DAVIS: RIVER TERRACES IN NEW ENGLAND. ait 
seems to me an unnecessary conclusion, unless rapid elevation up to a 
recent date be postulated also, as is perhaps implied by Miller in a later 
sentence. Certainly, so far as increase of scarp height in New England 
valleys is concerned, it has not sufficed to prevent the broadening of the 
valley floor and the consumption of terraces at higher levels, so long as 
the terraces consist only of clay, sand, and gravel. 
It may be noted that if there had been a diminution of volume during 
the deepening of a drift-filled valley, the obliteration of stepping terraces 
would be delayed, but not prevented. It has already been explained 
that some diminution in stream volume is certainly probable. It may 
now be added that many valleys have, in spite of this very probable 
decrease of stream volume, already reached in one or another part of 
their length the late stage of terracing just described, in which all the 
descent from the highest terrace to the flood plain is concentrated in a 
single scarp, and that in many other parts of these valleys only a few 
basal terraces remain beneath the strong scarp of the high terrace. It 
thus becomes all the more probable that diminution of volume is not an 
important cause of the decrease in the breadth of the interscarp space, 
and that where stepping terraces occur, they must be in large part re- 
ferred to some special and local cause. Such a canse is found in the 
presence of rock ledges, as suggested by Miller; and to that element of 
the problem we may now turn. 
DerenpeD Terrace Cusps: Earty Srace. It has thus far been 
tacitly postulated that no buried ledges should be discovered by the 
wandering river. Such, indeed, is the condition usually assumed in the 
cross-section of a series of typical terraces, as in Figure 1. Let a new 
series of terraces now be developed, in which ledges shall here and there 
be discovered as the river degrades its valley floor to greater and greater 
depths. It is evident that the number of such ledges may vary greatly. 
They might be numerous and frequently encountered by a terracing 
river in a narrow valley with rugged rock walls and bottom ; they might 
be almost absent and hardly ever discovered in a broad valley that had 
been heavily aggraded. In all cases it is important to note that the 
slope of a ledge face will seldom be as steep as the average slope of a 
terrace front, which may be as much as 30° in freshly cut scarps. 
As before, the river wanders about freely so long as it is working 
on unconsolidated sands and clays; and thus several low terraces 
may be formed in the manner already described. But when a ledge 
is encountered in the river bank, as at the left forward edge of Fig- 
ure 23, the rock is practically indestructible. The stream willin a 
VOL, XXXVIII. — NO. 7 3 
