DAVIS : RIVER TERRACES IN NEW ENGLAND. 311 



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 valle}-, 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 cause is found in the 

 presence of rock ledges, as suggested by Miller ; and to that element of 

 the problem we may now turn. 



Defended Terrace Cusps : Early Stage. 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 will in a 

 vol. xxxviii. — no. 7 3 



