture. 

 There remains to be discussed the narrow strip of dunes which lies 

 close along the Mississippi river and which is affected also by water 

 action. The river dunes are well developed in the Hanover and 

 Oquawka areas along the Mississippi river, whose swift current and 

 shifting channel have been chiefly responsible for their origin. They 

 are much less prominent along the Illinois river, whose sluggish cur- 

 rent possesses but little power of erosion. The first stages in the 

 vegetational history of the dunes were observed only in the Hanover 

 area ; the last, from and including the development of the oak forest, 

 only in the Oquawka area. 



In the first two areas, the sand deposits lie at an average height 

 of 15-30 feet (5-10 m.) above the swampy, alluvial flood-plain. The 

 ri\-er meanders across its flood-plain from side to side, and in some 

 places flows directly at the foot of a sand hill. Under these condi- 

 tions a river dune may be formed. Erosion by the river carries 

 away the sand from below, and that portion of the sand above the 

 high-water mark of the river, and consequently removed from the 

 direct erosive action, stands at a steep slope, the angle of which de- 

 pends upon the wind, the rate of erosion, and the vegetative cover- 

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

 keeps loose and dry. Below ordinary high-water mark the sand is 

 worked over by the water and lies at a gentle slope, forming a broad 

 or narrow beach. The wind, which is generally from the west, re- 

 moves sand from the lower slope, and to a less extent also from the 

 steeper portion, carries it up the slope, and piles it in a long dune 

 parallel with the river and several feet higher than the general 

 level of the sand. As long as the river continues erosion on that 

 part of its banks, the whole slope moves gradually back; if the wind 

 constructs the dune as rapidly, or more rapidly, than the river erodes 



