Gravel Beaches 



185 



ding planes in sedimentary rocks, foliations 

 in schists, and joints in some igneous rocks. 

 However, the generally high degree of uni- 

 formity of shape gives rise to the long- 

 pondered question of whether gravels are 

 worn flatter on beaches or whether the origi- 

 nally flat pieces are selectively sorted away 

 from their more spherical original compan- 

 ions. Arguments and evidence for both 

 points of view exist. In order to check the 

 erosional question, about 1000 cobbles were 

 painted white and replaced on a sandy 

 beach. After a few days the painted cobbles 

 were sought out after undergoing heavy 

 losses from wave action and little boy col- 

 lectors. Although the results are statistically 

 incomplete, the paint was the most worn off' 

 on the sides of flat pieces but more or less 

 uniformly around spherical ones. In a later 

 study Dobbs (1958) placed 375 painted cob- 

 bles on a gravel beach. After an hour only 

 10 specimens could be found. Counts were 

 made of the number of impacts that these 

 cobbles had undergone, with the following 

 results: round rocks, 185/sq cm uniformly 

 distributed; flat surface of flat cobbles, 125/ 

 sq cm; and rounded edges of flat cobbles, 

 292/sq cm. The hypothesis seems reasona- 

 ble that on beaches consisting only of cob- 

 bles the cobbles tend to be tumbled over 

 each other so that all sides are worn, espe- 

 cially the rounded edges, whereas cobbles 

 scattered about a predominantly sandy 

 beach rest on one side and are sand-blasted 

 on their upper surface. In favor of the sort- 

 ing point of view is the fact that even on 

 beaches consisting only of cobbles flat pieces 

 are prevalent high on the beach as though 

 lifted by advancing waves, whereas spherical 

 ones are commonest low on the beach as 

 though they tend to roll seaward in the back- 

 wash of waves (Fig. 159). 



Within the gravel beaches a preferred ori- 

 entation of individual cobbles is usually well 

 exhibited. The short axis dips landward be- 

 cause the force of wave impact tilts the 

 discoidal specimens (Fig. 159). Since the 

 beaches are highly permeable, most of the 

 water in the waves seeps downward through 

 the beach, leaving so little of it to return to 

 the ocean atop the beach that it does not 



P^40 



DISTANCE-FEET 



Figure 1 59. Some characteristics of gravel beach at Bluff 

 Cove, west side of Palos Verdes Hills. Each histogram is 

 based on a composite of 100 measurements at 1 1 stations 

 located 15 meters apart along the beach. Note the in- 

 crease of median diameter (intermediate axis) and de- 

 crease of sphericity from bottom to top of beach. Three 

 berms are shown by the beach profile. Adapted from 

 Briggs (1950). Petrofabric diagrams of C-axis (short axis) 

 with land on right-hand side are adapted from West 

 (1950) and are based on 100 to 125 cobbles at each of 3 

 stations, with cross-hatched area representing 1 per cent 

 and double-hatched area 5 per cent of cobbles. 



greatly modify the grain orientation on the 

 higher levels of the beaches. The loud rat- 

 tling of cobbles low on thf" beaches shows 

 that modification of grain orientation does 

 occur there, however, during the backwash 

 of waves. 



Gravel as well as sand beaches are com- 

 monly interrupted by two kinds of moder- 

 ately large features. One, the berm, extends 

 along the length of a beach (Fig. 160), 

 whereas the other, the cusp, is transverse to 

 the beach. Eight separate berms, or step- 

 like flattenings of the foreslope, have been 

 noted on a gravel beach at Punta San Telmo, 

 in Baja California, but three or four appear 

 to be the maximum in southern California. 

 Because the berms usually extend the entire 

 length of a beach, they must be produced by 

 an agent that is uniform along much of the 



