SAN JOAQUIN VALLEY. 
11 
are affected by one or more minor flexures which run obliquely out 
from the main ranges, and it has been pointed out by Anderson and 
1 ack 1 that all of the oil fields on the west side are associated with 
folds of this kind. In other words, all of these fields lie either on 
such an anticline or in the syncline back of it, or extend over both 
anticline and syncline. The Midway-Sunset field extends over 
several minor folds. The reason for the accumulation of oil only 
under these structural conditions need not be considered here, but 
it is important to note that all of the fields on the west side are 
characterized by this type of structure, for it undoubtedly influences 
the circulation of the oil-field waters. 
UNDERGROUND WATER . 2 
A large proportion of the surface water entering San Joaquin 
Valley sinks below the surface and joins the underground circulation, 
which follows the surface flow in general direction but at a far slower 
rate. The rate of movement of underground water is governed by 
several factors, important among which are the gradient of the slope, 
the shape and size of the materials through which the water must 
flow, and the freedom of the outlet by which it escapes. In San 
Joaquin Valley all these factors militate against rapid flow; the gra¬ 
dient is low, the materials generally fine, and the outlet at Carquinez 
Straits is narrow. As a result of these conditions the ground water 
is under artesian head in a wide belt along the center of the valley. 
Near the north end of the valley the waters are practically ponded 
and the deeper waters are highly mineralized, and even in the southern 
part the main valley circulation probably is abnormally sluggish. 
Owing to the sharp contrast in meteorologic and geologic conditions 
between the east and west sides of the valley the ground waters 
present some interesting chemical variations. As the Sierra is a 
region of moderate rainfall, and as the Temblor and Diablo ranges 
are semiarid, practically all the surface water entering the valley 
is brought down by streams on the east side. As this water is the 
drainage of a region underlain by granites and other relatively in¬ 
soluble silicate rocks, it is low in mineral content and carries chiefly 
carbonates. The west side of the valley, on the other hand, is a 
region of very scanty rainfall, nearly all of which sinks beneath the 
surface before reaching the valley proper and circulates through a 
great series of sedimentary formations. These strata contain a 
large amount of disseminated gypsum and other sulphates, which 
1 Anderson, Robert, and Pack, R. W., Geology and oil resources of the west border of the San Joaquin 
Valley north of Coalinga, Cal.: U. S. Geol. Survey Bull. 603, pp. 116-121, 1915. 
2 For a complete account of the underground waters of San Joaquin Valley, see Mendenhall, W. C., 
Dole, R. B., and Stabler, Herman, Ground water in San Joaquin Valley, Cal.: U. S. Geol. Survey Water- 
Supply Paper 398, 1916. 
