RECESSION MORE RAPID THAN SHIFTING 209 



of active cliff-cutting, the migration of the shoreline has been more rapid 

 than shifting due to subsidence alone would have been. 



The diagram represents only an initial, broad, unbroken slope of the 

 land. If this slope be diversified by valleys and ridges, the latter will 

 be truncated to the level of A E. The dotted line parallel to B A in the 

 diagram, may be taken to represent the depth of the valley bottoms. As 

 the cliff recedes it necessarily increases in height. The new-made sea 

 bottom approaches continually nearer to the valley bottoms which at 

 first were beneath its level in bays. Xhe bays become smaller and fewer, 

 the cliff more continuous ; the maturity of the shore is advancing. 

 Finally, with the extension of the plane inland, its level may fall below 

 that of the deepest valleys, the cliff will be continuous, and the subaque- 

 ous cut terrace will have no depressions to be filled with the sediments 

 of the newer formation. From this line landward the surface of uncon- 

 formable contact between the older and newer formations will be approxi- 

 mately a plane regardless of initial topography. 



This stage, in which denudation cuts below the valley bottoms, may 

 or may not be reached. Cliff recession is accomplished against accumu- 

 lating difficulties. Not only does the increasing height of the cliff yield 

 an increasing measure of detritus for each foot of recession, but each cliff 

 is becoming longer ; the bays in which the shore drift was formerly stored 

 in the form of spits, bars, and wave-built terraces, become fewer and 

 smaller, and the material won from the cliffs must be disposed of by the 

 slower process of dragging offshore. At any point of the line A E the 

 waters may prove unequal to the task of cutting a higher cliff. They 

 must then be content to push the shore landward at the same rate at 

 which subsidence would shift it without cliff-cutting. 



CASE II. RECESSION PROGRESSIVEL Y RET A RDED UNTIL ITS RA TE EQ UALS THA T 



OF SHIFTING 



Assume as a second case that the rate of cliff recession is progressively 

 retarded to a minimum. If the initial ratios be taken as in case I — that 

 is, the migration of the shoreline greater than the shifting, as defined 

 above — then the surface of marine denudation will be begun, as before 

 with a lower slope than that of the land surface. However, as the rate 

 of cliff recession is diminished'it will approach that of the shifting, which 

 it will finally equal. The surface of denudation in the meantime curves 

 upward, as represented by the line A E in the diagram. When the rate 

 of recession has become equal to that of shifting, the slope of the plane 

 of marine denudation will equal that of the land surface. The two will 

 be parallel and the receding cliff will have a constant height. This 

 height will be such as to allow recession at the same rate at which the 

 shoreline would move landward by submergence alone. 



