!9 16 ] Martin: Pliocene of Middle and Northern California 223 



described under another formation name. The general stratigraphic 

 and lithologic features of each formation will be briefly outlined 

 before the faunal correlation is attempted. 



MERCED OF THE TYPE LOCALITY 



Physical Features. — The type section of the Merced Series occurs 

 on the San Francisco Peninsula about eight miles south of the 

 Golden Gate. The area covered by these beds lies in the form of 

 an elongate trapezoid extending diagonally across the peninsula 

 from Seven Mile Beach southeast to San Francisco Bay. The south- 

 western boundary is a nearly straight line. From Mussel Rock it 

 follows a course southeast to Lake San Andreas and thence east- 

 ward toward San Mateo. The northeastern limits of this area are 

 marked by a fault scarp along the south flanks of San Bruno 

 Ridge. Professor Lawson has shown that this fault scarp, Avhich 

 is somewhat obscured by the valley alluvium, is the line along 

 which movement took place, letting the Merced Series down against 

 the Franciscan of San Bruno Mountain, and that this downthrow, 

 along the northeastern border, gave the latter its monoelinal structure. 



The general stratigraphic characters of these sediments were 

 described by Professor Lawson as follows : 



From the basement at Mussel Rock the strata of the Merced Series are 

 well exposed in ascending sequence to Lake Merced. The edges of the strata 

 form the sea-cliff for the entire distance. The sea-cliff is in active recession, 

 so that fresh exposures of the rocks are afforded throughout the section. 

 The strike is for the most part more or less transverse to the shore, and 

 the latter is a simple, nearly straight line. The rocks are tilted, generally 

 at high angles, and have a monoelinal structure for the entire length of the 

 section. . . . There is no repetition of strata, and fault structure is rep- 

 resented only by very minor dislocations. In a word, the section is ideally 

 simple, and is eminently susceptible of approximately accurate measurement 

 for the thickness of the series. There is but one drawback, and that con- 

 sists of two landslides which scar the face of the cliff. . . . The cliff at its 

 highest is about 720 feet above the shore, and at several places the strata 

 may be seen extending from the shore to the top of the cliff with uniform 

 dip.is 



These beds are resting upon the eroded surface of the Fran- 

 ciscan Series, which is a prominent formation in the Coast Range 

 Mountains. The contact between these two formations is marked 

 by a very distinct change in lithology and a very pronounced dif- 



i8 Lawson, A. C, Post-Pliocene Diastrophism of the Coast of California, 

 Univ. Calif. Publ. Bull. Dept. Geol., vol. 1, p. 143, 1893. 



