DAVIS: RIVER TERRACES IN NEW ENGLAND. 293 
valley itself, or still more with a whole cycle of erosion, in which a 
mountain mass is reduced to a plain of degradation. 
While slow uplift is thus seen to be consistent with the production 
of many terraces, it is not consistent with their preservation, for 
it does not explain the diminution of the interscarp space from the 
higher to the lower levels. Indeed, the present rivers might tend to 
develop broader flood plains by strong lateral swinging at the faint 
grades now assumed than they had developed at the stronger grades 
during the earlier stages of possibly more active uplift and heavier 
load ; and the broad low flood plains would necessitate the undercutting 
of all or nearly all the earlier high-level terraces by the present stream, 
and the concentration of nearly all the separate scarps in a single 
high-terrace front, as in Figure 2. Examples in which this condition 
has been actually attained are to be found in the valleys of various 
rivers, as will be more fully set forth on pages 328, 342, and 344. 
Single high-scarped terraces are indeed so common as to warrant the 
conclusion that high-level and intermediate terraces would nearly 
always be destroyed by the swinging of the river at a lower level, 
but for the occurrence of some special conditions by which they are 
preserved. 
TERRACES CARVED BY STREAMS OF DriminisHina Loap. A graded 
river may be caused to degrade as well by diminishing its load as by 
increasing its slope, volume remaining constant. A diminution of 
load since the stage of glacial retreat is highly probable, for not only 
the streams that issued from the ice sheet but those also which 
washed the freshly-exposed drift-covered land surface were in all 
probability highly charged with detritus in late glacial and early 
postglacial time. Indeed, increase of load may have been almost as 
potent a cause of filling the valleys with washed drift as was the 
depressed attitude of the land in the north and the consequent en- 
feebled slope of the south-flowing rivers. As the ice disappeared and 
as the land surface was more or less covered with vegetation, the 
load of the streams should have been lessened, and they must there- 
upon have set to work to degrade the valleys that they had just 
before been aggrading, even if no change of slope had taken place. 
This process, if working alone, must have been very gradual, and 
might therefore have allowed plenty of time for lateral swinging and 
terrace carving. But, as before, no explanation is here found for the 
production of stepping terraces. On the contrary, when the diminu- 
tion of load was further advanced the rivers would degrade their valley 
