l62 



MARINE DEPOSITS 



always stratified, though in cases when deposition is continued 

 for long periods without interruption, thick masses, not obviously 

 divided into layers, may be accumulated, but this is exceptional in 

 those parts of the sea where deposition is most rapid. 



i. Littoral Deposits are laid down between high and low tide 

 marks and a little beyond the low-water mark, in very shallow 

 water. These are made up of the coarsest materials, boulders, 



Fig. 63. — Basin of the Gulf of Mexico, showing the submerged margin of the 

 continental platform and the steep descent of the bottom at the 100-fathom line. 

 Vertical scale much exaggerated. (From a model by the U. S. Coast Survey.) 



coarse gravel, and sand, though mud may accumulate in holes 

 and sheltered situations, even within this zone. The principal 

 grinding action of the surf is exerted between tide marks, and the 

 undertow and tidal currents continually sweep out the finer parti- 

 cles to deeper and quieter water, leaving the coarser fragments 

 behind. Mineralogically, these coarser fragments may be of any 

 kind, depending upon the rocks of the coast and the material sup- 

 plied by neighbouring rivers, but most frequently they are quartzose, 

 which is due to the superior hardness of that mineral. 



