FISHER: RIVER TERRACES. 
11 
sea, and indicate an interruption in the cycle of river development. 
The rock valleys represent the work accomplished, before the ice age, 
by the mature rivers in sculpturing our hills and valleys. 
During the glacial period, the valleys became filled with abundant 
drift material, and the subsequent changes of level in the earth’s sur¬ 
face forced the rivers to erode, stratify, and carve the unconsolidated 
material into the form of terraces. Postglacial time has allowed the 
rivers to swing widely in their valleys and thus remove more and more 
of the unconsolidated material, a task which is as yet very incomplete 
because of the rock ledges, which have served as barriers, preventing 
erosion and so protecting the higher alluvial plains. Should the rivers 
succeed in removing all the valley drift,— or in cutting down to the 
floor of the preglacial rock valleys,— they would not have advanced 
beyond the preglacial stage of their cycle of development. Davis (’02, 
p. 285) has clearly shown that “terraces are examples of the forms 
assumed by waste that still remains in its stopping-place after part 
of its volume has been swept forward again.” 
Terrace 'pattern .— A river terrace is a plain terminated backward 
by an escarpment which rises to the terrace above, and forward by 
Fig. A. — Typical section of a terraced river valley in New England. Shows a 
broad rock valley, aggraded with drift and partially degraded by a swinging 
river checked in its lateral cutting here and there by the discovery of rock 
ledges. 
an escarpment which falls to the terrace below. The surface of a 
terrace is rarely level, but slopes gently toward the sea, since it has 
been deposited by the gradually falling river. Further, the terrace 
floor slopes toward its river bed as an even or as an undulating surface. 
Occasionally the plain rises toward its river on account of accumula¬ 
tions from heavy floods. 
The most significant line of the terrace is not the conspicuous 
front edge where the plain drops by an escarpment to a lower terrace, 
but rather, the line to the back and at the base of the rising escarp- 
