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STUDIES FOR STUDENTS 



forces — the rivers building out by irregular delta mouths, inclosing 

 lagoons and providing new tidal marshes; the sea cutting off head- 

 lands and sweeping the material along the coast, forming spits which 

 shut off new lagoon spaces. 



The chief maintenance of the httoral belt is, however, due to 

 vertical land movements, especially subsidence of a fiat land surface, 

 but one still showing sHght relief. This may produce extensive 

 estuaries, and the waves, by throwing up barrier beaches in shallow 

 water at some distance from the land, may form a continuous series 

 of lagoons and salt marshes, as is illustrated by the present condi- 

 tion of the seaward margin of the coastal plain of the eastern United 

 States from Long Island southward. The questions of immediate 

 importance in the present connection are those of the width and 

 areal extent of the Httoral zone and possible fluctuations in impor- 

 tance in past times, owing to the prevalence of conditions not now 

 operative, such as absence of tides in ancient protected seas, or 

 hypothetically greater oceanic tides due to a possibly greater near- 

 ness of the moon. It is first necessary to collect the facts for the 

 argument, by observing the various shore conditions as they exist 

 today. Arranging these with respect to the tidal range, a representa- 

 tive set is as follows, the information being taken largely from the 

 charts of the United States Coast and Geodetic Survey: 



OBSERVED RELATION OF TIDES TO THE LITTORAL ZONE 



TIDE I TO I -5 FEET (eXAM. MOBILE BAY AND MISSISSIPPI DELTA, GULF OF MEXICO; 

 STORM TIDES THE ONLY IMPORTANT ONES) 



Mud-flats at mean low tide, o.i to 0.25 mile wide in protected places. 



Salt marshes. — Frequently absent. Around Mobile Bay a few up to 2 miles 

 wide. On the delta of the Mississippi (General Chart No. 19) they average 27 

 miles in width, cut through by the fresh-water channels and showing a poorly 

 developed system of channels for tidal drainage. 



TIDE 2.5 FEET (eXAM. GARDINER' S ISLAND AND OYSTER PONDS ) 



Mud-flats few in number. At mean low tide 0.33 to 0.66 mile wide in 

 protected places. 



Salt marshes average 0.25 mile wide. 



TIDE 4 TO 6 FEET (VICINITY OF NEW YORK AND NEW HAVEN) 



Mud-flats from 0.16 to 0.33 mile wide, of hmited development. 



Salt marshes in protected inlets behind barrier beaches 0.5 to i .0 mile wide. 



