140 



it, the whole complex migrates slowly landward. If the river erodes 

 more rapidly than the wind piles up the sand, the dune will soon be 

 destroyed and only a bare slope remain. On the other hand, if the 

 river shifts its channel, or for some other reason ceases erosion, the 

 whole exposed surface will ultimately be fixed with vegetation and be- 

 come static. From a physiographical standpoint, therefore, the whole 

 dune consists typically of two divisions (PI. XVII, Fig. 2) : the lower, 

 termed the middle slope, consists of sand now being uncovered and re- 

 moved by the wind and erosion ; and the upper, called the upper slope, 

 of sand deposited by the wind, and removed by undermining through 

 erosion. From the standpoint of vegetation, several associations may 

 be distinguished which are in part correlated with the physiography. 



The lowest portion of the dune, near the river and within reach 

 of high water, is marked by vegetation of a semi-hydrophytic nature. 

 When visited in June, 1908, the river was very high, and only the 

 tops of the half-submerged plants could be seen. These were Populus 

 dcltoides, Sali.v longifolia, Fra.viiius pcnnsylvanica, var. lanceolata, 

 Gleditsia triacanthos, and Ulmus americana, typical sand-bar or 

 river-bottom plants. The herbaceous vegetation of a later season is 

 doubtless of the same ecological nature, probably including Bupato- 

 riiun scvot'uium, Xaiithiiun cormiuinc, and other species of similar 

 habitat. This vegetation has no relation to the typical dune vegeta- 

 tion above it, except in the presence of a few individuals of Panicum 

 znrgatiini, which had probably slid down from the slope above. 



The vegetation of the middle slope clearly belongs to the blow- 

 sand association, as described under the blowout fonnation. The 

 plant covering is sparse, becoming somewhat dense toward the bot- 

 tom. It consists chiefly of Cassia Chamacchrista and Diodia teres, 

 with smaller numbers of Croton glandulosus, var. septentrionalis, 

 Ambrosia psilostachya, and Crisiatella Jainesii. At wide intervals 

 are tufts of perennials, including Acerates viridiflora, var. lanceolata, 

 Panicum virgatwu, Lithospermnm Gmelini, Buphorhia coroUata, Cy- 

 periis Schzveinitzii, and Tcphrosia virginiana. The surface of the 

 sand is dotted with numerous pebbles, sometimes as much as 2 inches 

 in diameter. They apparently do not affect the vegetation, and there 

 are not enough of them to be called gravel. They evidently repre- 

 sent the accumulation of pebbles left by the sand blowing up into the 

 deposits above. 



The top of the middle slope is marked by the outcrop of a layer of 

 loamy sand (PI. XVII, Fig. 2), verv dark brown in color, rather 

 fine-grained, and conspicuously earthy in texture. The top of this layer 

 is well marked, but it gradually passes below into the typical orange- 



