246 



PHYSICAL GEOLOGY 



sive wave-cut plain. Plains of marine denudation, like base- 

 level plains, cut indifferently across beds of varying structure 

 and hardness. 



FIG. 264. 



Natural bridge on the coast of Califor- 

 nia, near Santa Cruz. 



TRANSPORTATION AND DEPOSITION ALONG THE SEASHORE 



The beach and transportation. When waves come in to 

 the shore obliquely, some of the bottom material is moved 

 up and at the same time along the beach. Carried out by the 



undertow, it is 

 again swept up 

 and along the 

 beach, and by a 

 continuation of 

 the process trav- 

 els alongshore by 

 a series of zigzag 

 paths. Further- 

 more, winds blow- 

 ing obliquely upon a beach generate a current which moves 

 alongshore, and is known as the shore current or littoral cur- 

 rent. Littoral currents are not strong, but when sand particles 

 and pebbles are lifted wholly or partially by the waves or the 

 undertow, the current is able to move them a slight distance 

 along the beach in the direction of its movement. 



The beach is the roadway along which the shore drift is 

 transported. (What determines its width?) The material 

 of unprotected beaches at the foot of sea cliffs is coarse, for 

 the water is agitated vigorously, and only relatively heavy 

 material can remain ; silt is carried seaward by the undertow, 

 or along the coast by shore currents to more sheltered places. 

 In sheltered bays and along low, protected shores, the beach 

 material is likely to be fine sand or mud. 



As the material of a beach is moved, sometimes to a depth 

 of several feet on exposed coasts during severe storms, the 

 particles are worn and crushed, and may be reduced at last 



