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BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



side. The short curves and blunt cusps may be blended so as to pro- 

 duce a convex terrace front for a little distance down-valley from the 

 ledge ; and this convex front will be separated from the long concave 

 re-entrant by a more or less pronounced angle at the defended cusp. 

 The reason of this may be easily understood from Figure 32. The ledge 

 was here first discovered by a down-sweeping meander, of whose first 

 work only the short scarp A remains. The meander was then com- 

 pressed so as to scour out a large concave re-entrant, and was with- 

 drawn from the channel, B, at its base by a short-cut (outside of the 

 diagram). A meander on the new course of the stream", swinging 



Fig. 32. 



westward, trimmed off the terrace front in successive lines, C, D, E ; 

 but as this meander had no definite relation to the ledge, and as the 

 general sweeping of its curve was down-valley, it did not trim the ter- 

 race front with any special regard to the down-valley side of the de- 

 fending ledge. In brief, the tendency of a stream to sweep its meanders 

 down-valley commonly l'esults in trimming off the terrace front close to 

 the up-valley side of a defending ledge, but it is only by chance that the 

 terrace is worn away close on the down-valley side of the ledge. The 

 pattern hei-e deduced may be matched in many actual examples. 



Defended Cusps : Later Stage. After a ledge has once been dis- 

 covered by the swinging river, there is much probability that the for- 

 ward reacli of its under slope will present an obstruction to the stream 

 every time it swings again towards the valley side. For example, let 



