34: 



AQUEOUS AGENCIES. 



Section 2. — Ocean. 



Waves and Tides. 



Waves. — Waves produce no current, and therefore no geological 

 effect in deep water. The erosive effect of this agent is almost entirely 

 confined to the coast-line, but at this point is incessant and powerful. 

 The average force of waves on the west coast of Scotland for the sum- 

 mer months is estimated by Stevenson at 611 pounds per square foot, 

 and for the winter months at 2,086 pounds per square foot* In violent 

 storms the force is estimated at 6,000 pounds per square foot,f and 

 fragments of rock of many hundred tons' weight are often hurled to a 

 considerable distance on the land. These fragments hurled against the 



shore are the prin- 

 cipal agent of wave- 

 erosion. The ra- 

 pidity of the erosion 

 of a coast-line by the 

 action of waves is 

 determined partly 

 by the softness and 

 partly by the incli- 

 nation of the strata. 

 If the strata turn 

 their faces to the 

 waves, particularly if inclined at a small angle, the effect of the waves 

 is comparatively slight (Fig. 25) ; but, if the edges of the strata are ex- 

 posed to the waves, the erosion is much greater. For instance, if the 

 strata be horizontal, as in Fig. 26, then the strata are undermined and 





Fig. 25. 



Fig. 26.— Section of an Exposed Cliff. 



Fig. 2? 



form overhanging table-rocks, which from time to time fall into the 

 sea ; if the strata are vertical or highly inclined and their edges turned 

 to the sea, then an exceedingly irregular coast-line is formed and the 

 erosion is very rapid, as the force of the waves is concentrated upon the 

 re-entering angles. Fig. 27 is a map view of a coast, in which from a to 



Dana's Manual, p. 654. 



f Herschel's Physical Geography, p. 75. 



