General Structural History 



95 



Figure 81. Andesite pebbles rounded by wave action on ancient beaches. On the right are two pebbles from 1950 feet 

 above sea level on San Clemente Island: on the left are five pebbles from AHF 5227 at 2230 feet below sea level near 

 the top of a submerged hill at latitude 33°36.2', longitude 120° 14.4'. Weathering has removed part of the rounded 

 surface of the large pebble from land. The large one from the sea floor was broken in order to identify the kind of 

 rock (xO.6). 



is the broad regional warping shown by the 

 submerged terraces of the shelf; these ter- 

 races were probably formed during eustatic 

 low sea levels of the Late Pleistocene. The 

 lowest and presumably oldest of these is only 

 about 250 feet deep near Los Angeles but 

 about 480 feet deep far to the south and the 

 west. The fact that the maximum lowering 

 of sea level during the Pleistocene was prob- 

 ably about 400 feet and that the shelf-break 

 beyond the northern and southern limits of 

 Chart I is probably about 400 feet indicates 

 that the continental borderland northeast of 

 the approximate 400-foot contour of the 

 shelf-break must have been warped upward 

 and the one to the south and west of that 

 contour downward. Thus, the 400-foot con- 

 tour must be the approximate hinge line of 



the post-Late Pleistocene warping. Warp- 

 ing, however, was not confined to the late 

 history of the continental borderland. 



Islands are obviously concentrated in the 

 northern half of the continental borderland, 

 and most of the deep basins are in the 

 southern half. A more complete analysis 

 was made by plotting the height above sea 

 level or the depth below sea level of the top 

 of each major mountain against distance 

 south of the northern border of Chart I (lat. 

 34° 30'). The same procedure was followed 

 for the bottoms of the basins and for their 

 siUs (Fig. 82). It is evident that all three 

 features systematically decrease in altitude 

 toward the south (Emery, 1953). The 

 steepest part of the curve for the tops 

 of mountains is due to the presence of the 



