131 



veiling valleys are for the most part filled vith extensive deposits of 

 muck overlying sand and occupied by swamp vegetation, with Iris 

 z'crsicolor, TypJia latifolia, Rhcxia virginica, Houstonia cocnilea, 

 Populus tveninloidcs, and other species of similar habitat prefer- 

 ences. Outside these depressions, and accordingly above them, lies 

 the bur oak association, above which in turn is the black oak asso- 

 ciation, occupying the crests of the ridg'e. In every case the bur 

 oak type is characteristic of the more mesophytic sand near the water- 

 level. According to all established principles of succession the drain- 

 age of the intervening swamps would cause a downward migration 

 of the bur oak association, provided other features of the environ- 

 ment were favorable to it. It is, of course, hardly probable that the 

 bur oak would extend very far out upon the deposits of muck. If 

 the swamps were composed of sand instead of muck, it is very prob- 

 able that the whole area would be occupied by the bur oak association 

 as rapidly as the lowering of the water-level permitted. In the 

 Kankakee area the interdunal depressions are occupied by meadows, 

 which are doubtless veiy similar to the Amboy swamps, having a 

 number of species in common, and probably representing a further 

 stage in the succession on muck or peat. The ridges are all covered 

 with forest, but in the short trip made through the area the distinc- 

 tions made between the black oak and bur oak associations were not 

 recognized. 



The whole leads to the first alternative mentioned, that the de- 

 pressions in the Winnebago area have never been occupied by the 

 black oak association, and that the bur oak association, which now 

 occupies them, represents the present culmination of a past hydro- 

 ph}rtic to mesophytic succession, which has been so far completed 

 that scarcely a trace of it is now in existence. 



This conclusion is supported 1)y the presence of a single small 

 pond occupying a depression in a partially cleared field. The few 

 j^lants remaining indicate that the surrounding vegetation was of 

 the bur oak type. The swamp vegetation at its margin is scant}, 

 consisting of Scirpus validus and Stcironciua lanccolatuin, outside of 

 which are successive zones of Populus trciiiuloidcs and Solidago 

 graijiiiiifolia. Further details were not noted. 



The development of the muck soil in the depressions of the Am- 

 boy and Kankakee areas and its absence in those of the Winnebag'o 

 area must also be explained. The latter areas are essentially fluviatile ; 

 their depressions are not far above the beds of the Green and the 

 Kankakee rivers, respectively, and the lowering of the water-level is 

 entirely dependent upon changes in the river level and upon dcposi- 



