DAVIS: RIVER TERRACES IN NEW ENGLAND. 329 
swinging of this kind that the high terrace has been worn back to its 
present outline. 
Where the rivers have withdrawn from the high-scarped terrace, flat 
fans have been formed at the outlet of the minor lateral valleys of small 
brooks, or beneath little gulleys of wet-weather wash. The fan of 
Powdermill brook, for example, forms a low barrier, X, Figure 39, across 
a deserted channel of Westfield river, and thus determines a swampy 
depression just northeast of Westfield station. The further course of 
the brook follows the marshy deserted channels of Westfield river at 
the base of the scarp for over a mile. 
It would be difficult to find better illustrations of the deductions 
presented on page 310 than are offered by this beautiful basin. The 
two chief streams, far from exhibiting any incapacity to open their 
valley floors, have now widened them to a greater breadth than ever 
before. Whatever decrease of capacity may be dne to decrease of 
stream volume and of stream slope, and whatever increase of work may 
be due to the more active wash of side streams on account of gain in 
height of valley sides, the main streams are certainly more competent 
to corrade laterally now than they have ever been, and there is every 
probability that they will in the future continue to widen their basin 
still further by intermittent attacks on its border until restrained by 
defending ledges or by the hand of man. Indeed, so nearly complete 
is the obliteration of all terraces above the level of the present basin 
floor, one might be tempted to conclude that the Westfield and Little 
rivers never produced any extended series of flood plains in this division 
of their course at higher levels than those of modern times, until an ex- 
amination of the western division of the Westfield terraces proves that 
flood plains must have been produced at various levels in the eastern 
division as well as elsewhere. 
Evidently then, as far as this example goes, it affords no evidence 
that the production and preservation of terraces is due to any incom- 
petence arising from decrease in the volume or from other changes in 
the habits of our New England streams. Terrace preservation must be 
due to some control external to the streams; and of this we find imme- 
diate proof on looking at the eastern and western enclosure of the broad 
basin just described. 
The basin is enclosed on the east by the approach of a defended spur, 
A, Figure 38, on the north towards a free spur, B, on the south, beyond 
which a subordinate basin, C, is again opened. The defended spur 
carries a terrace plain at a height of 200 feet, and the highest plain 
