DAVIS : RIVER TERRACES IN NEW ENGLAND. 



307 



that are produced by the chance intersection of the curved terrace 

 fronts. In the two-sweep or two-swing cusps, two patterns may be 

 produced, according as the higher scarp is on the up-valley or down- 

 valley side of the cusp, as in Figures 12 and 13. The higher scarp is 

 evidently the younger of the two that unite in the cusp ; it is con- 

 tinued in direction, but with less height beyond the point of intersection. 

 From the appearance of a letter Y in the shaded scarps of the diagrams 

 forms of this kind may be called two-swing (or two-sweep) cusps with 

 an up-stream or a down-stream Y-stem, as the case may be. All the 

 elements of such cusps are variable. The location of the cusp is at the 

 chance intersection of two lines that have no particular relation to each 

 other. The angle of the Y may vary through a large range. The 

 heights of the scarps have no definite relation, except that the higher 

 single scarp must be the sum of the other two. 



Fig. 12 



A series of one-sweep cusps may occur with some regularity along a 

 valley side ; but two-sweep and two-swing cusps cannot be expected to 

 show so definite an arrangement, unless under the control of something 

 more systematic than the action of the wandering stream. In Figure 

 11, for example, two series of one-sweep cusps in the middle of the dia- 

 gram stand in normal down-the-valley order; but all the two-swing 

 cusps are located indifferently to one another. So on the left side of 

 Figure G ; but on the right side of that figure, three one-sweep cusps on 

 successive terraces are placed in line, one forward of the other, as if in 

 some way systematically related or subject to some common control. 



In view of these various ideal combinations, it is evidently desirable 

 to analyze the special configurations that may be due to river action 

 alone, in order to detect more surely the patterns that must be referred 

 to some other cause. 



A later-made one-sweep cusp may occasionally chance to stand in 

 front of an earlier-made one-sweep cusp, as in the further part of Fig- 



