1916] 



Dickerson: Tejon Eocene of California 



465 



contact with the Chico, the Sobrante or basal sandstone of the Monterey 

 group being represented by only a thin layer of yellow incoherent sand- 

 stone, mapped with the Claremont. There are no conglomerate beds at 

 the base of the Monterey in this section, for here the Eocene rocks, 

 which are more than 4000 feet thick 9 or 10 miles farther northeast, either 

 were not deposited or, as seems more probable, had been completely 

 removed in pre-Monterey time. One justification for the view that the 

 Eocene rocks once extended over the region of the Berkeley Hills and 

 were removed in the Eocene-Miocene interval is that this part of the Coast 

 Ranges contains no representative of the San Lorenzo formation (Oligo- 

 cene), which occurs to the thickness of 2500 feet in the Santa Cruz Quad- 

 rangle. In San Lorenzo time the Berkeley Hills region was probably a 

 zone of erosion. 



The fifth section in which the Monterey is exposed in superposition 

 upon older rocks is at Selby, on San Pablo Bay, in the Napa Quadrangle 

 a few miles north of the San Francisco Quadrangle, where a well-defined 

 unconformity is revealed in a cliff. The surface upon which the Mon- 

 terey rocks rest is a wave-cut terrace, perforated by many holes made 

 by boring molluscs. The strata in which this terrace is cut are soft 

 black shales, which are probably Martinez in age, for Martinez fossils 

 have been found in the sandstones that adjoin them on the north. These 

 shales have a southerly dip of about 70°, and the surface of the per- 

 forated terrace and the superimposed sandstones of the Monterey dip 

 in the same direction at about 60°. It is evident that the shales were 

 e!evated above sea-level and inclined at about 10° to the horizon when 

 they were truncated to form the terrace. The sandstone of the Monterey 

 contains a fossil fauna which, in the opinion of Professor J. C. Merriam, 

 is that of the middle Monterey. The Tejon is apparently absent here, 

 although it is abundantly represented only a few miles to the southeast, 

 along the strike of the rocks. The Monterey sea evidently did not extend 

 over this part of the region until middle Monterey time, and therefore 

 part of the erosion is referable to early Monterey time. In general, how- 

 ever, the historical facts that are so clearly manifest at Selby are con- 

 sistent with and support the interpretation of the section in the Berkeley 

 Hills, where the Monterey rests directly upon the Chico. 



The middle Miocene, Turritella ocoyana zone, in the vicinity of 

 Stone Canon in Monterey County rests directly upon the Franciscan 

 and upon remnants of Chico without any intervening Eocene strata. 

 This region is west of the Coalinga District where the Tejon is fairly 

 well exposed. Fairbanks 85 reported no Eocene in the San Luis Quad- 

 rangle, but the Miocene rests upon granite, Franciscan, Knoxville or 

 Chico indifferently. He described conditions as follows : 



With the close of the Cretaceous and the advent of the Tertiary a 

 marked change took place. The region of the Coast Ranges began to 

 rise and the ocean was excluded from the greater portion of it. The 

 water continued, however, to occupy Sacramento and San Joaquin Val- 

 leys, maintaining an outlet to the south across Ventura and eastern 



85 Fairbanks, H. W., San Luis Folio, U. S. Geol. Surv. Folio No. 101, 

 p. 9, 1904. 



