DAVIS : RIVER TERRACES IN NEW ENGLAND. 



315 



of the curve is held fast ; the apex of the curve bends outward and cuts 

 a curved re-entrant in the terrace front next up-stream from the ledge, 



Fig. 29. 



Fig. 30. 



Fig. 31. 



as in Figure 29. The meanders further up-stream continue their advance 

 and the meander uext to the ledge is therefore compressed to a relatively 

 strong curvature, as in Figure 30. 

 The defended cusp is now sharp- 

 ened. It may come to point 

 somewhat up the valley. The 

 compressed meander cannot slip 

 by the ledge ; there is no escape 

 for the stream save by a short- 

 cut across the narrowing flood- 

 plain lobe at time of high water ; 

 thus the condition of Figure 31 

 will in due time be developed. 

 A rather sharp cusp, one of whose sides faces up the valley, will be 

 produced, and the great concave scarp adjacent to it will have an 

 abandoned channel at its base. 



Terrace Fronts near Defended Cusps. The difference between 

 the behavior of slipping and of compressed meanders may be inferred to 

 depend on the position of the obstructing ledge with regard to the apex 

 of the meander. If the ledge is discovered near the apex of the meander, 

 the stream may slip past the obstacle, as in the example first given. 

 If the ledge is encountered near the point of river inflexion — that is, 

 on the tangent between two meanders, — compression of the meander 

 up-stream from the obstacle is likely to result. 



In both these cases the defended cusp is likely to be associated with 

 several short curves and free blunt cusps for a little distance down- 

 valley ; while a rather long re-entrant curve usually joins its up-valley 



