282 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
I. General Statement. 
THEORIES OF River TERRACES. The terraces carved by streams in the 
washed drift of our valleys have been frequently studied and described 
since the beginnings of geological investigation in New England. In 
nearly all cases more attention has been given to terrace pattern as 
seen in vertical cross-section than as presented in horizontal plan. The 
cross-section is usually represented as in Figure 1, in which the 
depth of the rock-floored valley is made greater than that of the new 
valley carved in the drift filling. A. notable feature of such terrace 
sections is that the open space measured across the valley between the 
scarps of the low-level terraces is narrower than that between the scarps 
of the high-level terraces ; and this fact has frequently given rise to 
the supposition that the volume of our streams to-day is less than that 
of the streams by which the high-level terraces were carved. It will 
however be shown from what follows that the characteristic cross-section 
of a terraced valley in which the river has not yet reached rock bottom 
exhibits few stepping terraces, and is fairly represented by Figure 2; 
while if stepping terraces are present a characteristic section, on which 
the most significant points for a mile or more up and down the valley 
are projected, would show that the base of many of the terraces is 
determined for short distances (ten to fifty feet) by a rock ledge, as in 
