Coasts 



15 



least several decades in the Hawaiian Islands 

 and elsewhere in the world. 



Wetting and drying action by water in and 

 just above the intertidal zone is slow, but it 

 accomplishes a large amount of erosion, 

 particularly in southern California where 

 many sea cliffs consist of sedimentary rocks. 

 Prolonged soaking of shale in water is a 

 well-known method of causing the shale to 

 break down into its constituent particles. 

 This action is probably not so much the re- 

 sult of dissolving of a cement as it is the re- 

 sult of base exchange, hydration, or swelhng 

 of the grains so that they become loosened 

 and able to be washed away. For sand- 

 stones solution of calcareous cement is, of 

 course, more likely. Exfoliation by forma- 

 tion of thin fragile scales is common and 

 serves as an indicator of these processes. 

 Expansion of scales and loosening of grains 



PERCENTAGE FREQUENCY 



10 20 30 40 50 



54 DATES AUG. 1956 



res NOV. 1943 



res AUG. 1940 



Figure 10. Wave damage to house at Redondo Beach. 

 Photographed January 17, 1953, after a small storm. 



Figure 1 1 . Frequency distribution of dates cut in Eocene 

 and Cretaceous sandstone chffs at La Jolla in 1940, 1943, 

 and 1956. It appears that about 6 years are required for 

 weathering to obliterate the dates. If the average depth 

 of each is '/g inch, a rate of weathering of about 1 foot in 

 600 years is indicated. Numbers indicate total numbers 

 of observations. 



by growth of salt crystals has been suggested 

 in the literature, but neither field nor lab- 

 oratory tests seem to support the suggestion. 

 These processes operating in the intertidal 

 and spray zones produce pittings in the 

 rocks and also a general wasting away of the 

 surface. The latter was estimated at about 1 

 foot in 600 years (Emery, 1941a) for sand- 

 stones at La Jolla on the basis of frequency 

 distribution of dated inscriptions cut into 

 the rocks (Fig. 11). 



Biological activity in the intertidal zone is 

 a much overlooked means of erosion. It 



