DAVIS: RIVER TERRACES IX 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 pi'esent 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 Diminishing Load. 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 w T ith 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 



