Shelves and Bank Tops 



35 



local high areas that once were islands or 

 stacks such as the center of Santa Monica 

 Bay (Terry and Stevenson, 1957). 



Multiple terraces at shallow depths occur 

 elsewhere in the world — Japan, Guam, Aus- 

 traha, the Persian Gulf, Britain, and the Gulf 

 and Atlantic coasts of the United States 

 (Emery, 1958a). The shallowest terrace (50 

 to 60 feet) is even more widely known. This 

 widespread distribution is strongly indicative 

 of eustatic origin in relatively recent times. 

 According to Flint (1947, p. 437) the lower- 



represent separate glacial ages of the whole 

 Pleistocene Epoch or whether they were 

 formed during only part of this time. 



The terraces that together make up the 

 mainland and island shelves and the bank 

 tops are wider and flatter than most of those 

 now exposed on land. Many are between 

 1 and 5 miles wide, in contrast, for example, 

 to the 0.1- to 0.5-mile width of the high ter- 

 races of the Palos Verdes Hills (Fig. 37) and 

 of San Clemente Island. The greater flat- 

 ness of the submerged terraces (usually less 



PALOS VERDES H/LLS SANTA CATALINA ISLAND SAN CLEMENTE ISLAND 



_ 14 15 16 17 18 19 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 



Figure 36. Elevations of terraces between 300 feet above and 700 feet below sea level at margins of Palos Verdes Hills, 

 Santa Catalina Island, and San Clemente Island. Thin vertical lines show depths or heights that were sounded or shown 

 on land contour maps. Wide vertical bars show depth ranges of individual terraces or relatively flat areas having 

 definite landward and seaward edges; medium-width bars indicate terraces having less definite edges. Top and bottom 

 limits of terraces on each profile have been connected by lines to show the lateral extent of terraces. 



ing of sea level during the maximum extent 

 of glaciers was about 400 feet, to a level 

 slightly shallower than the deepest terrace 

 off southern California and slightly shallower 

 than Shepard's (1948, p. 143) figure of 430 

 feet for the average depth of the outer edge 

 of the continental shelf around the world. 

 The Pleistocene age of the terraces that is 

 indicated accords with the fact that some of 

 them, at San Pedro Bay and along the coast 

 from Ventura to Santa Barbara, truncate 

 structures in Pliocene shales. The question 

 stiU remains of whether the shallow terraces 



than 1°) as compared to the emerged ones 

 (usually more than 3°) is a reflection of the 

 lesser width of the latter as well as of their 

 greater mantling by sedimentary materials. 

 These diff'erences may be expressions of re- 

 peated movements of sea level across the 

 area, each time planing off'more rocks to make 

 the flat shelves, in contrast to single brief 

 times of cutting of the very high and the very 

 deep terraces. The small depth variation of 

 the shelf-break and the fact that the shallow 

 submerged terraces are present in areas 

 where high terraces on land or very deep ones 



