186 



Sediments 



-OFFSHORE- 



HIGH TIDE- 



-FORESHORE - 



-BACKSHORE- 



^^KK^KKKwkk ^' : : ^^;::^: ^\^-^\\\w<\K\^ 



^; ^\\\V<.VV\\\\\\\'\\\^ ^^\\\^\^\^^^^^^^:^^^ 



Figure 160. Terminology and 

 some of the characteristics of 

 water and topography of 

 beaches. See also Shepard 

 (1948, p. 82) and Wiegel (1953). 



coast, such as storm waves at a particular 

 stage of the tide. 



The second feature, cusps, has been in- 

 triguing to many geologists. At Santa 

 Monica Bay the gravel cusps range from 

 irregular masses to isosceles triangles having 

 points facing the ocean. They average 15 

 meters wide, 10 meters long, and 50 cm high. 

 They are separated by sand embayments 

 averaging about 20 meters wide. The cusps 

 consist of well-sorted gravel with the smallest 

 size at the apex. Several hypotheses have 

 been formulated to account for cusps, among 

 which are the following: breaching of a 

 gravel bar (Jefferson, 1899), interference of 

 two sets of waves (Branner, 1900), erosion 

 of an embayment at an initial depression in 

 a continuous gravel beach (Johnson, 1919, 

 pp. 457-486), burial under sand followed by 

 exhumation (Shepard, 1935), and erosion of 

 embayments by swash followed by refraction 

 of the swash to the sides and deposition of 

 the eroded material as a cusp (Kuenen, 

 1948). None of these hypotheses appears to 

 be based on actual observation of formation, 

 probably because the cusps form quickly and 

 then endure usually for at least several days. 

 By a clever bit of detective work Schupp 

 (1953) managed to photograph the develop- 

 ment of a cusp from beginning to end (Fig. 

 161). Examination of the beach at the north 

 end of Santa Monica Bay at intervals over 

 a 2-year period showed that the cusps are 

 most abundant during the summer and fall 

 when waves are relatively small and during 

 the neap stage of the tidal cycle. They also 

 appeared to form at high tide. After sev- 

 eral failures Schupp picked a favorable pe- 

 riod, at night, and observed at first a bare 

 sand beach (sloping 20° rather than the 

 usual 12°), atop which a 4.5-hour rising tide 



deposited a thin veneer of fine gravel that 

 culminated in a 8-cm layer of medium-to- 

 coarse gravel. During the first 2.5 hours of 

 falling tide cusps were formed through ero- 

 sion of the embayments by wave backwash 

 which also deposited some of the eroded 

 material at the seaward side of the embay- 

 ment to build the apex of the cusp. The 

 change from bare sand to gravel cusp re- 

 quired only 7 hours. Whether this explana- 

 tion is true for other gravel cusps in the 

 region is unknown, but quite possibly some 

 others are of different origin. 



Recognition of ancient gravel beaches is 

 simple where shells are still present or where 

 some of the cobbles contain pholad borings. 

 If only unbored cobbles and pebbles remain, 

 there may be some difficulty in distinguish- 

 ing whether a large blanket conglomerate 

 was of beach or alluvial origin. However, 

 where former beach deposits are narrow, a 

 sequence may be expected from angular talus 

 debris at the base of the land side through 

 spheroidal to diskoidal cobbles at the top of 

 the deposit. Such a sequence is more or less 

 completely represented in exposures of the 

 raised terraces of the Palos Verdes Hills and 

 elsewhere along the coast. Beach gravels 

 (Fig. 81) also occur on the sea floor (Clem- 

 ents and Dana, 1944; Emery and Shepard, 

 1945); the sequence is less well exhibited 

 than on land. 



Sand Beaches 



Unlike beach cobbles and pebbles, almost 

 all beach sand grains are monomineralic 

 with the maximum diameter limited by the 

 original size of the minerals in the source 

 rocks. The beach sands also usually have a 

 lower degree of roundness and a higher de- 



