220 



DYNAMICAL GEOLOGY. 



ing cliifs, wave-action makes shore-platforms, by shearing away the rocks of 

 coasts down to a horizontal surface near low-tide level, and in the process 

 cliffs are undermined and set back. These effects are produced where the 

 rocks are of moderate firmness, — where they are not too hard to yield rather 

 easily to the waves, and not so weak as to be torn up by the gentler attack 

 of low-tide movements. As the tide rises, the earlier waters quietly cover 



19L 





Rocks detached by wave-action, Mount Desert, on an old beach at a height above the sea of 220 feet. 



Shaler, '89. 



the rocks. Then the waves move in ; but the rocks below are under protec- 

 tion, and only those of the cliff or wall take the force of the blow. There 

 is hence, in such cases, a level of no wear near low-tide level. The level of 

 greatest wear is that of the stroke of the breaker at or above high tide. 



