342 



GEOLOGY, 



of the undertow diminishes rapidly from the breaker-hne outward as 

 the depth of the water increases. 



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

 developed as illustrated by Fig. 300, where ab represents the direction 



^ of the incoming wave, he the direction of the 



^A littoral current, and bd the direction of the 



^VM rr undertow. Where they strike the borders 



of land, the wind-waves, therefore, generate 

 two other movements, the undertow and the 

 httoral 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 he. 

 The effect of other combinations can be 

 Fig. 300.— Diagram showing readily inferred. These various combina- 

 relative directions of wave, tions are of consequence in the transporta- 



undertow, and shore-cur- . . r i ^u • 



j,g^+ tion 01 debris. 



WORK OF THE WAVES. 



Erosion. 



The general effects of the waves and the other movements to which 

 they give rise along shores are (1) the wear of the shores; (2) the trans- 

 portation for greater or less distances of the products of wear; and (3) 

 the deposition of the transported materials. 



By waves and undertow. — In the dash of the waves against the 

 shore, the chief wear is effected by the impact of the water and of the 

 debris which the water carries. Lesser results are accompHshed in 

 other ways. 



When the land at the margin of the water consists of unconsohdated 

 material, or of fragmental material but slightly cemented, the impact 

 of the water is sufficient to displace or erode it. If weak rock be asso- 

 ciated 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 the strong. The impact 



