2l6 



PHYSICAL GEOLOGY 



Fig. 207. — An ancient plain of marine denudation, with the former islands stand- 

 ing above the plain as hills, is shown in the diagram. The accordant level of the hills 

 indicates an ancient peneplain. (Modified after S. W. Cushing.) 



The New England Marine Plain. — As has been seen (p. 215), the presence of cliffs 

 at the shore line shows that locally marine erosion may become more effective than 

 subaerial (the work of weather, wind, and streams). The sea, however, can work only 

 against the shore, while the effect of stream erosion and of the weather is to reduce the 

 whole surface of the land (p. 114). The work of the sea, though powerful, is limited 

 to the shore line, and plains produced by marine erosion are of small extent compared 



with the extensive plains carved by 

 the subaerial agencies. It has been 

 suggested, however, that at certain 

 times, planation by the sea may 

 become more effective than usual 

 over much broader areas. After a 

 region has been worn down to such 

 an extent that the soft beds of rock 

 are reduced to base level, leaving the 

 harder as hills, subaerial erosion 

 works very slowly, and the amount 

 of sediment carried to the sea by the 

 streams is so small that the littoral 

 currents expend little energy in 

 moving it. Under such conditions 

 the surface of the land is lowered 

 very slowly, while marine erosion is 

 relatively much more effective. If 

 such a stage is combined with some 

 submergence the sea has an added 

 advantage, and its action is concen- 

 trated on the residual hills and 

 uplands remaining from subaerial 

 erosion. The land bordering the 

 Atlantic coast of North America is 

 thought by some to have been under 

 conditions such as these during a long period of time (Cretaceous and Tertiary). 1 

 The uplands of New England and New Jersey and the resistant ridges of the Appa- 



I [G. 208. — An ancient shore line. The rim 

 of the cave is 65 feet high. East coast of 

 India. (Photo. S. W. Cushing.) 



Barrell, J., — Bull. Gcol. Soc. Am., Vol. 24, No. 4, 1913, pp. 688-696. 



