GEOLOGICAL IMPORTANCE OF SEDIMENTATION 433 



water, and can attain any degree of permanence only where they 

 face a gently shelving land mass. 



The waves not only tend to unify and extend the shallow sub- 

 marine platform, but tend to cut away its highest portions to a cer- 

 tain depth, dependent upon the power of the waves. Thus, by 

 taking nautical charts it is observed that over any district where there 

 is open water the bottom maintains a certain depth close up to the 

 off-shore beach. Facing the open oceans this is usually from 30 to 

 40 feet, but in more protected places, such as Long Island Sound, it 

 may be but 6 to 12 feet. Thus, up to the line of surf the submarine 

 platform extends without any confusion with the littoral zone. 



EXTENT AND CHARACTER OF THE LITTORAL ZONE 



The littoral region, as has been shown, is rather sharply delimited 

 from the marine by the trimming action of the sea, marked along a 

 shelving shore by the line of barrier beaches. Where a bold land 

 meets the sea, it is merely the lowest exposed portion of the sea-cliff. 

 On the landward side, however, the littoral is not so regularly defined, 

 but consists of irregular tidal lagoons consisting of three portions; 

 the mud-flats, exposed at low tide, the salt marshes flooded only at 

 high tide, and a rather abrupt transitional mud-slope between them. 

 Both the mud-flats and tidal marshes are cut through by tidal 

 channels, those on the marsh being characterized by meanders. 



The littoral finds its greatest development in estuaries or where 

 the land meets the sea in the form of a plain, either a base-plain of 

 erosion, or a river plain of aggradation. The littoral does not show 

 a tendency, like the two previous regions, to extend its limits, since 

 the flood-tide tends to leave sediment upon the tidal marshes, building 

 them up to the extreme tidal limit ; and, on the other hand, the tidal 

 scour of ebb-tide tends to remove sediment to the open sea. On 

 the contrary, the forces of both land and sea tend to fill up and 

 obliterate the littoral. On the one hand, the river deposits and the 

 wash from the land creep out over the tidal flats, and, on the other, 

 the sea, by wearing the beach deposits smaller and by removing the 

 finer shore material outward to a greater depth, tends to push its 

 beaches farther inland. 



The littoral zone is partly maintained by the contest of the two 



