GEOLOGY OF THE COAST SYSTEM OF MOUNTAINS. 11 
lower horizon of the beds on the Santa Cruz Coast, as compared with the beds nearer 
San Francisco, indicates a transgression of the Merced sea from the south. The upper 
portion of the Merced section contains so large a proportion of molluscan remains of exist- 
ing species that it has been regarded by Arnold as Pleistocene rather than Pliocene. 
The accumulation of the Merced series to the great thickness above indicated in 
middle and northern California proves local depressions of the coast of over a mile below 
sea level in later Pliocene time. Similar orogenic deformation was in progress at the 
same time on the eastern side of the barrier corresponding to the then axis of the Coast 
Ranges. These movements gave rise to great troughs from which the sea was excluded, 
but which were occupied by fresh water, and filled with sediments equal in volume to 
those of the marine troughs to the west of the barrier. The greater part of these fresh- 
water beds are comprized in the Orindan formation, which may be the equivalent of the 
Cache Lake beds of the Clear Lake district ! and of the Paso Robles in the southern Coast 
Ranges. They have an extensive distribution on the eastern side of the Coast Ranges, 
and in the vicinity of the Bay of San Francisco there intervenes between the base of the 
Orindan and the San Pablo a formation of white pumiceous tuff entirely similar to 
that at the base of the Merced series in Sonoma County, but containing here fresh-water 
fossils. This tuff attains a maximum thickness of about 1,000 feet and is known as the 
Pinole tuff. Thruout the Orindan there are occasional intercalated strata of volcanic 
tuff of moderate thickness. The Orindan lacustrine period was brought to a close in 
the region of the middle Coast Ranges by volcanic eruptions which resulted in extensive 
flows of lava and showers of ashes. Upon these lavas lake basins were later established 
and some hundreds of feet of fresh-water deposits (Siestan formation) accumulated 
in them, which were in turn buried by other lavas. 
The accumulation of the Merced marine beds and the corresponding lacustrine and 
volcanic rocks was brought to a close by an acute and widespread deformation regarded 
as part of the general mountain-making movements which ushered in the Pleistocene in 
western North America. Asa result of these movements, the Merced and Orindan basins 
were folded and faulted, and the basement upon which their contained strata had been 
laid down was lifted in part from a position over a mile below sea-level to one far above 
sea-level. The Pliocene formations were brought within the zone of active erosion and 
the evolution of the present geomorphic features of the Coast Ranges was inaugurated. 
When the degradation of the folded Orindan strata was well advanced, a lake basin was 
established across the edges of these strata and in it accumulated the various fresh- 
water beds and voleanic lavas and tuffs comprizing the Campan series. Ata time within 
the Pleistocene when the geomorphic evolution of the coast had been well advanced to 
its present condition, the coastal belt was deprest 1,000 to 2,000 feet lower than it is at 
present, and then uplifted in stages marked by marine terraces along many parts of the 
coast. Since this there have been oscillations of the region about the Bay of San Fran- 
cisco, the net result of which has been a depression allowing the sea to invade the valley- 
lands and thus make the magnificent harbor to which San Francisco owes its existence. 
In the foregoing sketch of the formations of the Coast Ranges and their historical 
significance, it is desired to emphasize particularly the remarkable series of subsidences 
and uplifts which have affected the coastal region from the beginning of the Franciscan 
to the present. This record of oscillation is in marked contrast to the comparative sta- 
bility of the Sierra Nevada. Except for a marginal strip of its foot-hill slopes, the 
region of the present Sierra Nevada has not been submerged beneath the sea. During 
the geological ages in which the Coast Range region has been repeatedly deprest to 
receive marine sediments, the sum of the maximal sections of which amounts to 65,000 
feet of strata, the western edge of the Sierra Nevada region has probably never been 

1 Described by G. F. Becker, U.S. G.S. Monograph xi, pp. 219-221, 238-242, 
