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. 



