164 



MARINE DEPOSITS 



tion cease at that point. If the rate of subsidence be very slow, 

 deposition may shoal the water and thus extend the seaward range 

 of the coarse materials, forming broad belts of them, while, if the 

 rates of deposition and sinking be nearly equal, the coarser de- 

 posits will be restricted to long and narrow bands, running paral- 

 lel with the coasts. If, on the other hand, subsidence takes place 

 at a rapid rate, the water will be deepened, the coast will retreat, 

 and where coarse materials were before gathered, only the finer 

 will now be accumulated. Thus in the same vertical line will be 

 formed rocks which indicate very different depths of water. 



2. Shallow-water Deposits. — Beyond the low-tide mark, and 

 for a distance out to sea which depends upon the slope of the 



FIG. 65. — Diagram illustrating the change of materials on the sea-bottom and 

 the dovetailed edges of sandy, clayey, and calcareous beds. Owing to the great exag- 

 geration of the vertical scale, the beds appear unduly irregular. 



bottom, the sea-bed is ordinarily covered with sand. If, as along 

 the eastern coast of the United States, the water deepens very 

 gradually and the 100-fathom line is far from the shore, sand will 

 be found 100 to 150 miles out, growing finer and finer with the 

 increasing depth. Throughout this whole belt wave action is 

 exerted on the bottom, though to a very insignificant extent in 

 the deeper parts, but the stratification is regular and uniform, and 

 the materials are widely and evenly distributed. The characteristic 

 deposit of this zone is siliceous sand, though under exceptional 

 circumstances, considerable areas of muds and clays may be 

 formed. Thus, south of Block Island is a large triangular patch 

 of clay which invades the sand area, and several large mud holes 

 occur off the entrance to New York Bay. The action of the waves 

 and shore currents transports the sand along the coast for long 



