GEOLOGY. 
67 
uplift, did not occur at the close of Eocene time, but it is probable 
that orogenic movements did bring to a close the period during 
which the Eocene sediments were laid down by raising the strata 
slightly above the sea and preventing for a time further deposition. 
How long this time was is not known, but it corresponds approxi¬ 
mately with the Oligocene. 
LOWER MIOCENE PERIOD. 
The movements immediately following the deposition of the 
Eocene caused no appreciable disturbance in the Eocene strata, and 
when sedimentation recommenced over the same area in lower 
Miocene time neither the old nor the new strata preserved any posi¬ 
tive evidence in their relative position that a time break had occurred. 
The great masses of coarse conglomerate forming the base of the 
lower Miocene portion of the group record a change to conditions 
of very shallow water, and the abrupt change of faunas indicates 
that a long time interval separated their deposition from that of the 
subjacent Eocene. It is most probable, however, that the post- 
Eocene movements, which were gentle, were also somewhat local, 
and that in portions of the Santa Ynez Mountains to the east of the 
region under discussion sedimentation was more nearly continuous. 
At about the close of the Oligocene period the Eocene basin was 
again depressed; deposition of sediments, almost entirely of detrital 
origin and very similar to those previously laid down, ensued in a 
widening area covered by the sea; and subsidence of the land gradu¬ 
ally continued. The Yaqueros formation, which resulted from this 
period of depression, represents the greater part of lower Miocene 
time. 
MIDDLE MIOCENE PERIOD. 
The middle Miocene (Monterey) shale formation is one of striking 
individuality, and conditions of unusual character prevailed during 
its period of deposition. At the beginning of middle Miocene time 
the land sank over a large part of the region of California now occu¬ 
pied by the Coast Ranges and fairly deep water conditions became 
prevalent. The wearing away of extended land areas ceased as they 
became submerged, and the material for the formation of coarse 
detrital deposits was no longer plentiful. Two varieties of deposits, 
which were largely, of organic origin, were the chief ones to be formed 
during the long period- that followed. These were the laminated 
limestones and the much more abundant siliceous shales. Silt of 
extremely fine grain, both of siliceous and argillaceous nature, was 
swept into the sea waters, probably from considerable distances, and 
settled down to form a considerable proportion of the deposits; but 
sand and other coarse detritus found their way only at rare intervals 
