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 was 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, farther 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 tbe 
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 
