DAVIS: RIVER TERRACES IN NEW ENGLAND. 319 



have been given an importance that their restricted occurrence hardly 

 warrants. 



Distribution of High-Scarp and Low-Scarp Terraces. Ledges 

 may be gradually disclosed at various points up and down the valley, 

 each one having an effect in cusp-making and terrace-keeping appro- 

 priate to its position with respect to the valley axis. The frequent 

 swinging of the meander belt from side to side during the slow degrada- 

 tion of the valley floor requires that the discovery of every ledge lying 

 well within the belt of wandering should be made soon after the stream 

 has degraded the valley floor to the level of the ledge top. If the valley 

 floor is deepened ten feet during a complete swing of the meander belt 

 to and fro, it would be very unlikely that a ledge within the breadth of 

 swinging should escape discovery until the valley floor was worn down 

 twenty feet below the ledge summit. It would be impossible for the 

 discovery to be postponed so long that the stream should first encounter 

 the ledge fifty feet below its top, unless the ledge were situated rather 

 far to one side of the valley axis, where it might not be encountered by 

 the down-sweeping meanders every time the meander belt swung across 

 the valley. The more likely case is that an actively swinging, slowly 

 degrading stream will discover the upper part of every ledge lying 

 well within the belt of wandering, and that thereafter the stream will 

 frequently swing against the slope of the ledge at lower and lower levels 

 as the valley floor is deepened ; unless, indeed, another ledge, nearer the 

 axis of the valley, and with a lower summit than the ledge already dis- 

 covered in its neighborhood, prevents the river from swinging laterally 

 so far as it had at higher levels. 



This specialized conception of the terracing process leads to some 

 reasonable deductions as to the distribution of high-scarp and low- 

 scarp terraces. They are summarized in Figure 36. A low-scarped 

 terrace is formed near the western border of the belt of wandering 

 shortly after degradation has begun (see further side of figure). After 

 six swings the river discovers a ledge (also on the further side of the 

 diagram) somewhat within the belt of wandering. Then all the terraces 

 of earlier swings lying back of this ledge will be preserved. On every 

 later westward swing the river is halted nearer and nearer to the 

 axis of the valley. Thus a flight of stepping terraces is formed in con- 

 nection with a series of defended cusps ; but on account of the absence 

 of ledges on the near side of the diagram and of the increased breadth of 

 wandering as the later stage of terracing is approached, the river de- 

 stroys all traces of the earlier terraces in the foreground, where the 



