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



The land surface is thus pared down to a uniform depth. If this 

 depth is greater than the former relief, the resulting cut surface will be 

 a plane or a broadly rolling surface, as described below. This condition 

 is represented by taking the upper of the two dotted lines in the figure 

 to represent the depth of valley bottoms. If the depth of paring be less 

 than that of the valley bottoms (which may then be represented by the 

 lower dotted line), the valley bottoms will escape the paring process and 

 their lower parts will continue to indent the denuded surface. The dimi- 

 nution of relief will, of course, be, equal to the constant cliff height. 



In general, if denudation cuts below the valley bottoms, the entire 

 surface of the cut terrace will be of fresh rock. If erosion fail to cut so 

 low, the valleys will be filled, and the weathered rocks of the valley 

 bottoms will thus be preserved on the lower side of the contact. 



Figure 2.— .Recession retarded. 



Under the conditions of case II the maturity of the shoreline will at 

 first be advanced, but a limit of development will be reached, at which 

 the rejuvenating effects of submergence will balance the advancement 

 in the cycle due to erosion. If the down-cutting does not extend to the 

 valley bottoms, there will be on the site of each headland a cut terrace 

 whose surface will, in the main, be a plane ; but the sites of the valleys, 

 and therefore of the bays, will show depressions in the older formation, 

 filled by sediments of the newer. If marine denudation cuts below the 

 valley bottoms, cliffs along the shoreline will be continuous, and (with 

 one important modification, given below) the old land surface is planed 

 down to a level. 



The modification necessary to the general statement that when cliffs 

 are entirely continuous along a sinking coast the cut terrace thus pro- 

 duced has a plane surface, may be stated thus : In the submergence of a 

 surface of hills and valleys the coastline can not well be straight. Low 

 cliffs recede more rapidly than high ones ; hence where the land is low 

 the shore is pushed more rapidly landward. Reentrant curves are thus 

 produced, whose possible depth is limited by several factors. In general, 



