ACTION OF THE TIDES ON SAND BEACHES 6 



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by the action of the tidal currents, which are the principal 

 agents serving to distribute sediments next the shores. The 

 action of the tide is in brief as follows : In tlie profounder 

 depths of the ocean, say where it is three miles deep, the 

 water in a tidal swing moves to and fro for only a few hun- 

 dred feet of distance twice each day. This rate of movement 

 is so slow that it does not disturb the finest mud. As the 

 tidal wave moves in toward the shallower parts of the ocean- 

 floor, the swing of the sea increases with every stage in the 

 decrease of the depth, until on the higher part of the conti- 

 nental shelf the current which it creates becomes strong 

 enough to move the grains of sand to and fro with each 

 oscillation which the heavenly bodies communicate to the 

 water. In the shoals next the shore the movement is often 

 strong enough to roll even the pebbles about. 



In order to conceive the effects of this tidal movement, 

 the student should imagine a billiard-table slightly tilted at 

 one end, and a number of balls pushed with the same amount 

 of force up and down the slope. He will easily understand 

 that the balls will, when thus pushed up and down the table 

 with equal impetus in each direction, gradually work toward 

 its lower end, and this for the reason that the force of gravita- 

 tion acts ever to diminish their upward movement and to 

 increase their downward journey. The seaward slope of the 

 continental shelf may then in the mind's eye be given the 

 place of the billiard-table, the grains of sand that of the balls, 

 and in lieu of the force applied by the hand we have the alter- 

 nating push of the tide. There, age by age, the swinging of 

 the water in the tidal flow and ebb, each movement acting 

 with equal force, gradualh', with the daily journe)'s, works the 

 particles of sand from the shore where they are formed, out 



