10 OIL-FIELD WATERS IN SAN JOAQUIN VALLEY, CAL. 
age. At the loot of the Sierra these rocks are overlain by practically 
unaltered Tertiary sediments, which dip gently to the west and dis¬ 
appear beneath the sand and gravel of the valley proper. These 
Tertiary formations are not everywhere present, but are well devel¬ 
oped in the neighborhood of the Kern River field. The lower of 
the Tertiary formations is made up in part of diatomaceous shale, 
which is probably the source of the oil; the upper division consists 
of coarse sand, gravel, and clay, into which the oil has probably 
migrated, and in which it is now obtained. 
The oldest rocks on the west side of the valley are probably of 
Jurassic age and comprise a series of altered igneous and sedimentary 
rocks. This formation forms the backbone of the Diablo Range 
north of Coalinga and probably extends beneath the surface for 
some distance to the south. It is overlain on the valley side by a thick 
series of Cretaceous shale and sandstone, from the upper part of which 
oil in commercial quantities is produced in the Oil City district of 
the Coalinga field. The Cretaceous is overlain by rocks of Eocene 
and Oligocene (?) (earlier Tertiary) age, above which is a mass of 
sandstone and diatomaceous shale belonging to the middle Tertiary. 
Shale, sandstone, and conglomerate of later Tertiary age (upper 
Miocene and Pliocene) overlie these formations, in most places un- 
conformably, and these are in turn overlain by similar Quaternary 
sediments. The later Tertiary deposits, which are practically un¬ 
consolidated, were probably laid down under alternating marine, 
brackish, and fresh-water conditions; the series as a whole is charac¬ 
terized by many minor unconformities and by the lenticularity of 
its individual beds. 
In the Coalinga district the oil is believed to have formed in the 
Oligocene (?) shale and to have migrated up into overlying beds, 
which are partly middle Tertiary and partly younger. In the 
Temblor Range fields to the £outh most of the oil probably originated 
in the thick beds of diatomaceous shale of middle Tertiary age; 
it is now found in the irregularly stratified rocks of the later Ter¬ 
tiary. As the lower part of these lenticular masses constitutes the 
main oil measures everywhere except in part of the Coalinga field, 
the correlation of individual oil and water sands by a study of the 
well logs is very difficult. 
This whole sedimentary series, resting in what may be considered 
broadly as a great eroded anticline, constitutes the Diablo and 
Temblor ranges. The older rocks, closely folded, form the main 
mountain mass; the younger rocks, more gently folded but dipping 
in general away from the axis, form the lower flanks and foothills. 
The oil-bearing strata outcrop well down at the foot of the range, 
where the prevailing dips are toward the north or northeast at a 
comparatively low angle. In many places, however, these strata 
