FACTORS AND CONDITIONS OF THE PROBLEM 207 



surface nearer to a plane, and that the general tendency of shorelines is 

 to straighten themselves; hence to reduce relief. The possibility of the 

 opposite effect depends on unequal strength of the shore rocks, and can 

 occur only when wave erosion is cutting out bays in the weaker rocks 

 of a rugged coast, thereby making the coastline more irregular. Even 

 under these conditions of unequal strength of rocks, increase of relief is 

 not universal. In the following discussion this phase of marine denu- 

 dation will not be considered. 



There is one further exceptional and unusual case which will not be 

 considered in the paragraphs to follow. That is the case in which no 

 erosive work whatever is performed by the oncoming waters. This may 

 be exemplified by a preexisting land surface of such gentle seaward 

 slope as to require the construction of offshore barriers in order to steepen 

 the beach profile. But even in this case observation may sometimes 

 detect a small degree of cliff-cutting along the new shoreline before the 

 barrier appears. The usual case, and the only one needing considera- 

 tion here, is that in which submergence is attended by the paring down 

 of hilltops. 



Subsidence alone causes the shore to shift landward at a rate deter- 

 mined by the slope of the land and the rapidity of crustal movement. 

 This change of position of the shoreline will be spoken of as shifting. 

 But the actual migration of the shoreline will depend partly on this hori- 

 zontal shifting and partly on the rate of the attendant cliff-cutting. The 

 actual rate at which the cliffs recede and the shoreline migrates may be 

 greater or less than this shifting. The total movement of the shoreline, 

 regardless of causes, will be spoken of as migration. 



If the position of the water-level be newly assumed against an irregular 

 land surface, the first cliff erosion causes recession which is more rapid 

 than the mere shifting which could be caused by any possible rate of 

 subsidence. In other words, no supposition is allowable which would 

 engulf an island so promptly that the waves could not etch its descend- 

 ing slopes. This forging ahead by cliff-cutting becomes less and less 

 rapid as the cliffs become higher, the amount of detritus yielded by each 

 foot of recession becomes greater, and the disposing of the waste becomes 

 an increasingly difficult task. By continual cliff-cutting against a land 

 sloping shoreward, the cliffs will eventually become so high and their 

 rate of recession so slow that the latter will just equal the rate at which 

 the shore would shift landward by submergence alone. 



When high cliffs have once been developed it may happen, as ex- 

 plained below, that their recession over a given space will be slower 

 than mere shifting of the shoreline over the same space would have been 

 had no cliffs been cut. The extreme of this condition is found where a 



