Shelves and Bank Tops 



37 



shelves were cut by multiple lowerings of sea 

 level during the Pleistocene, at least in some 

 parts of the region, but that the present well- 

 defined submerged terraces date only from 

 Wisconsin time. 



Some support for a Wisconsin age of ter- 

 race cutting is provided by lithothamnoid 

 encrusting calcareous algae found in cores 

 from depths of 334 to 387 feet just beyond 

 the deepest terrace off the Palos Verdes Hills 

 (Emery, 195Bfl). The present depth is too 

 great for the algae to live today, considering 

 the turbidity of the water; moreover, the 

 algae occur in layers covered by as much as 

 10 feet of silt, sand, and clay. It is beheved 

 that they lived when the sea level was lower 

 — probably low enough to have cut the 

 deepest terrace. Radiocarbon ages of four 

 samples of the algae show them to be 17,000 

 to 24,500 years old (Emery 1958^), presum- 

 ably indicating a similar age for the deepest 

 terrace. This makes the deepest terrace 

 younger than the + 75-foot terrace on the 

 nearby shore, dated as older than 30,000 

 years (Kulp, Tryon, Eckelman, and Snell, 

 1952). Moreover, carbon- 14 dates compiled 

 by Shepard and Suess (1956) show that the 

 sea level rose from minus 150 feet to the 

 present position during the past 12,000 

 years, supporting the ages derived from the 

 algal encrustations. Of interest here is the 

 finding of nearly 700 stone mortars and a 

 few metates and other artifacts in water 20 

 to 100 feet deep off Solano Beach, La Jolla, 

 Point Loma, and Imperial Beach (Tuthill 

 and Allanson, 1954), of at least one mortar 

 off Santa Barbara (Wallace and Kritzman, 

 1956), and of one or two off Malibu and 

 Redondo Beach (Conrad Limbaugh, per- 

 sonal communication). Although the 

 mortars conceivably were carried out in 

 boats and dumped, it is more probable that 

 they mark village sites submerged beneath 

 the post-Pleistocene rising sea level (Carter, 

 1955). If so, then many other sites remain 

 to be discovered under the sea, and archae- 

 ologists must be prepared to take to the 

 water. 



The outer flat terrace differs from all the 

 shallower ones in that its outer edge marks 

 the depth of abrupt change of slope that 



has been called the shelf-break by Dietz and 

 Menard (1951). This is the true edge of the 

 shelf. If any of the terraces are correlatable 

 over long distances, the one at the shelf- 

 break is the most certain, owing to its ease 

 of identification. Its depth ranges from 

 about 250 feet near Los Angeles to about 

 480 feet far to the south and to the west. 

 Contours of depth of the shelf-break in Fig- 

 ure 38 show a gradual deepening from 

 northeast to southwest throughout the 

 borderland. Variations in depth of wave 

 base in the region are insufficient to account 

 for the differences in depth of the shelf-break, 

 for as pointed out by Dietz and Menard 

 (1951) effective wave base is that depth at 

 which energy begins to be transferred to the 

 bottom and the wave velocity begins to de- 

 crease. For all practical purposes this depth 

 is only about 30 feet. Instead, the depth 

 variation must be taken as a measure of the 

 extent of regional warping after the terrace 

 was cut. 



Although the shelf-break yields the most 

 reHable information about warping of the 

 coastal region, some additional information 

 is provided by the shallower terraces. The 

 outer edge of each terrace is shallower where 

 crossed by profiles off most of the mainland 

 and the nearshore islands of Santa Rosa, 

 Santa Cruz, and Anacapa than where crossed 

 by profiles off the outer islands and banks. 

 Average depths for each terrace in both 

 areas (Table 3) show that the difference in 

 depth is progressively greater for deeper 

 than for shallower terraces. In fact, the 

 ratio of depth difference to depth offshore 

 exhibits only a small range, 26 to 40 per 

 cent. 



Table 3 



Depths of Outer Edges 

 OF Submerged Terraces, feet 



