240 PHYSICAL GEOLOGY 



come before reaching the coast, all influence the strength of 

 the waves. (How does each factor affect the result?) The 

 force of waves has been measured in connection with cer- 

 tain engineering enterprises. On the coast of Scotland and 

 among the outer Hebrides, storm waves sometimes exert a 

 pressure of nearly three tons per square foot. (2) The rate 



FIG. 256. Wave erosion near Santa Cruz, Cal. The parallel channels 

 in the foreground are the result of rapid wear along joint lines. (U.S. 

 Geol. Surv.) 



of wear is influenced by the character and structure of the 

 rocks at the shore. Soft rocks wear faster than hard ones, 

 soluble rocks faster than insoluble ones, rocks with many 

 joints (Fig. 256) and openings faster than rocks with few. 

 (Other things equal, which structure would occasion most 

 rapid wear, (a) horizontal beds, (6) beds dipping abruptly 

 toward the sea, (c) beds dipping away from the sea? Why?) 

 (3) Finally, the rate of wear is influenced by the number and 

 character of the tools of the waves. The shallower the water 

 immediately offshore, the greater the number of tools that are 

 likely to be accessible to the waves. But, on the other hand, 

 if the water be very shallow for any considerable distance 

 from the shore, the velocity of the waves will be so reduced by 

 friction with the bottom that, on arriving at the shore line, 

 they will be unable to erode effectively. It may be noted 



