THE WORK OF THE OCEAN 



299 



developed as illustrated by Fig. 244, where ab represents the direc- 

 tion of the incoming wave, be the direction of the shore (or. littoral) 

 current, and bd the direction of the 

 undertow. Where they strike the 

 borders of land, the wind-waves, 

 therefore, generate two other move- 

 ments, the undertow and the littoral 

 current. Any particle of water near 

 shore may be affected by any two 

 or by all three of these movements 

 at the same moment. The effect of 

 littoral current and undertow is to 

 give a particle of water on which 

 both are working a direction between 

 the two, as be. The effect of other 

 combinations can be readily inferred. 

 These various combinations are of 

 consequence in the transportation of 

 debris. 



Fig. 244. Diagram showing 

 relative directions of 



wave, undertow, 

 shore-current. 



and 



Work of the Waves 



The general effects on shores of the waves and the other move- 

 ments to which they give rise are (1) the wear of the shores; (2) 

 the transportation of the products of wear; and (3) the deposition 

 of the transported materials. 



Erosion. In the dash of the waves against the shore, the wear 

 is effected chiefly by the impact of the water and of the debris which 

 the water carries, but lesser results are accomplished in other ways. 



When the land at the margin of the water consists of uncon- 

 solidated material, or of fragment al material but slightly cemented, 

 the dash of the water is sufficient to displace or erode it. If weak 

 rock is associated with resistant rock within the zone of wave-work, 

 the removal of the former may lead to the disruption and fall of the 

 latter, especially when weak rock is washed out from beneath strong. 

 The impact of the water is competent also to break up and remove 

 rock which was once resistant, but which has been weakened by 

 weathering. Rock affected by joints is likewise attacked with 



