Coasts 



21 



which sea cliffs can be eroded, the erosion 

 of rocks other than alluvium takes place ex- 

 ceedingly slowly in southern California. 

 This has been shown by a comparison of 

 old and new photographs of many sea cliffs 

 (Shepard and Grant, 1947). Slowness of 

 present-day erosion is also indicated by the 

 presence of large talus slopes at the base of 

 many sea cliffs (Figs. 5 and 8). Some of the 

 slopes are fairly old, as indicated by the es- 

 tablishment of abundant vegetation on their 

 surface. Shepard and Grant suggested that 

 present slow erosion may be a result of a 

 decreased rate of cliff erosion, owing to in- 

 creased width of the cut terrace, to lower 

 waves in recent years, and to long intervals 

 between storms violent enough to erode the 

 sea cliffs. The latter suggestion appears to 

 be particularly plausible; however, one other 

 possibility seems worthy of consideration: 

 that the ocean cannot easily reach the base 

 of the sea cliffs and their talus slopes since 

 the 6-foot lowering of sea level about 3000 

 years ago (Kuenen, 1954; Cloud, 1954; 

 Antevs, 1953). Other evidence of this lower- 

 ing is presented in the tropics by a raised 

 and now largely dissected reef level and by 

 a nip commonly present about 6 feet above 

 present sea level. In southern California it 

 may be represented by the low rock plat- 

 forms that are awash or are only a few feet 

 above high-tide level and are cut across dip- 

 ping beds of basalt, sandstone, breccia, and 

 siliceous shale (Figs. 7, 22, and 23). Because 



MARINE 

 EROSION 



LANDSLIDING 



8. 

 SHEET WASH 



Figure 21. The amount of direct stream erosion during 

 the cutting of a valley is small compared with the effects 

 of landsliding and sheet wash; similarly, the amount of 

 direct marine erosion during the cutting of a sea cliff is 

 small compared with the effects of landsliding and sheet 

 wash. 



of their preference for very hard rocks, the 

 low platforms are most common at rocky 

 points between which marine erosion has re- 

 moved areas of softer rocks and thereby pro- 

 duced irregular shorelines. 



Low rock platforms are common through- 

 out the world, but many writers (for ex- 

 ample, Bartrum, 1935; Jutson, 1939) have 

 ascribed them to storms, terming them storm 

 terraces. They have reasoned that, because 

 the low platforms are practically free of 

 debris, they are being cut at the present time. 



Figure 22. Low rock terrace 

 ("storm terrace") in Miocene 

 basalt sill near Resort Point, 

 Palos Verdes Hills. White pro- 

 jections above basalt consist of 

 more resistant siliceous lime- 

 stone veins in basalt. Terrace 

 extends about 100 feet to the 

 left of the picture. Photo- 

 graphed May 1956. 



