DAVIS: RIVER TERRACES IN NEW ENGLAND. 285 
latter may bear a veneer of river drift on their surface, as in Figure 5. 
Moreover, drift terraces are in nearly all cases developed with much 
more irregularity of pattern than is the case with the terraces of other 
kinds. A single drift terrace — unless it be the highest one of a series 
— is seldom traceable many miles along a valley side; its length may 
be only a few hundred yards. Terraces of other kinds are usually much 
more persistent. 
Our drift terraces differ, furthermore, from other terraces in the place 
that they occupy in the geographical cycle. They are not products of 
normal erosion during an undisturbed still-stand of a land mass, but are 
the consequence of some relatively short-lived episode during which a 
greater or less departure is made from the normal progress of a cycle. 
. The terraces of New England occupy well-opened rock-floored valleys of 
earlier origin, and thus imply the previous attainment of maturity in the 
cycle which witnessed the development of our hills and valleys. The 
glacial period witnessed certain modifications of the preglacial valleys 
and closed with the accumulation of abundant drift in them, as well as 
with certain changes of level by which the rivers were prompted to wash 
the valley drift away. Postglacial time has allowed the rivers to enter 
well upon this task : yet, even when the task has been completed, the 
normal cycle of erosion in New England will not have advanced far 
beyond its preglacial phase ; so brief are the glacial and terrace episodes 
compared to the time required for baseleveling a region of resistant 
rocks. 
Systematically considered, river terraces may be best associated with 
the forms assumed by the waste of the land on the way to the sea. 
Flood plains and alluvial fans are representative examples of the form 
assumed by land waste while it is stopping on its way down a valley. 
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 Patterns. Before entering upon the consideration of the 
process of terracing it will be well to examine briefly the more character- 
istic elements of terrace pattern, especially as seen in horizontal plan. 
The plain or floor of a drift terrace frequently presents a rapid variation 
in width, usually terminating in points at its up and down stream ends, 
as in Figure 6. The borders are prevailingly formed of curves of greater 
or less length, but of tolerably uniform radius, concave to the stream 
and frequently uniting in cusps. When several cusps are grouped, one 
back of the other, so as to form a strong salient, they may be called a 
terrace spur. Convex borders fronting the stream occur but rarely. 
