
Igor] . PHYSIOGRAPHIC ECOLOGY OF CHICAGO 107 
plain is the mesophytic forest, but here, as well as on the river 
bluffs, the climax may be but temporary. Retrogression is 
almost sure to come in connection with terrace formation. 
While it is true that deposition is the main feature of flood plains, 
it is also true that erosion has not ceased; the downward cutting 
of the river once more causes vertical banks, though this time 
in its own flood plain. This action is seen in fig. rg which 
shows the beginning of the new erosive phase, and its indication 
in the falling elm. There has doubtless been lateral erosion here 
also, since elms are not usually marginal trees. /%g. 75 shows 
the erosion of the flood plain still farther advanced; this bank is 
just opposite the willow vegetation shown in fig. zz, hence there 
is deposition 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 vegeta- 
tion destroyed directly, 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 herbaceous 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 meandering 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. 76 shows the remnant of one of 
these oxbows; on the farther side are old and dying willows, 
trees that look back to the well drained river margin. On either 
Side of the pond are seen clumps of the button bush ( Cepha/lan- 
thus occidentalis), one of the most characteristic plants of 
undrained swamps. Thus the willows are antecedent and the 
button bush subsequent to the formation of the cut-off. Fig. 77 
shows a portion of the same, in which the willows and even the 
pond itself have gone, and only the marginal button bush is left, 
