328 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



descend to the main low-level basin, everywhere present concave re- 

 entrants, whose curves unite in cusps — usually two-sweep cusps — of 

 greater or less acuteness ; and this shows that the streams have re- 

 peatedly swung against the terrace scarp, under-cutting it and pushing 

 it back, after the present grade had been essentially reached. The 

 curved re-entrants are of somewhat larger radius on the north than on 

 the south, as if they had been scoured by the Westfield and Little 

 rivers, respectively. With the significant exception of certain points on 

 the cast and west, to be described below, all these cusps, at least twenty- 

 four in number, are free, undefended by ledges. We have, therefore, 

 here an example of a vigorous stream with a good-sized branch working 

 in a broad deposit of loose drift, and free to sweep, swing, and wander 

 over a large area. A late stage of terracing has been reached, for the 

 wide plain is nearly or quite i-educed to grade with respect to a rela- 

 tively permanent local baselevel in the trap-ridge notch on the east. 

 The detention of further degradation by the trap barrier is a factor of 

 importance ; for many recent swings of the streams must have on this 

 account tended to destroy earlier terraces by reducing them all to one 

 level, instead of tending to make new ones at lower levels. Whatever 

 flood plains may have been produced during the excavation of the 

 present basin floor, the streams have now so well taken advantage of 

 their opportunity for lateral corrasion or " sapping " that terraces at 

 high and intermediate levels are nearly everywhere obliterated, and even 

 the low terraces are as a rule destroyed by the broad swinging of the 

 streams at their present grade. 



Westfield river is at present nowhere working against the base of the 

 high terrace on the north ; its actual course lies about half a mile to 

 the south of the scarp, but several of its former courses along the ter- 

 race base are clearly revealed in a series of shallow swampy troughs, 

 the remains of channels from which escape seems to have been effected 

 by repeated short-cuts or cut-offs. The river is now engaged at several 

 points in grading down to modern flood-plain level a broad and low 

 terrace, parts of which are not yet destroyed. 



Little river was in 1901 sweeping against its high terrace on the 

 south at two points a little east of the New-Haven and Northampton 

 Railroad. Here the usual cover of vegetation has been removed from 

 the scarp, the sands are under-cut, and the face of the scarp is sliding 

 intermittently into the stream. Small sand dunes are formed at the 

 top of the sliding bank by the northwest winds which sweep the sand 

 up from below. It is evidently enough by a repetition of sweeping and 



