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THE VEGETATIONAL HISTORY OF A RIVER DUNE.* 



Henry Allan Gleason. 



Sand regions and dunes have long been a popular field for 

 the plant ecologist, and studies of them have been of great 

 importance in developing and widening our ideas of plant 

 ecology. Of the many interesting features of the Illinois 

 sand areas, the single physiographic structure, the river dune, 

 has been chosen for description, since it illustrates in a strik- 

 ing manner the action of water and wind, the role of vegeta- 

 tion in sand-binding, and some important phases of succession. 



Three of the chief sand areas of Illinois lie along the east 

 bank of a river; the Havana area parallels the Illinois River 

 for many miles south of Pekin, the Hanover area extends 

 along the Mississippi River in Jo Daviess and Carroll counties, 

 and the Oquawka area borders the same river in Mercer and 

 Henderson counties. In each of these the river dune is de- 

 veloped to some degree, but it is especially prominent in the 

 last two, which are the only ones referred to in this article. 



The sand deposits constitute the so-called second bottom, 

 which extends from the swampy, alluvial first bottom, the 

 modern flood-plain of the river, inland usually to the bluffs. 

 While they are always more or less undulating, their general 

 level is fairly constant. This level in the Hanover area is 

 approximately twenty feet above high-water mark in the 

 river. The river meanders over its modern flood-plain from 

 one side to the other, and when it flows at the eastern mar- 

 gin, directly against the deposits of sand, the conditions are 

 such that a river dune may be formed. Erosion by the river 

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

 sand above the river action stands at a steep slope, the angle 



*The field work, a portion of the results of wbich are here presented, was car- 

 ried on during the summer of 1908 through the aid of a grant from the Botanical 

 Society of America. 



