200 



PHYSICAL GEOLOGY 



general, the mechanical action of the waves is not perceptible at 

 depths greater than 600 feet. This last estimate is based upon the 

 occurrence of ripple marks to be found upon the sand of the ocean 

 bottom. 



Storm waves sometimes travel great distances, even thousands of 

 miles, preserving their length and velocity, but diminishing in height 

 until they become gentle swells. 



The Breaking of Waves. — As a wave nears a shelving shore its 

 length is decreased and its height increased. The breaking of the 



wave is the result of friction with the 

 bottom, which retards the lower part, 

 while the crest, continuing with its 

 previous speed, finds itself without 

 support and " breaks." This tumbling 

 crest is called a breaker or roller. 

 Since waves of the same height break 

 in about the same depth of water, a 

 line of breakers is formed. If the 

 ocean bottom descends gently, the 

 water of the breakers rushes upon the 

 shore, and gravity then draws it back 

 down the beach and along the bottom 

 beneath the incoming wave as the un- 

 dertow. On pebbly (shingle) beaches 

 the grinding of the pebbles as they 

 are moved forward by the waves and 

 carried back by the undertow can 

 be heard, even when the waves are 

 small. 

 When waves strike a coast obliquely, a shore current (p. 217) is 

 produced (Fig. 186), and on coasts where the prevailing direction of 

 the storms is fairly constant, the importance of currents of this origin 

 in transporting sand and gravel is very great. 



Force of Storm Waves. — The force of waves varies with their 

 height, but it is difficult to reduce the force of impact with which a 

 breaking wave strikes a cliff to an exact mathematical calculation. 

 Their strength is, however, influenced by the force of the wind which 

 generates them, by the depth of the water over which they have 

 moved, and by the distance which they have traveled. Experiments 

 at Cherbourg, France, showed that the force of storm waves on that 



Fig. 186. — Diagram showing the 

 directions of the various currents pro- 

 duced by a wave moving in the direc- 

 tion AB, a shore current BE, and 

 undertow BC, and a reflected wave 

 BD. 



