DAVIS: RIVER TERRACES IN NEW ENGLAND. 345 
river to open a broad flood plain enclosed by strong scarps, as typified. 
in Figures 2 and 22; while further south the valley exhibits the com- 
plete control exercised upon a swinging river by the chance discovery 
of ledges during the progress of degradation. This process is shown 
to have been gradual by the preservation of flood-plain remnants at 
various heights, wherever ledges are present to defend their bases ; yet 
so complete is the destruction of the plains at intermediate levels just 
above Concord that one might there infer that the river had had no op- 
portunity to swing laterally until the opening of the present flood plain 
was begun. 
