572 NOAH FIELDS DRAKE 
Lawson has shown? that a considerable part of this region 
has been eroded to base level and when this area was elevated 
to a plateau, erosion followed the weak lines, dissecting the 
plateau until only the tops of ridges are now left as evidence of 
the once leveled region. The larger ranges and valleys, how- 
ever, appear to be of orogenic rather than erosive origin. Local 
shifting of land elevations or the irregular tilting of some small 
blocks of the earth’s crust has divided some of the larger val- 
leys such as Russian River—Petaluma Valley, and the Santa 
Clara-San Benito Valley.. In both cases the adjoining valleys 
are continuous, but from tilting of earth-crust blocks the valleys 
are slightly divided so that their drainage runs into the ocean or 
bays at different places. An elevation of the southern end of 
the Santa Clara Valley has turned the San Benito River to one 
side so that it now flows through a narrow outlet into the Bay 
of Monterey instead of continuing in the straight and open val- 
ley to the north and emptying into San Francisco Bay. Simi- 
larly the southward continuation of the Russian River Valley 
leading into San Pablo Bay is so tilted as to throw the Russian 
River drainage to one side through a narrow outlet into the 
ocean. Such local tilting of the earth’s crust causing the flood- 
ing of valleys is the origin of San Francisco Bay, Tomales Bay,’ 
and probably? Monterey Bay. This shifting has been so late 
that the effects of subsidence are plainly shown in adjoining val- 
leys. Tomales Bay‘ is clearly one of these drowned valleys. 
It is a long riverlike bay that is about three-quarters of a mile in 
width and fifteen miles long. The valley of the bay continues 
to the southward until it reaches the ocean again. 
The great valley of Californa.—The California Valley, lying 
between the described mountain systems, is a low, level area 
about 400 miles long and 50 miles wide. The width of the val- 
ley is quite regular throughout, but is somewhat greater at the 
southern end and a little north of the center. At this latter 
* Bull. Dept. Geol. Univ. Calif., Vol. I, No. 8, pp. 242-244. 
? A. C. Lawson, Bull. Dept. Geol. Univ. Calif., Vol. I, pp. 263-269. 
3A. C. Lawson, Bull. Dept. Geol. Univ. Calif., Vol. I, p. 59. 
4A. C. Lawson, Bull. Dept. Geol. Univ. Calif., Vol. I, No. 8, p. 264. 
