522 RALPH ARNOLD 
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- 
t 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, zb7d., Vol. IT, 
pp. 9 ff.; Ralph Arnold and Robert Anderson, U.S. Geol. Survey Bull. 322, pp. 35 ff. 
