318 GEOLOGY 



kilometers), so that the zone of littoral deposits, though narrow, 

 covers a very considerable area. 



Mechanical deposits in shallow water beyond the littoral zone. 

 These deposits are made between the littoral zone and the 100- 

 fathom line, and cover an area of about 10,000,000 square miles. 1 

 Their composition is much the same as that of the littoral deposits 

 with which they are continuous, except that they are finer. At 

 their lower limit they pass insensibly into the fine deposits of the 

 deep sea. Coarse materials, such as gravel and sand, prevail, 

 though in depressions and inclosed basins, and out toward the 

 oceanward edge of the zone, muddy deposits are found. Some 

 of the deposits are composed wholly of inorganic debris, but organic 



Fig. 261. Diagram showing the interwedging of gravel, sand, and mud beds. 



remains are freely mingled with others. The mechanical effects 

 of tides, currents, and waves are everywhere present, but become 

 less and less well marked as the 100-fathom line is approached. 

 The forms of vegetable and animal life are numerous, though the 

 former decrease as depths which make the sunlight feeble are 

 approached. 



As a rule, no definite line marks the seaward terminus of the 

 coarse detritus, since coarse material is carried farther out when 

 the waves run high (and the undertow is strong) than when they 

 are feeble. In calm weather, therefore, fine sediment may be 

 deposited where coarse had been laid down in the preceding storm, 

 only to be covered in turn by other deposits of a different character. 

 Thus gravel grades off into sand, with more or less overlapping or 

 interwedging, and sand grades off into silt in the same way. This 

 is diagrammatically illustrated by Fig. 261. 



Since coarse deposits may extend far out from land whore the 

 waves are strong and the water shallow, and since the /one of 



1 Murray, loc. cit., pp. 187-188. 



