CHICAGO AND VICINITT. 



31 



the erosion of the flood plain still farther advanced; this bank is just 

 opposite the willow vegetation shown in Fig. 11, hence there is depo- 

 sition on one side and cutting on the other. A river may thus swing 

 quite across its flood plain, destroying all that it has built, including 

 the mesophytic forest. Not only is the vegetation destroyed directly, 



Fig. 16. — An oxbow lake in the flood plain of Thorn creek. The willows are antecedent, 

 dating back to a stream margin, while the shrub (Cephalanthus) and herb vegetation is asso- 

 ciated with the present undrained condition. 



as shown in fig. 14, but also indirectly, since the lowering of the river 

 causes the banks to become more xerophytic. In place of the her- 

 baceous mesophytes, Equisetum and other relatively xerophytic forms 

 may appear, though the trees usually live until directly overthrown by 

 the river. 



One more phase of river activity may be briefly sketched. In mean- 

 dering over a flood plain, serpentine curves, or oxbows, are frequently 

 formed. In time the river breaks across the peninsula, and the oxbow 

 remains as a crescentic lake. The conditions radically change almost 

 immediately, and the river life is replaced by pond life, The change 

 is even more striking on the margins, where the old plants pass away 

 and the forms of undrained swamps come in. Fig. 16 shows the rem- 



