236 ^ The American Geologist. October, i«.7 
Faulting as well as folding may cleterniine the original courses 
of the main streams but be followed hy extensive erosion if 
elevated above the ocean sufficiently. In the latter eases I 
would not consider the valley, for the purposes of this discus- 
sion at least, as of structural origin. 
I will now enter upon a description of some of these valleys 
beginning at the south, and attempt to discriminate the phen- 
omena of the post-Pliocene elevation from the otiiers and also 
the indications of a very recent subsidence. The streams 
which tlow westward to the ocean across San Diego county are 
baseleveled in their lower portions having cut sharply defined 
valleys across the mesa. Near the ocean the^y have broad al- 
luvial bottoms the depth of which is not known. It must 
however in many cases be several hundred feet judging from 
the abrupt nature of the valle}^ walls. The streams entering 
the ocean from the San Diego river southward to the Mexican 
line have not as far as known cut through the Pliocene to ex- 
pose any older sedimentary rocks. The lower valley of the 
Tia Juana river is particularly broad and here the alluvium or 
Pleistocene is apparently very deep. The thickness of this de- 
posit in the valley bottom is something less than an earlier 
Pleistocene elevation. How much less it is impossible to saj- for 
that elevation would have thrown the coast line nearly 12 miles 
to the west. There has taken place however a recent subsid- 
ence of moderate amount which complicates matters. In dis- 
cussing the movements of the coast of southern California 
Lawson failed to recognize the fact of this subsidence, which 
is very marked at the mouths of many of the smaller streams. 
From Del Mar northward nearl}^ all the valleys are liooded, the 
tide waters entering some of them a distance of nearly two 
miles, while the depressions of their mouths are often below 
level of low tide. Tiie sea cliffs are also actively being eaten 
away. The streams about the bay of San Diego are not appar- 
ently flooded but it is quite possible that this bay as well as 
Falso bay has been formed by this very recent depression. 
The fact that we do not know just how much the recent sub- 
sidence has been makes it difficult to discriminate with cer- 
tainty between the erosion of the early Pleistocene and that 
preceding this depression, but I think we are justified in as- 
suming that the broad and tlat baseleveled valleys with allu- 
