526 RALPH ARNOLD 
in the southern Coast Ranges, especially, are largely derived from 
granitic rocks and are usually coarser at the base, becoming finer and 
LecennD Teh] See ae 
Sl arene a > ea |e 
FRESHWATER (5 Siam (- | - 
Le 
Wee 
| Upper Miocene 
a =: 
SO ———— —— 
Fic. 4.—Map showing hypothetical 
distribution of land and water on the 
Pacific Coast during upper Miocene 
time. 
finer toward the top, possibly indi- 
cating a subsidence greater than the 
concomitant sedimentation. Excep- 
tions to the rule of coarse basal 
sediments are not uncommon, how- 
ever, and in. the. Santa Cruz 
Mountains and also in eastern 
Monterey County, Cal., the uncon- 
formable deposition of fine shale 
directly upon. older rocks is a well- 
marked phenomenon. ‘This, of 
course, indicated a sudden and 
rather deep submergence of the 
areas in question at the initi ation 
of the upper Miocene. Conditions 
favoring the life of diatoms, so 
marked in the Monterey, continued 
over part of the Monterey diato- 
maceous shale territory during the 
upper Miocene (Santa Margarita 
and Fernando formations). The 
areas of maximum deposition 
during the period were apparently 
on the southwestern side of the San 
Joaquin Valley in western Fresno 
County and in central Ventura 
County, Cal., where thicknesses of 
over 8,000 feet of sediments, belonging largely to the upper Miocene, 
occur. 
EROSION AND VOLCANIC ACTIVITY 
The peneplanation of the Klamath Mountains and the Sierra 
Nevada was probably completed during the upper Miocene, the 
detrital material from these land areas forming the great deposits in 
the San Joaquin and Sacramento valleys and the coastal belt of 
northern California. Erosion was practically continuous in these 
