336 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



this re-entrant from the south has formed a fan on the high terrace plain 

 and again on the floor of the re-entrant, but it is now dissecting the 

 fans. No B-C re-entrant has been carved out, perhaps because till was 

 there discovered. Several ledges were encountered at lower levels 

 between G and H, against all of which the stream has swung most 

 faithfully. The valley floor would surely be wider to-day, had these 

 ledges not existed. A fine re-entrant wa3 swept out between the 

 defended cusps C and D, when the river ran at a height about ten feet 

 over the modern flood plain, and another effort was here made to widen 

 the valley floor at its present level, but as ledges are now discovered at 

 H and J, further forward than C and D, the lower re-entrant has not 

 quite consumed all of the earlier flood plain. A low terrace, caught on 

 ledges J and K, stands in front of the re-entrant between D and E. The 

 projection of the strong but low cusp at J as compared to that of the 

 blunt but high cusp at D is one of the best illustrations of the effect of 

 ledges that is found in this little valley. The river must have slipped 

 past the ledge at D, as well as past most other defending ledges here- 

 abouts ; but a compressed meander must have been caught for a time 

 on the ledge at J. Down-valley from E, a modern swing of the stream 

 has under-cut all the earlier terraces, and a full-height scarp is the 

 result. 



These terraces are even better than those of the "Westfield for purposes 

 of field illustration, inasmuch as defended cusps here occur in abundance 

 on both sides of the valley. The narrowing of the interscarp space, as 

 the valley floor was degraded to lower and lower levels, is manifestly 

 due to the presence of the ledges. That the river was continuously 

 acting as a graded but degrading stream is sufficiently proved by the 

 fine flight of stepping terraces at M. That the preservation of the 

 successive terraces is not due to any shrinking of the stream from its 

 first intention as to valley widening, is proved by the vigor with which 

 it has opened the modern flood plain to as great a width as the numerous 

 ledges permit. 



It was on seeing — in October, 1900 — the relation of the defended 

 cusp of the little terrace at F to the corresponding defended cusp of the 

 next higher terrace a little farther back at A, that the value of ledges 

 in determining terrace pattern and in preserving the upper terraces from 

 later attacks of the stream first came to my mind. The manner in 

 which this explanatory idea first took shape was as good an example of 

 the sudden invention or birth of theory as I have ever experienced, for 

 the theory was essentially complete at the moment of its first conscious 



