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 plain 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. 
