THE OCEAN AND ITS WORK 



237 



deposits of this origin. Such impressions cannot be retained on a beach 

 unless deposits are accumulating on it ; otherwise the record of one day 

 would be obliterated by the tide or waves of the next. The presence 

 of these characteristics of littoral deposits affords evidence that certain 

 ancient rocks were deposited in the littoral belt. 





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Fig. 230. — Rill marks on a modern beach. They 

 resemble, and in ancient beds have sometimes been mis- 

 taken for, seaweed impressions. (U. S. Geol. Surv.) 



The thickness to which littoral deposits accumulate depends upon 

 whether the coast is stationary or is sinking. If the subsidence is 

 slow and of long duration, the deposit may accumulate to a great 

 thickness ; hundreds and even thousands of feet of sediments having 

 been deposited in this way in the past. 



Shoal-water Deposits 



Extent and Character of Deposits. — Shoal-water deposits extend 

 from the outer face of the littoral deposits to a depth of about 600 



