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N. M. FENNEMAN — EFFECT OF CLIFF EROSION ON FORM 



broad horizontal surface is reached by the receding cliff. Over such a 

 surface shifting would, of course, be instantaneous if the sealevel were 

 as high as the level of the plain. When there is a cliff separating these 

 two levels the rate of migration of the shoreline is, of course, that of 

 cliff recession and no more. 



Illustrative Cases of submergence Conditions 



in general 



The above conditions of submergence are more conveniently discussed 

 by assuming certain cases and indicating them diagrammatically. No 

 one case can represent the conditions during the whole history of a sub- 

 mergence, but the entire history and the forms resulting may best be 

 described in terms of these cases. 



CASE I. SHORE RECESSION MORE RAPID THAN SHIFTING BY SUBMERGENCE 



Assume the rate of cliff recession to be constant and greater than the 

 shifting due to subsidence. In figure 1 let B A represent the original 



Figure 1. — Recession more Rapid than Shifting. 



land slope. B C is a certain assumed amount of vertical sinking. C A 

 is then the amount of horizontal shifting of the shoreline corresponding 

 to the assumed amount of sinking. D A is the actual amount of migra- 

 tion of the shoreline, due partly to cliff-cutting. 



The tendency of the waves is to cut away the higher land and reduce 

 it to a plane just as though there were no subsidence. In this case, 

 however, the plane would not be horizontal, but inclined at an angle 

 depending on the ratio of A D and D E — that is, the relative rates of 

 cliff recession and subsidence. The water's edge, which was at the outset 

 at A, advances toward E, and in the meantime occupies all positions 

 along the line A E. This line, therefore, represents the sloping surface 

 of a denuded bench having a gentler seaward inclination than that of 

 the former land surface. This result is due to the fact that, on account 



