Sand Beaches 



187 



gree of sphericity than the gravels. The low 

 roundness is a result of both the low impact 

 as compared with that of the heavier cobbles 

 and the cushioning effect of the surround- 

 ing water film as compared with the negli- 

 gible effect of air for wind-blown sands 

 which are usually well rounded. 



As pointed out long ago by Reed (1928), 

 most of the Mesozoic and Cenozoic strata 

 as well as unconsolidated recent sediments 

 contain such a high percentage of feldspar 

 that they should be termed arkoses or arko- 

 sic sands. Other supporting analyses of 

 ancient sedimentary rocks of the region are 

 given by Woodford (1925), Hertlein and 

 Grant (1944), and Emery, Butcher, Gould, 

 and Shepard (1952). Among the recent sedi- 

 ments the beach sands of Coronado Strand 

 near San Diego consist of approximately 50 

 per cent feldspar and 50 per cent quartz, as 

 compared with the 20 per cent minimum 

 feldspar required by Krynine (1948) and the 

 25 per cent required by Pettijohn (1957, p. 

 291) for use of the term arkose. The high 

 feldspar-to-quartz ratio is doubtlessly a re- 

 flection of the large proportion of feldspar 

 in the source batholiths, the arid conditions 

 of weathering, and the rapid transportation 

 and deposition in the region. 



More diagnostic of provenance are the 

 heavy minerals, although unfortunately this 

 branch of sedimentology has received less 

 attention in the region than it deserves. 

 Regional studies of the mineralogy of all 

 southern California beaches have not yet 

 been made, but some general information is 

 provided by a comparison of several local 

 studies. Trask (1952) found that the beaches 

 between Monterey and Santa Barbara can be 

 divided into three subequal units: the north- 

 ern third with heavy minerals (62 to 125 

 microns) dominated by hornblende, the mid- 

 dle by augite, and the southern one by epi- 

 dote. The latter third includes the entire 

 coast between Point Conception and Santa 

 Barbara, where the chief source of the min- 

 erals must be the Tertiary sedimentary rocks 

 of the Santa Ynez Mountains (Fig. 162). 

 Handin (1951) examined the minerals in the 

 same size fraction of beach samples collected 

 between Santa Barbara and Point Fermin 



and found that heavy minerals in the central 

 part of this span of beaches are dominated 

 by augite. This span corresponds to the 

 position of the Santa Monica Mountains, 

 whose streams carry large percentages of 

 augite. Near Point Fermin the chief heavy 

 mineral is hornblende, which is even more 

 dominant southward to Mexico (Inman, 

 1953) and San Diego (Emery, Butcher, Gould, 

 and Shepard, 1952). It appears reasonable 

 to divide the beaches of southern California 

 into three heavy mineral regions: epidote 

 between Point Conception and Ventura, 

 augite between Ventura and Santa Monica, 

 and hornblende between Santa Monica and 

 Mexico. The presence of these zones is 

 strongly suggestive of distinctive composi- 

 tion of the corresponding chief areas of 

 provenance. Mineral composition can thus 

 be used to identify the source of the beach 

 sands. For example, Trask considered that 

 the gradually diminishing content of augite 

 in beach sands southward from the augite 

 source area near Morro Bay indicates the 

 downcurrent passage of sand around Point 

 Conception to Santa Barbara. 



Many studies of the grain-size distribution 

 of beach sands have been made, but nearly 

 all of these are for only small parts of the 

 region and are of uneven quality and density 

 of samples. In order to provide a more uni- 

 form basis forjudging the character of the 

 beaches, a special suite of samples was col- 

 lected at 5-mile (8-km) intervals between 

 Point Conception and the United States- 

 Mexican border during a period of neap tide 

 in late November 1956. These samples were 

 composites of the zone between the high 

 tidemark and the approximate midtide level. 

 They exhibit a range in median diameter 

 from 150 to 590 microns and average 240 

 microns (Table 12, Fig. 163). This is lower 

 than the average of 350 microns obtained by 

 combining the results of many independent 

 beach studies (Emery, 1954e) but is a more 

 valid representation for the region. Irregu- 

 larities in grain size are apparent in the 

 regional picture. Some areas of coarser- 

 than-usual sand are present at the mouths 

 of rivers such as the Ventura and Santa Clara 

 and of Ballona Creek, as though these 



