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 moment of its formation, the stream is withdrawn by a short-cut or a 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 15. 



Fig. 20, 



Ideal Terrace Patterns: Late Stage. 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 highdevel 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 



