Coasts 



19 



Figure 19. Diurnal variation of water characteristics of 

 a solution basin in calcareous sandstone at La Jolla 

 (Fig. 17). Dashed line shows data for water of ocean 

 near the basin. Lowest part of figure shows the variation 

 in calcium combined with carbonate, bicarbonate, and 

 borate ions, as computed from measurements of tem- 

 perature, chlorinity, pH, total carbon dioxide, and alka- 

 linity. The dashed line indicates the concentration of this 

 calcium if no solution or deposition of it had occurred. 

 Cross-hatching shows times of flooding of basin at high 

 tide. Redrawn from Emery (1946, Fig. 12). 



from the pool locally strengthens the sur- 

 rounding rock enough so that raised rims 

 are left when the rock surface farther from 

 the pool is eroded away. 



Analyses made at Guam and Bikini 

 (Revelle and Emery, 1957) show that the 

 oxygen content of the water bears an in- 

 verse relationship to carbon dioxide, as 

 would be expected from the processes of 

 respiration and photosynthesis. The diurnal 

 change of these ions and of pH occurs to a 

 much greater extent in the tide pools than 

 in the open sea because of the greater ratio 

 of bulk of plant and animal tissue to bulk 

 of water in the pools. With lateral enlarge- 

 ment several pools may join together; even- 

 tually their growth intersects a joint and the 

 pools become drained. The process starts 

 anew at a low point of the empty pool, so 

 that by the time several generations of pools 

 have existed much rock has been removed. 

 Although large pools exist only on relatively 

 flat surfaces, small thimble-like holes on 

 near-vertical sea cliffs are occupied by a 

 small amount of water and associated plants 

 and animals; these are being enlarged by the 

 same process. The great role of biochem- 

 ical erosion is signified by the abundance 

 along the shore of its products. Moreover, 

 the organisms responsible for it work during 

 the long periods of calm seas when many 

 other erosional processes are inactive. The 

 rate of deepening of the solution basins of 

 La Jolla is estimated at 1 foot in 1000 years. 



All these erosional processes are restricted 

 to a zone only slightly greater than the range 

 between low and high tide. None of them 

 erodes directly the tops of high sea cliffs. 

 Yet, they do cause the undercutting which 

 finally results in landsliding of the whole sea 

 cliff" so that all the material of the cliff" comes 

 within reach of the ocean for comminution 

 and redistribution. An examination of the 

 coast mostly from shipboard showed the 

 presence of landslides along at least 25 miles 

 (40 km) of coast, about 40 per cent of the 

 length of shore illustrated by Figure 20 and 

 probably about 80 per cent of the shore 

 frontage of all landslides in the entire region. 

 The distribution shows little relationship to 

 degree of wave exposure and only a mod- 

 erate relationship to steepness of slope; 

 therefore, we must infer that the major con- 

 trol is that of lithology and structure. For 

 example, the largest slide known in the re- 



