Landslides 



311 



Table 30 



Shipwrecks per Decade off Southern 

 California 



Decade Number of Wrecks 



Later 

 1946-1955 

 1936-1945 

 1926-1935 

 1916-1925 

 1906-1915 

 1896-1905 

 1886-1895 

 Earlier 

 Total 



11 

 90 

 67 

 53 

 23 

 21 

 6 

 5 



14 

 290 



Many were buried during land-fill enlarge- 

 ment of Terminal Island in Los Angeles 

 "Harbor. Whether any of the wrecks, recent 

 or ancient, in harbors or open sea, treasure- 

 laden or empty, are worth the trouble and 

 expense of salvage is an open question, but 

 their very existence serves as a perennial in- 

 vitation to the poor but romantic landsman, 

 as indicated by occasional newspaper stories 

 of the departure from port of another party 

 of eager treasure seekers. No mention is 

 ever made of their successful return! 



Shorelines 



Landslides 



The rugged nature of much of the coast 

 of southern California is reflected in num- 

 erous landslides along its sea cliffs (Fig. 20). 

 Deep valleys that have been eroded into the 

 coastal areas behind these cliffs are such 

 great obstacles to building of roads that in 

 some areas highways have been built at the 

 foot of sea cliffs, in spite of the danger 

 from landslides and from wave erosion. 

 This danger has been increased by the re- 

 moval of ancient slides and talus slopes in 

 order to provide space for a right of way 

 and some material for a road bed. The 

 three chief areas where highways he between 

 cliff and sea are the east half of the Santa 

 Monica Mountains at Pacific Palisades west 

 of Santa Monica, the west end of Santa 

 Monica Mountains west of Point Dume, 



and midway between Dana Point and San 

 Clemente. 



By far the most dangerous of the three 

 areas is the one immediately west of Santa 

 Monica where cliffs about 60 meters high 

 consist of Pleistocene alluvium at the east 

 and soft Middle Miocene shales at the west. 

 The alluvium is poorly consolidated, and 

 most slides in it are of the soil or debris fall 

 and the slump types (Sharpe, 1938; Varnes, 

 1958). Not only are the shales poorly con- 

 solidated but they dip seaward so that most 

 of the slides in them probably start as block 

 glides that soon become thoroughly broken 

 up and transformed into slumps. Both 

 ancient and modern slides have occurred 

 (Fig. 20, Section 7) and, although they have 

 not been the largest in the region, they are 

 especially destructive, owing to steepness of 

 the slopes. In 1932 two slides, one of 30,000 

 cu meters and the other of 45,000 cu meters, 

 covered the highway. Control was estab- 

 lished by digging tunnels for drying with hot 

 air (Hill, 1934). After about 6 years of 

 operation the blowers were stopped and the 

 tunnels and equipment allowed to deterio- 

 rate. At present the shales have again be- 

 come saturated, largely through the heavy 

 watering of lawns, so that renewed move- 

 ment can soon be expected. About 1 km 

 farther west a slide of about 40,000 cu 

 meters occurred as a slump in the Miocene 

 shale during March 1958. The debris partly 

 blocked the highway but was soon cleared 

 up. Just as the last of the debris was being 

 moved another slide of more than 80,000 cu 

 meters took place, burying an engineer and 

 some of the equipment used on the first 

 slide (Fig. 239). It was decided that re- 

 moval of the new slide might cause stiU 

 more slides, so instead the highway was re- 

 built around the toe of the second shde. 

 These and many other smaller landslides 

 west of Santa Monica have been examined 

 by Roth (1959), who found most to be of 

 the slump type, with smaller ones of the 

 soil avalanche type. Although it is evident 

 that lubrication and weighting by water 

 is a prime factor in original and subsequent 

 movements, intensive watering of lawns by 

 residents and municipalities occurs at an in- 



