OCEANS AND LAKES 



237 



estuaries, and in narrow passages between islands, tidal cur- 

 rents may be of great strength, and sometimes sweep quan- 

 tities of sediment back and forth and erode the beds and 

 sides of their channels. Tides aid the work of wind waves 



FIGS. 253, 254. High tide and low tide on the coast of Maine at North 

 Haven. The rocks exposed at low tide but underwater at high tide are 

 heavily covered with seaweed. Such vegetation often helps to protect 

 rocky coasts against wave erosion. (Bailey Willis.) 



by lifting and lowering them, and so increasing the width of 

 their zone of attack (Figs. 253 and 254). 



THE SHORES OF THE OCEAN 



The shores of the ocean are zones of great activity. Here 

 is the meeting place of land and air and sea. The principal 

 coast-line features and offshore deposits are discussed in 



