298 



GEOLOGY 



greater depths, for ripple-marks, which indicate agitation of the 

 water, are said to have been found at depths of 100 fathoms. 



When a wave approaches a shelving shore, its habit is changed. 

 The velocity of the undulation is diminished, while the velocity 

 of the advancing particle of water in the crest is increased; the 

 wave-length, measured from trough to trough, is diminished, ami 

 the wave-height is increased; the crest becomes acute, with the 



Fig. 243. Shore wave breaking on east wall of Hastings. (From Wheeler's 

 The Sea Coast; by permission of Longmans, Green and Company.) 



front steeper than the back, and these changes culminate in the 

 breaking of the crest, when the undulation proper ceases. Waves 

 of a given height break in about the same depth of water, and the 

 line along which incoming waves break is the line of breakers. The 

 line of breakers is in deepest water and farthest from shore when the 

 waves are strong. The return of the water thrown forward in the 

 crests of waves is accomplished by a current along the bottom 

 called the undertow. The undertow is sensibly normal to the coaM 

 when uninfluenced by oblique waves, and is efficient in removing 

 the products of erosion. 



When waves advance on the shore obliquely, a shore-current is 



