DAVIS: RIVER TERRACES IN NEW ENGLAND. 289 



north-flowing rivers was turned over the divides into the valleys of 

 south-flowing rivers. This may have heen the case while the ice still 

 covered the northern basins, their waters (as far as they had any) then 

 running as subglacial streams, which may have been forced to ascend 

 slopes and cross divides. Effective constraint may also have been pro- 

 vided after the ice had at least in part withdrawn from the northern 

 basins, but when it remained in sufficient force to obstruct their normal 

 outlets, thus forming lakes whose overflow ran across a pass in the 

 divide to some southern valley. Another cause for increased volume of 

 our south-flowing rivers was the importation into their basins of a con- 

 siderable snowfall that was received on the ice sheet over some northern 

 basin. A fourth cause for increased volume of our rivers lies in a pos- 

 sibly greater precipitation during the later stages of the glacial period 

 than at present. A fifth cause lies in a relatively rapid meltiug of the 

 retreating ice sheet. It is eminently possible that these various causes 

 may have contributed effectively to an increase in river volume while 

 the New England valleys were aggrading with drift ; but it does not 

 follow that volumes decidedly larger than those of to-day were continued 

 into the period of terracing. 



Except where direct evidence is given by curvature and arc of high- 

 level terrace scarps, a formerly greater volume of the terracing streams 

 should be regarded only as a possible, not as an actual occurrence. It is 

 especially desirable that large bulk and coarse texture of terrace deposits 

 should not be too readily accepted as evidence of former greater volume 

 of streams ; for bulk of deposits is a function of time as well as of 

 rate of action, and texture is a function of slope as well as of 

 stream volume and velocity. Hence until time and slope are shown to 

 have been insufficient to account for bulk and texture of deposits, it is 

 not compulsory to account for them by greater stream volume. 



Even if decrease of volume has been of general occurrence during the 

 period of terracing, it has nevertheless not been in control of terrace 

 development ; for if it had been, stepping terraces should be much more 

 abundant than they are to-day. As a matter of fact, the diagrams by 

 which terraced valleys are ordinarily represented give an exaggerated 

 idea of the prevalence and perfection of these graceful forms. It is 

 rare to find a long flight of stepping terraces on both sides of a valley ; 

 it is rare to find a flight of terraces continued for any long distance 

 along a valley side ; when more than three or four low steps are to be 

 counted, it is usually only for a moderate fraction of a mile that they 

 persist. A large part of the length of our terraced valleys is bordered 



