30 



Physiography 



SIX YEARS OF SEASONAL CUT AND FILL 

 ALONG SCRIPPS PIER 



Tirn OUT 



Figure 30. Profiles showing winter and summer changes 

 of the beach and shallow sea floor along the pier at 

 Scripps Institution of Oceanography, La Jolla. From 

 Shepard (19506). 



lays its eggs in the sand of southern Cah- 

 fornia beaches (Fig. 127, p. 144). The incu- 

 bation period of two weeks is just completed 

 by the next time of spring tides when the eggs 

 are washed free of the sand and immediately 

 hatch. An error of a few days in the laying 

 of the eggs would result in premature expo- 

 sure and loss. 



The analogy drawn in a previous section 

 between marine erosion of sea cliffs and 

 stream erosion of valleys has its counterpart 

 (Fig. 32) for marine deposition of beach 



sands and stream deposition of flood plain 

 alluvium. Again, however, we must think 

 in terms of the horizontal changes by marine 

 agents and the vertical changes by the 

 streams. In other words, the planes of action 

 of shore and stream processes are 90° apart. 

 The long gradual slope of a stream at grade 

 has its parallel in a long straight or broadly 

 curving beach, along each segment of which 

 the amount of sand entering from up-beach 

 or up-current and from land exactly equals 

 the amount that either is transported out of 

 the segment over a long period of time in a 

 down-beach direction or is lost to the open 

 sea. Where the beach consists of fine sand, 

 it is long and nearly parallel to the trend of 

 wave fronts, and thus it is an area of only 

 moderate or slow longshore drift. One com- 

 posed of gravel is usually short and trends 

 at a large angle to approaching wave fronts, 

 whose fast longshore drift has winnowed 

 away the sand. These relationships have 

 their parallels in streams, wherein the seg- 

 ments having high velocity are floored by 

 gravel (if gravel is available) and those hav- 

 ing low velocity are floored by sand or finer 

 sediments. In addition, the rocky points 

 separating some beach segments are areas of 

 high turbulence and little sediment, just as 

 are waterfalls and rapids for streams. The 

 great turbulence of winter storm waves which 

 reduce the width of beaches also has its 

 counterpart in the turbulence of flooding 

 streams which places much of the stream bed 

 in suspension and thereby temporarily re- 

 duces the thickness of alluvial fill in valleys. 

 Estuary and lagoon fillings of mud accumu- 

 lated in closed areas of quiet water and sep- 

 arated from the open ocean by baymouth 

 bars have their paraUels in muds that even- 

 tually fill lakes and then are covered by 

 layers of sand. 



Sediment suppUed to the beach by streams 

 ranges from fine- to coarse-grained. The 

 fine-grained sediment is carried away in sus- 

 pension by longshore currents and finally it 

 diff'uses seaward; the medium-grained sedi- 

 ment slowly moves along the shoreline by 

 saltation, and the coarse may be left as a lag 

 deposit. Trunk streams similarly dispose of 



