eldridge] PETROLEUM FIELDS OF CALIFORNIA. 307 
which it is named, lying somewhat nearer the northern end. Three 
areas of oil development exist, which may be designated the Oil City 
field, the Kreyenhagen field, and the Avenal field, but the first only 
is of special productiveness. Coalinga is accessible by rail from the 
main lines of both the Southern Pacific and the Atchison, Topeka 
and Santa Fe railroads. 
The topographic features of the region are those of a high, rugged 
range, bordering a desert. The line of mountain and desert passes 
southeast from Coalinga in a direct course for 30 miles, but immedi- 
ately north of the town there is the reentrant angle of a valley and syn- 
cline which separates the main range from one of the diagonally 
transverse spurs and anticlines that are such conspicuous features of 
the structure of the Coast Range. It is at the southeast end of this 
anticline that the Oil City petroleum field has been developed. 
The formations involved in the anticline embrace at least 1,000 to 
2,000 feet of massive concretionary sandstones of Tejon (Eocene) age, 
overlain by 800 to 1,000 feet of purple and gray shales, clays, thin 
sandstones, and limestones, that have also been referred by some to 
this period; 100 or 200 feet of clays and sandstones that may prove to 
be Lower Miocene ; 200 feet of siliceous shales typical of the Monterey 
(Upper Miocene); and, unconformable with these, a great thickness 
of conglomerates, sandstones, and clays, recognized by their fossils to 
be San Pablo (Middle Neocene). The conglomerates of the San Pablo 
in this region contain pebbles of quartz, black chert, jasper, serpen- 
tine, siliceous shale, and sandstone, the matrix being of the same 
materials; the sandstones, which are coarse, are chiefly quartzose; 
the clays are generally gypsiferous. 
The Oil City field, as already suggested, is developed about the 
southeastern terminus of one of the diagonally tranverse anticlinal 
spurs that extend from the Coast Range into the valley of the San 
Joaquin. The axis here dips rapidly to the southeast, and within 10 
miles of the higher crest of the range evidence of the fold has com- 
pletely disappeared beneath the valley deposits. The line of junc- 
tion between mountain and desert on the northeast side of the fold 
extends for 20 to 30 miles without conspicuous break. With the excep- 
tion of severe crumpling in the immediate vicinity of the axis, accom- 
panied perhaps by some faulting, and a comparatively gentle flexure 
on the southern periphery of the uplift in the vicinity of Oil Creek, 
the anticline appears to be unaffected by minor folds. The measures 
exposed in the heart of the anticline are the massive Tejon sandstones. 
Encircling these are the overlying shales, and these in turn are fol- 
lowed by the heavy and resistant conglomerates and sands of the San 
Pablo. 
The oil-bearing horizons of this field are two: one, a sandstone in 
the lower portion of the shales that are by some regarded as the upper 
member of the Tejon; the other, the lower sandstones and conglomer- 
