WAVES AND TIDES. 



35 



Fig. 28. 



h the waves strike the edges, while from a to c they strike the faces of 

 the same rocky strata. The arrow shows the direction of the dip of the 

 strata. The difference in the form of the coast-line is seen at a glance. 



Waves, cutting ever at the shore-line only, act like an horizontal saw. 

 The receding shore-cliff, therefore, leaves behind it an ever-increasing 

 subaqueous platform which marks the amount of recession. This is 

 shown in the section (Fig. 28), in which s is the present shore-line, I the 

 water-level, a b the platform, s' the original shore-line, and s' b c the 

 original slope of bottom. The recession of the shore-line and the for- 

 mation of the shore platform have been accurately observed in Lake 

 Michigan (Andrews). 

 Level platforms termi- 

 nated by cliffs, therefore, 

 when found inland, 

 sometimes indicate the 

 position of old shore- 

 lines. 



Tides.— The tide is 

 a wave of immense base, and three or four feet in height in the open 

 ocean, produced by the attractive force of the moon and sun on the 

 waters of the ocean. The velocity of this wave is very great, since it 

 travels around the earth in twenty-four hours. In the open ocean it 

 produces very little current, only a slow transfer of the water back and 

 forth, too slow to produce any geological effect;* but in shallow water, 

 where the progress of the wave is impeded, it piles up in some cases 

 forty to fifty feet in height, and gives rise to currents of great velocity 

 and immense erosive power. By this means bays and harbors are 

 formed, and straits and channels are scoured out and deepened. Tides 

 also act an important part in assisting the action of waves upon the 

 whole coast-line. The action of waves on exposed cliffs quickly forms 

 accumulations of debris at their base, composed of sand, mud, shingle, 

 or rocky fragments (Fig. 26), which receive first and greatly diminish 

 the shock of the waves upon the cliff. The incessant beating of the 

 waves upon this debris reduces it to a finer and finer condition, and 

 the retreating waves bear much of it seaward ; so that, even without 

 the assistance of any other agent, the protection is incomplete, and the 

 erosion therefore progresses. But if strong tidal currents run along the 

 coast, these effectually remove such debris and leave the cliff exposed 

 to the direct action of the waves. 



Examples of the Action of Waves and Tides. — The coasts of the 

 United States show many examples of the erosive action of waves and 

 tides. The form of the whole New England coast is largely determined 



* Herschel's Physical Geography, p. 64. 



