28! 



BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



I. General Statement. 



Theories of Biver 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 ir. horizontal plan. The 



Fig. 1. 



cross-section is usually repi'esented 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 measiu*ed 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 



Fig. 2. 



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 



