212 



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



of denudation will have a steeper slope than that of the land. This is 

 impossible unless an initial cliff be assumed. This case therefore be- 

 comes important only as a later stage of a process begun under other 

 conditions. 



In the diagram, B C represent a cliff whose recession has become less 

 than the simple shifting of the sinking shore would be without a cliff. 

 The recession is equal to CD during a vertical subsidence equal to D E. 

 During the same subsidence the shifting of the shore with no cliffs 

 would have carried the water's edge from A to E through a horizontal 

 distance equal to DA. 



Here, again, cliff-cutting is assumed to continue at a uniform rate; 

 hence the cut slope is a plane, and the cliff height has been reduced to 

 zero at E. In actual occurrence the rate is accelerated as the height is 



Figure 3.— Recession less Rapid than Shifting^ 



reduced, and the line CE becomes a curve, convex upward and never 

 actually intersecting the line A B so long as any land remains above 

 water. Assuming the conditions of this case after cliffs have once been 

 developed, the surface of the cut terrace will rise landward, intersecting 

 first the deeper valleys and then the shallower, resulting in the greater 

 separation and subdivision of headlands and the isolation of islands. 

 The rejuvenating effects of sinking are more rapid than advancement 

 in the cycle of erosion, and the features of maturity give way again to 

 those of youth. 



GENERALIZA TION 



Abandoning straight lines, but using the principles stated above, a 

 truer picture of the forms actually resulting from submergence may be 

 obtained by assuming land forms of familiar type. Assume, for exam- 

 ple, a horizontal land, but having a seaward slope near the shore. Its 

 surface may have something of the compound curve shown in figure 4. 



