LOS ANGELES DISTRICT: STRUCTURE. 155 
Aside from their scientific value, these bones have the added sig¬ 
nificance of indicating that the brea-forming conditions which are 
now prevalent in this field were in operation during a large portion 
of Pleistocene time. In view of the great number of years during 
which the gas and oil have been escaping in this field, it seems rather 
remarkable that such prolific deposits of petroleum are still to be 
found in the underlying strata. These facts indicate the almost 
incomprehensible amount of oil that was originally contained in the 
Tertiary rocks over certain areas of the Coast Range belt. 
STRUCTURE. 
GENERAL FEATURES. 
The structural features in the region about Los Angeles are depend¬ 
ent on two systems of disturbances. The older of these consists pri¬ 
marily of folds, with jvhich are usually associated minor faults. This 
system dominates the Miocene strata and was probably largely devel¬ 
oped during the post-Puente (late middle Miocene) readjustment, 
which had such a widespread effect throughout the Pacific coast of 
the United States. The axes of disturbance of the Miocene system 
trend, approximately, N. 60° W. The younger system, in which 
faults are apparently the dominant features, was developed during 
late Pleistocene time and affects all the formations in the area. The 
general trend of the axes of the Pleistocene system is east and west. 
Minor faults with planes striking in various directions are common 
throughout the district. 
The most prominent structural feature in the district is the great 
flexure in the Puente sandstone and shale which lies northeast of the 
business portion of Los Angeles and trends N. 60° W. This will be 
referred to as the Elysian Park anticline. (See PI. XX, sec. E-F.) 
This anticline might almost be regarded as an elliptical structural 
dome, as it appears to plunge at both its northwest and southeast 
ends. Not far from the northwest extremity of the anticline where 
it approaches the fault zone lying along the southern base of the 
Santa Monica Mountains the fold develops into a fault. Near the 
axis of the anticline the beds in the southern limb dip gently, the 
slope becoming steeper and steeper, however, toward the south 
until, in the region of the shaly beds, dips of 60° are not uncommon. 
The northern limb is obscured by erosion, but the rocks composing 
it appear to dip at angles as great as 40°. Toward the south end of 
the anticline minor flexures are developed on its flanks and these in 
turn are squeezed up into what appears to be one or more closely 
folded overturns in the hills 1| miles east of Eastlake Park. Other 
folds of considerable extent and at least one prominent fault trending 
approximately in the same general direction as the Elysian Park anti- 
