16 



Physiography 



takes place by both mechanical and chem- 

 ical action. The most direct form is that of 

 borings made by worms, pholads, chitons, 

 limpets, and sea urchins (Barrows, 1917). 

 Each of these animals bores or dissolves its 

 way into the rock surface for protection, 

 making a hole of characteristic shape during 

 the process (Figs. 12-16). The mass of rock 

 removed is unknown but very large, perhaps 

 surpassing that of some of the more widely 

 recognized processes of marine erosion. 

 Borings are not restricted to soft sediments 

 but have been found in gneiss, andesite, and 

 chert. In shales near Point Fermin, pholads 

 are especially effective, having bored so 

 thoroughly that more than 50 per cent of 

 the rock in specimens from many low-tide 

 areas has been removed and much of the re- 

 maining rock between holes has been 

 loosened to form gravel because of the join- 

 ing of several adjacent holes. Many snails, 

 although not borers, contribute materially 

 to erosion of rocky shores through their 

 activity in grazing for blue-green algae that 

 grow on the rock surface. Their peculiar 

 patterns of scraped trails are common 

 (Seilacher, 1953), and sand excreted by the 

 snails has proved on analysis to have the 

 same grain-size distribution and mineralogy 



Figure 12. Boulder beach at Whites Point, Palos Verdes 

 Hills. The large block of Miocene siliceous shale under 

 the hammer was rafted to shore by kelp, as shown by 

 remnant of holdfast and by encrusting coralline red algae 

 and bryozoans. Cup-shaped holes are the work of sea 

 urchins which erode by moving their spines. 



Figure 1 3. Miocene siliceous shale from Whites Point, 

 Palos Verdes Hills, bored by pholads that make bottle- 

 shaped holes which are larger at depth than at the en- 

 trance. Boring by the pelecypod is accomplished by 

 mechanical twisting of the shell against the bottom of 

 the hole, aided probably by biochemical action. Centi- 

 meter scale for comparison. 



as the rock on which the snails grazed 

 (Emery, 1946; North, 1954). North esti- 

 mated the erosion by the snails at about 1 

 foot per 1200 years. 



Indirect or biochemical erosion produces 

 solution basins (Emery, 1946) which occur 

 in limestone, sandstone (Fig. 17), and even 

 basalt (Fig. 18) in southern California. 

 These are shallow flat-floored pools having 

 irregular outlines and usually steep walls. 

 Some have raised rims, but most are sur- 

 rounded by flat surfaces or by a miniature 

 karst topography that is indicative of ad- 

 ditional solution outside the pools. Solution 

 occurs because the carbon dioxide liberated 

 by the plants and animals living in the pool 

 combines with the water to form carbonic 

 acid. Enough carbonic acid accumulates at 

 night, when none is used by photosynthesis, 

 so that bicarbonate ion exceeds carbonate 

 ion (Fig. 19). At the existing temperature, 

 salinity, and pH the water can contain more 

 calcium carbonate, bicarbonate, or borate 

 than was originally present when the water 

 entered the pool, so calcium carbonate is 

 dissolved from the rock underlying the pool. 

 Solution of calcium carbonate is also shown 



