26 



Physiography 



-3000 



~2500- 



B 



CONTOURS IN FEET 



Figure 26. Portion of coast just west of Point Dume showing approximate relative effects of marine and stream ero- 

 sion. C, present topography of land and sea floor. B, topography drawn by connecting seawardmost points of each 

 contour of C. This eliminates the incisions produced by stream erosion. A, smooth topography drawn in by elim- 

 inating the wave cut the terrace B. This eliminates the effects of marine erosion and serves as a possible expression 

 of the original structural topography. 



Subtraction of the volume of rock below the contours of B from that below the contours of A shows that marine 

 erosion removed about 3.6 cubic miles of rock from this area; subtraction of the volume of rock below contours of 

 C from that below the contours of B shows that stream erosion subsequently removed about 5.5 cubic miles of rock. 

 Thus, in this area streams have removed at least 50 per cent more rock than has marine erosion. Stream erosion prior 

 to terrace cutting should further increase the role of stream erosion in comparison with marine erosion. In other por- 

 tions of the coast having through-flowing streams, the role of stream erosion has been even greater than here. 



water movement, some sand is transported 

 along the beach. Because most of the waves 

 in southern Cahfornia come from the north- 

 west during winter and from the west or 

 southwest during summer, the sand contrib- 

 uted all along the mainland coast might be 

 expected to accumulate in the central part 

 of the broad embayment, between San Diego 

 and Ventura, supplementing the effect of 

 greater stream contribution in this area. 

 This conclusion is supported by the fact that 

 the beaches are more or less progressively 

 wider going southward and eastward from 

 north of Point Conception to Point Concep- 

 tion, to Ventura, to Los Angeles, and north- 

 ward from Ensenada, Mexico, to San Diego 



to Los Angeles. In addition, since the winter 

 waves are larger than the summer ones, we 

 might expect the longshore drift to be pre- 

 dominantly southeastward, and again this 

 belief is supported by the accumulation of 

 sand against the north or west sides of nearly 

 all groins of the coast. An eastward move- 

 ment of sand at least between Point Concep- 

 tion and Santa Barbara is also shown by its 

 mineralogy with respect to source areas 

 (Trask, 1952). Local northward or westward 

 movement of sand (Shepard and Inman, 

 195 \b) results from irregularities of trend of 

 the shorehne and from seasonal variations 

 of wave direction. 



Only minor leakage of sand out of the 



