TERTIARY FAUNAS OF THE PACIFIC COAST 239 



usually more noticeable toward the base. The Modelo formation 

 of Ventura County, the probable equivalent of the Monterey, con- 

 tains two important coarse sandstone zones. In the region of Mount 

 Diablo the Vaqueros and Monterey formations comprise alternations 

 of sandstone and shale. In Washington and Oregon the whole lower 

 Micoene is largely sandstone with some associated shale, A gradual 

 gradation between the two formations is the rule, although their 

 contact is often sharply marked and in some places is an angular 

 unconformity.' The thickness of the Vaqueros is as much as 3,000 

 feet, that of the Monterey over 5,000 feet, a total for the whole of the 

 lower half of the Miocene of over 8,000 feet. 



CONDITIONS OF DEPOSITION 



The deposition of the lower Miocene (Vaqueros) sediments was 

 inaugurated over much of the submerged territory, along the shores 

 of islands of sharp relief. Erosion and deposition were rapid within 

 local basins, especially in the region from the Santa Cruz Mountains 

 southward to San Luis Obispo County, and still there were localities 

 within these areas of intense sedimentation where deposition was 

 slow. It is the belief of the writer that these variations were depend- 

 ent, at least in part, on the positions of the areas in question relative 

 to the steep or low slopes of tilted fault blocks. 



Over those portions of southern California, such for instance as 

 in Ventura County, where the sea supposedly occupied the present 

 land-area during the Oligocene, the conditions during the Vaqueros 

 (lower Miocene) were quite different from those northward in the 

 Coast Range archipelago. Instead of the littoral conditions accom- 

 panied by rapid and coarse sedimentation of the latter province there 

 was in the Ventura County area deep water with slower deposition 

 and finer sediments, especially in the earlier Miocene. 



The lower middle Miocene (Monterey) shale formation is one of 

 striking individuality, and conditions of unusual character prevailed 

 during its period of deposition.^ The land which had begun to 

 subside at the beginning of Miocene time, later, at the inauguration 

 of the middle Miocene, sank over a large part of the region of Cali- 



1 Branner, Newsom, and Arnold, Santa Cruz Folio. 



2 For a fuller description of the Monterey see A. C. Lawson and J. D. L. C. 

 Posada, Bull. Dept. Geol. Univ. Cal., Vol. I, pp. 22 ff. ; H. W. Fairbanks, ibid., Vol. II, 

 pp. 9 ff. ; Ralph Arnold and Robert Anderson, U. S. Geol. Survey Bull. ^22, pp. 35 ff 



