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PHYSICAL GEOLOGY 



feet, and to a distance of ioo or more miles from shore, that is 

 to the outer edge of the continental shelf (p. 195) ; and cover an 

 area of about 10,000,000 square miles. They pass almost im- 

 perceptibly on the one hand into the coarser littoral deposits, and 



on the other into the fine 

 deposits of the deep sea. 

 They are similar in character 

 to the littoral deposits, but 

 are finer. Shoal-water de- 

 posits, in common with those 

 of the littoral zone, are often 

 ripple-marked and preserve 

 the tracks of such animals as 

 worms and shellfish. Sun 

 cracks and the tracks of land 

 animals are absent. Cross- 

 bedding (p. 235) is often well 

 developed in the sand near 

 shore where horizontal strati- 

 fication was interfered with 

 by currents (Fig. 228). In 

 general it may be said that 

 these sediments are coarsest 

 near shore and become pro- 

 gressively finer away from it. The reason for this is evident, as the 

 following example shows. When a river enters the sea the force 

 of its current is immediately checked, and the coarser sediment 

 which it carries is deposited, sand is swept out to a greater distance 

 and spread over a wider area, while the fine clay travels still farther 

 and covers a much larger tract of the sea bottom. The sediments 

 moved by the waves and ocean currents are similarly afFected ; 

 shingle and gravel accumulate close to shore, sand is carried farther 

 out, and clay is most widely spread. 



Limestone. — Beyond the reach of the clay lime ooze accumulates. 

 This statement should not be taken to mean that limestone may not 

 accumulate near shore. Near coral reefs lime carbonate is accumu- 

 lating to-day, and there is much reason to believe that during certain 

 periods of the past, limestone of great thickness was deposited near 

 shores which bordered lands so low that the streams were able to 

 bring to the sea little besides the lime carbonate which they carried 



Fig. 231. — Mud or sun cracks. 

 (U. S. Geol. Surv.) 



