Beaches 



313 



from swimming pools, and breakage of water 

 and sewer pipes are important causes of 

 movement (Roth, 1959). In spite of the 

 warning and denial of grading and building 

 permits by the Department of Building and 

 Safety of the City of Los Angeles, owners 

 have insisted on building in dangerous areas, 

 with past and future damages amounting to 

 millions of dollars. In areas such as that 

 between Dana Point and San Clemente 

 some homes have been built at the very 

 edge of the cliff so as to provide the best 

 view, but so far the main losses have been 

 the slow undermining of the houses followed 

 by their removal before falling down the 

 cliff. 



The situation with respect to residences 

 has been especially serious along the south 

 side of the Palos Verdes Hills. Near Point 

 Fermin a sHde began in 1929 (Miller, 1931) 

 where Middle Miocene shales dip seaward 

 toward a vertical cliff. A dozen or so houses 

 atop the slide block had to be moved. Re- 

 newed movement in 1940, 1941, and prob- 

 ably subsequently has so broken up the 

 strata that slump has produced backward 

 rotation. This slide is a classic one, being 

 pictured in many textbooks of geology (for 

 example, Emmons, Thiel, Stauffer, and Al- 

 hson, 1955, Fig. 125) and in technical studies 

 of shdes (Varnes, 1958). Farther west on 

 the same coast is the largest slide of southern 

 California from the point of view of area. 

 This one, known as the Portuguese Bend 

 slide, began in ancient times as a block 

 glide down seaward-dipping bentonitic 

 Middle Miocene shales. Movement of an 

 area of about 2.5 sq km produced a hum- 

 mocky topography that is easily recognized 

 as of slide origin even on a topographic map 

 of 1916 (Nelson, Zinn, Strahorn, Watson, 

 and Dunn, 1919), and was described as 

 such by Miller (1931) and by Woodring, 

 Bramlette, and Kew (1946) in their geologi- 

 cal study of Palos Verdes Hills. In some of 

 the depressions lake sediments accumulated 

 with internal unconformities that bear wit- 

 ness to repeated movements of the slide. 

 Doubtlessly each movement continued until 

 a toe pushed into the ocean was large 

 enough to provide sufficient resistance to 



counterbalance the weight of the moving 

 mass. With the great residential develop- 

 ment of Los Angeles, the area of the slide 

 became subdivided, owing to its gentle 

 slope and beautiful view. About August 

 1956 renewed movement of the eastern haff 

 of the ancient slide began. Whether the 

 cause was the just-completed dumping of 

 about 175,000 tons of rock at the top of the 

 slide during road building or whether it was 

 due to excessive watering and leakage from 

 septic tanks, or both, has not been legally 

 estabished. Nevertheless, after 2 years the 

 movement has reached about 16 meters 

 horizontally with a 6-meter drop at the top 

 and a bulge at the toe (Merriam, in prepa- 

 ration). Of 156 houses in the area, only 40 

 remained occupied in January 1959, the 

 others having been moved away or ruined 

 through shear at the west side and compres- 

 sion at the toe. Continued movement of the 

 original area removed support from sur- 

 rounding areas so that additional movement 

 has occurred both to the west and north, 

 with still more areas likely to be involved in 

 the near future. Movement at the head 

 (north) resulted in the identification as an- 

 cient slide of areas not originally recog- 

 nized as such; these areas are not shown 

 on Figure 20. An attempt to stop the move- 

 ment of this mass of about 50 million tons 

 by insertion of a score of steel and concrete 

 pilings across the slip plane merely caused 

 them to be broken or rotated with no ap- 

 preciable effect on the slide. Slides of this 

 type are known elsewhere in the world 

 (MacPherson, 1952), but there seems to be 

 no effective way of stopping them. Pre- 

 sumably therefore future use of the area wiU 

 be restricted to park or golf course. 



Beaches 



The excellent sand beaches of southern 

 California are a notable resource of the 

 region, primarily for recreation. The sun- 

 bathing, swimming, and surf fishing are in- 

 ternationally known. On hot summer days 

 as many as a million people have been esti- 

 mated to visit the beaches, particularly those 

 near the larger cities. Crowds from Los 



