DAVIS: RIVER TERRACES IN NEW ENGLAND. 287 



presents a succession of curved re-entrants, separated by salients of 

 greater or less acuteness. It is well known that the curved terrace 

 fronts have been carved by the successive encroachments of a curved 

 stream which once swung against their base, and that the stream has 

 swung laterally at least as many times as there are terraces ; but the 

 behavior of the swinging stream has seldom been traced in detail. 



Although the plain and the descending scarp at its front are usually 

 taken together as bounding a terrace, these two surfaces are not ge- 

 netically connected in river terraces as they are in constructional lake- 

 shore or delta terraces. River terraces being of destructional origin, it 

 is the ascending scarp at the back of a terrace that should be asso- 

 ciated with the plain beneath and in front of it. The line along the 

 re-entrant edge between the plain and the ascending slope at its back is 

 the most significant of all terrace lines. The front line of a terrace 

 plain is of less significance, for it is determined merely by the slipping of 

 the sands and clays down to the line of the undercut scarp at the back 

 of the next lower terrace ; the front line of a terrace plaiu is therefore 

 of value only in so far as it represents the back line of the next terrace 

 beneath. 



Terrace scarps are steepest where the cutting stream has most re- 

 cently swung against their base. In a series of stepping terraces, the 

 youngest and steepest scarps are at the bottom of the flight ; but when 

 all the terraces of intermediate levels are destroyed by a chance lateral 

 swing of the stream so that it undercuts even the highest terrace plain, 

 then the whole descent from highest to lowest level may be fresh-cut 

 with sharp edges at top and bottom. In older terraces, the scarps 

 weather to a gentler slope, and the edges are rounded off. A convex 

 slope of erosion is thus formed above and a concave slope of deposition 

 below. The older the terrace, the greater the part of its front is occupied 

 by rounded slopes and the gentler is the slope of the shortened tangent 

 between them. At the same time, the salients or cusps between the re- 

 entrants of the scarps as seen in plan lose their original sharpness of 

 definition and become blunt and dulled. There has been no attempt to 

 show details of this kind in the accompanying diagrams. 



Gulches are often worn in terrace fronts by wet-weather streams, and 

 fans are spread on the terrace plain below. The abandoned stream 

 channels at the back border of a plain are usually taken as guides for 

 surface drainage, whose gathered waters dissect the plain where it is 

 cut off by the next lower terrace. A rather systematic drainage pat- 

 tern is thus developed, as in Figure 6. 



