Oscillations of /he Calif ornia Coast. — Fairbanlcs. :235 
from the land, where it begins to descend rapid!}'. While it 
does not seem that these valleys can be accounted for through 
structural conditions, the hypothesis of stream erosion does 
not at first sight appear well supported. However the fact 
that the mountains rise so ruggedly behind them to such 
hights, would, witJi increased elevation, result in the cutting of 
considerable canons across the plateau even if no large rivers 
debouched here. Their narrow deep character like those 
farther south would indicate a comparatively brief period of 
erosion. 
Evidence from the Present River Valleys. Our knowledge 
of the submerged valleys is not complete enough at present so 
that we can decide certainly concerning earlier elevations from 
this source alone. But there is another line of research which 
t ken together with what has been presented leads irresistibly 
to the conclusion of the much greater elevation of the Pacific 
coast at several periods in the course of its history. This oth- 
er question deals with the character of the present stream 
valleys where they debouche upon the coast. Nearly all the 
streams of the California coast are bordered by broad alluvial 
bottoms wh^re they enter the ocean and for many miles back. 
They are not only at their baselevel of erosion but are actual- 
ly flowing upon beds of unconsolidated sands and gravels of 
unknown depth. The bottom lands at the mouths of some of 
the rivers are 8 to 15 miles wide with no solid rock apparent 
anywhere except in the hills flanking them. What is the ex- 
planation of this fact? One of two solutions is possible, ei- 
ther they represent structural depressions which have been 
filled \x\) to the present baselevel, or ancient valleys eroded 
during some former elevation and subsequently depressed and 
filled with sediments. 
The main vallej'-s of the Coast ranges generally follow the 
structural features and if they all did so the question could 
not be greatly elucidated by a study of them. There are how- 
ever many exceptions to the rule, especially among the lesser 
valleys, some of the more important of which will be discuss- 
ed. Perhaps the use of the term structural depression ought 
to be more fully explained. By it I mean a synclinal trough 
originated by folding, l.ying either below the sea or baselevel, 
and which has subsequently been filled by sedimentation. 
