NEOCENE STRATIGRAPHY. 349 



South of Point San Pedro. — Wave erosion has formed 

 bluffs nearly the whole distance from Point Montara to 

 Capitola. The Quaternary forms the top of these bluffs 

 most of the way. Thus, at Purisima we have about 

 thirty feet of horizontal sand and gravel overlying about 

 fifty feet of the upper Miocene, the lower beds dipping to 

 the north. See plate xxiv. The gravel referred to proves 

 upon examination to be fragments of rock exactly similar 

 to the layer of volcanic ash in the Pliocene of Seven- 

 Mile Beach. These fragments of volcanic ash occur 

 abundantly not only in the Quartenary all along the coast 

 from Half Moon Bay to Capitola, but also in the Quater- 

 nary deposits in the little valleys opening out toward the 

 Bay of San Francisco. This leads to the suggestion that 

 the layer of volcanic ash on Seven-Mile Beach originally 

 extended all around the mountains, and that these frag- 

 ments tell what has become of it and explain why the 

 Pliocene is not found over a broader area. These frag- 

 ments argue strongly for a land period with subaereal 

 erosion between the Pliocene and Quaternary and sup- 

 port the evidence of the sanddunes at Mussel Rock. 

 These fragments of volcanic ash were used to some ex- 

 tent in recognizing the Quaternary. 



Fossils are not abundant in the Quaternary along the 

 coast, though at places Haliotis and some other shells are 

 quite plentiful. 



At Santa Cruz, besides the lower broad terrace already 

 mentioned, several others are observable. Dr. Lawson"^ 

 counted nine, the highest one 1201 feet above sea-level. 

 The terraces have their sea-cliffs, from the foot of which 

 the ground slopes gently to the next lower sea-cliff. 



Along the Bay of San Prancisco. — Along the bay the 

 Quaternary deposits have not been cut by the waves, but 



* Univ. of Cal., Bull. Dept. Geol., vol. i, p. 141. 



