32 



Physiography 



Figure 32. Topographic and textural analogies between marine and stream environments. Arrows show direction and 

 relative velocity of water currents. 



Shelves and Bank Tops 



The mainland shore is bordered by a shelf 

 which ranges in width from less than 1 mile 

 to 15 miles and averages 4 miles. Maximum 

 widths of more than 7.5 miles occur at four 

 places: the United States-Mexico border, 

 San Pedro Bay, Santa Monica Bay, and east 

 of Santa Barbara. Although each of these 

 areas is off the mouth of a main river of 

 southern California, the greater-than-usual 

 shelf widths cannot be the result of deltaic 

 deposition because rocky bottom is abun- 

 dant on each of the four shelves. 



Shelves around the islands vary from less 

 than 0. 1 mile to 22 miles wide. Reference to 

 Chart I shows that the shelves are narrowest 

 around the eastern islands (San Clemente 

 and Santa Catalina), are intermediate around 

 Santa Barbara Island, and are widest around 

 the five western islands. This variation in 

 shelf width may be due to less exposure of 

 the eastern islands to storm waves, aided by 

 the presence of more resistant rock on the 

 same islands. 



The surfaces of flat-topped banks are simi- 

 lar to shelves, except that they lack a border- 

 ing land area. Widths range up to 8 miles 

 but, unlike the shelves, they are limited by 

 the width of the block that was truncated. 

 Even so, most of the wider banks are west 



of the narrower ones, paralleling the distri- 

 bution of island shelf widths. 



Rocky bottom is present at many places 

 on the mainland shelves, is even more com- 

 mon on the island shelves, and is most 

 abundant on the flat bank tops. As shown 

 by fathograms and photographs of Figure 

 33, rock bottom is usually marked by irreg- 

 ular projections above the general level of 

 the shelf. The correspondence of very ir- 

 regular topography with rocky bottom per- 

 mits rapid identification of rocky areas on 

 fathograms. 



In addition to irregular rocky areas, the 

 shelves and bank tops have a series of step- 

 like terraces which are similar to those on 

 adjacent land areas: The terraces are easy 

 to recognize on sounding profiles made off 

 rocky points of the mainland and on island 

 shelves and across bank tops. On shelves 

 bordering low depositional mainland coasts 

 they are partly obscured by aprons of debris 

 derived from land (Fig. 34). In such areas 

 jet drilling through overburden (Thompson, 

 1957) or sonoprobe studies (Moore, 1957; 

 Moore and Shumway, in press) have aided 

 in the recognition of the terraces. Typical of 

 the shelf profiles are those around the Palos 

 Verdes Hills as shown by Figure 35. Profiles 

 for other parts of the region are given by 

 Emery (1958^). Several terraces can easily 



