DAVIS: RIVER TERRACES IN NEW ENGLAND. 309 
undercutting, A will be pushed into coincidence with B, thus forming a 
three-swing cusp. But it is very improbable that this temporary stage 
will be preserved. The undercutting will continue and the tempo- 
rary three-swing cusp will then be divided into two two-swing cusps, C 
and D. The three-swing cusp can be preserved only when, just at the 
moment of its formation, the stream is withdrawn by a short-cut or a 
cut-off, and such a coincidence must be of very rare occurrence. With- 
drawals of the stream may, however, happen likely enough before or after 
the momentary stage of the three-swing cusp ; and the various patterns 
thus producible are indicated by the full and broken lines in Figures 18 
to 21. The eight possible cases of this kind result from different com- 
binations of the up-valley or down-valley half of a meander with two- 
swing cusps of up-stream or down-stream Y-stems. Evidently, then, 
no combination of unguided sweeping and swinging meanders will pro- 
duce an orderly grouping of cusps such as is shown in Figure 19. 
IpEaL Terrace Patterns: Late Stace. When the causes that de- 
termine the degradation of a valley floor weaken and disappear, the 
stream will'repeatedly swing to and fro on about the same plane. Even 
the basal terraces of a series may then be almost completely swept 
away by the wandering river, as in Figure 22, and the whole descent 
from the high-level terrace to the existing floodplain may be, for con- 
siderable distances along the valley side, united in a single strong escarp- 
ment. The conditions under which this result may be brought about 
are: first, the attainment of nearly fixed values of volume and load, 
such as might be reached when a glacial climate had given way to a 
milder climate and the latter had become well established ; second, the 
cessation of any slow uplift by which degradation had been initiated 
or aided ; third, superposition of the stream on a strong rock sill on which 
corrasion is very slow. Under these conditions, a stream would almost 
cease to degrade its channel, and would then devote practically all its 
