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bulletin: museum of comparative zoology. 



curve is to remain practically unchanged, it can pass the ledge only by 

 withdrawing somewhat towards the axis of the valley ; it may thus, as 

 it were, slip by the obstacle that stands immovable in its way. The 

 stream is represented as having just slipped past the ledge in Figure 27, 

 and as having swept somewhat farther down the valley in Figure 28. 



Fig. 25. 



Fig. 26. 



All records of the first and third swing of the river are now destroyed, 

 so far as this part of the valley is concerned ; the terrace front shows a 

 high, defended, one-sweep cusp, a free two-sweep cusp with an up-stream 

 Y-stem, and a free one-sweep cusp. 



The ledge at the base of the defended cusp may come to be more or 

 less concealed by the sands that are washed down from the weathering 



Fig. 27. 



Fig. 28. 



scarp. In time it may be entirely covered, and its presence will then 

 be known only if a road cut or a boring reveals it. It is therefore quite 

 possible that some apparently free cusps are really defended cusps, with 

 their defending ledge ambushed beneath a thin cover of soil. 



Compressed Meanders and Sharp Cusps. Let it now be supposed 

 that the river in Figure 2G is unable to slip past the ledge. The front 



