144 



the later stage (in this case the dune thicket association) is well de- 

 veloped. 



Blowouts may be formed on the crest of the river dune in the 

 usual way, and extend transversely through it. They seldom reach 

 below the old soil bed which marks the limits of the middle slope. 

 Their vegetation is of the usual type, except that the lateral slopes 

 are frequently held by the plums and crabs of the thicket association. 



In some places in the Hanover area the river dune is occupied by 

 the black oak association. The erosion there is generally feeble and 

 the dune relatively stable. It seems probable that the oaks would also 

 develop on the dunes stabilized by the ordinary deposit association 

 if the thickets did not encroach upon them so rapidly. Smilacina 

 stellata, as already mentioned, is a characteristic member of the black 

 oak association, and its position on the dunes between the thicket and 

 the deposit associations possibly indicates a potential development of 

 the black oak forest at this place. 



In the Oquawka area the greater portion of the river dune is 

 forested, and in parts of it the development of humus and the in- 

 creased density of the ground cover has led to the establishment of a 

 mesophytic type of forest, described already (p. 137) under the mixed 

 forest association. This portion of the dune is no longer washed by 

 the river itself, but by some slug"gish bayous representing a former 

 channel of the river and separated from the present channel by a 

 number of densely wooded alluvial islands. At the foot of these 

 islands the channel bends eastward against the foot of the dune and 

 erosion is now proceeding rapidly. The plant covering is an efficient 

 protection against wind erosion, and the dune would be completely 

 stable if it was farther inland, but it can not resist the undermining 

 effect of the water. On a strip several hundred yards long the forest 

 has been completely destroyed (PI. XX, Fig. i), and the vegetation 

 now consists entirely of the blowsand and deposit associations. At 

 the north end of the deforested portion the destruction of the forest 

 is still proceeding. The effect of the erosion is first manifested at the 

 foot of the dune, and its influence gradually extends higher until 

 eventually the trees at the top are undermined. There is thus pro- 

 duced a triangular extension of the blowsand, extending like a wedge 

 along the river between the water below and the forest above. It is 

 now seen that the principal root development of the herbaceous vege- 

 tation extends but one or two feet (3-5 dm.) below the surface, and 

 binds the sand into a coherent stratum resting on the loo-e sand be- 

 neath (PI. XX, Fig. 2). The loose sand rests at as steep an anele as 

 possible, and irregular blocks of the surface layer become detached 



