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Where the rivers have withdrawn from the high-scarped terrace, flat 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 

 Powdermil] 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 due 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 heen, 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 



