EFFECTS OF CHANGES OF LEVEL. 45 



New Jersey, where the subsidence is at the rate of perhaps 2 feet in a cen- 

 tury, the formation of the usual sand barrier beaches is prevented for a 

 considerable section, with the result that the sea, save for the interference 

 of man, works back into the cliffs at the rate of several feet in a } r ear. 



While a process of subsidence is in general favorable to marine erosion, 

 that of elevation is probably yet more advantageous to the wave and cur- 

 rent work ; and this for the following reasons. When the land sinks, the 

 debris due to the surf remains in the possession of the sea and may be 

 used to build barrier beaches at a higher level. Off the coast of North 

 Carolina, where there is also a subsidence movement, because the amount 

 of sand is large, the beaches are still effective walls against the sea. 

 When the land rises, however, the beach material is constantly left behind 

 in the elevated coast lines, and at each successive zone of attack the sea 

 assails an unmasked shore. At present we appear to be in a period where 

 the land oscillations are relatively very slight; we therefore are in a posi- 

 tion where we would naturally underestimate the true measure of marine 

 erosion. Still, taken in a large way, we can easily see that the coastal 

 erosion is by far the most effective at the times, which we know to be fre- 

 quent, when the shore is moving upward or downward. This shoreward- 

 sloping bench may be taken as the result of the two main varieties of land 

 wearing— that due to the natural work of the rainfall, and that due mainly to 

 the stroke of the waves as they break upon the coast. In the equation which 

 determines the slope of the coastal bench, we have to reckon the effect 

 of many agents and conditions. Among these, the successive changes of 

 the base-level— i. e., the plane of the sea — are obviously of great importance. 

 As the surface of the bench gains in height, the capacity of the marine 

 agents become relatively diminished, for the reason that the marine cliff 

 grows higher and the waves have more deportation to effect for each unit 

 of the extension of the scarf into the land. On the other hand, with the 

 gain in height there comes a proportional gain in the wearing power of the 

 rain water, the capacity of which to do wearing work is directly related to 

 the height above the sea at which it comes upon the land. 



Although the conditions which are now found on the Atlantic coast of 

 the country are clearlv less favorable to erosion than the average, it is evi- 

 dent to the attentive observer that the amount of marine erosion which is 

 now done along the coast from Cape Hatteras to Canada equals if it 



