184 



Sediments 



m--' -'- ■ 











^^ ^J^y^ 









Figure 1 57. The longest gravel 

 beach in southern California 

 (about 1 km) forms a bar 

 across an estuary located just 

 north of Encinitas. The gravels 

 are supplied by erosion of con- 

 glomerates in the chfTs at either 

 side of the estuary mouth. 



limestone, and schist are locally represented. 

 The median diameters of gravels at 21 

 mainland beaches (La Jolla, Encinitas, Carls- 

 bad, Dana Point, Whites Cove, Vicinte Cove, 

 Malaga Cove, Redondo Beach, Santa Mon- 

 ica, Corral Beach, Sycamore Point, Ventura, 

 Rincon Point, Capitan, and Gaviota) were 

 found to range between 24 and 650 mm 

 (Emery, 1955a). Each station is character- 

 ized by low Trask sorting coefficients of 1.16 

 to 1.47 (average = 1.27) and symmetrical- 

 size frequency curves (Table 12, Fig. 156). 

 Within individual gravel beaches there usu- 

 ally exists a longshore decrease of grain size 

 and a decrease of sorting coefficient away 

 from the source of the gravels. This is well 

 illustrated at Vicinte Cove, Palos Verdes 



Hills, where basalt supplied by a sill is car- 

 ried mostly to the east by wave action and 

 is mixed with other boulders and cobbles of 

 siliceous shale (Fig. 158). During their 

 transportation these basalt cobbles become 

 rounder (wearing of corners) and more 

 spherical (volume approaching that of an 

 enclosing sphere). An idea of the speed of 

 transportion of cobbles is given by the fact 

 that painted cobbles were moved 20 meters 

 in 10 minutes by 1 -meter waves striking the 

 beach at Santa Monica. 



Individual cobbles usually are round or 

 subround, but they have only a moderate to 

 low sphericity where they are flat and diskoi- 

 dal. This is the original shape of some 

 fragments because of control exerted by bed- 



METERS EAST 

 40 60 



^BASALT S/LL AT VICINTE COVE 

 PERCENTAGE OF BASALT PIECES 



Figure 1 58. Variation in basalt 

 cobbles and boulders with dis- 

 tance from source (a sill) at 

 Vicinte Coves, southwest side 

 of Palos Verdes Hills. Round- 

 ness and sphericity estimated 

 visually from standard grain 

 outlines by Krumbein (1941) 

 and Rittenhouse (1943). 

 Adapted in part from Baldwin 

 (1956). 



