12 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
ment, since this line is determined by the carving action of the stream, 
while the conspicuous frontal edge of the terrace has oftentimes been 
affected by the simple processes of drainage. A characteristic feature 
of some terraces is a line of drainage at the back of the plain and at the 
foot of the rising escarpment. This line of drainage, or brook, flows 
down stream, cutting a gorge to the plain below and spreading there 
a fan-like delta deposit. 
The width of the terrace plain is variable. Commonly, terraces 
present, toward the river, scarps which here and there show a series of 
concave curves meeting in pointed cusps. 
It is clearly recognized that the curved terrace fronts have been 
sculptured by successive encroachments of a winding stream which 
at some time must have swung against the base of the rising scarp; and 
Fig. B.—Valley showing middle stage of terrace pattern. Terraces vary in 
width, have curved, concave scarps meeting in pointed cusps, and exist 
because of the discovery of defending rock ledges at lower and lower levels. 
it is believed that the stream has swung laterally at least as many times 
as there are terraces. 
Terrace scarps are steepest where the eroding stream has most 
recently done its work. In a series of stepping terraces, one would 
find the steepest slopes in the lowest and youngest, and the more 
gently and more deeply weathered slopes in the highest and oldest 
terraces. The oldest terraces not only weather to a gentler slope but, 
also, their edges are rounded off, which means that a convex slope of 
erosion is formed above and a concave slope of deposition below. 
Very excellent examples of this mav be found by a careful study of 
the terraces in our vallevs where the dissection of the highest and 
oldest terraces has been so excessive as fairly to separate them into 
individual plains severed by sharp and deeply incised valleys of 
erosion. At first sight they appear like delta sand plains with lobate 
margins. 
