108 SANTA MARIA OIL DISTRICT, CALIFORNIA. 
this work nearly all of the territory of the hills between San Luis 
Obispo Creek and the Arroyo Grande Valley, with the exception of 
a rather small area of Monterey volcanic ash, shale, and diatoma- 
ceo.us earth north of Pismo, is covered by the Pismo formation. 
This formation is composed of sandstone, some of which is asphaltic, 
and cherty diatomaceous beds, and is the equivalent of the lower 
part of the Fernando formation as described for the hills adjacent to 
the south side of the Santa Maria Valley. The Pismo is unconform- 
ably underlain by the Monterey shale, which outcrops on either side 
of it. 
STRUCTURE. 
According to Fairbanks, the Pismo area forms a low syncline, 
striking northwest and southeast, its flanks resting against the up¬ 
turned Monterey. 
OCCURRENCE OF THE OIL. 
The oil is derived from a great thickness of productive sands which 
probably represent the base of the Pismo and which rest upon the 
upturned and more or less contorted shale of the Monterey. Its 
occurrence in beds occupying a synclinal position is worthy of note, 
as ordinarily synclines are not highly productive. The Monterey is 
the oil-bearing formation in the Santa Maria district, and it is the 
ultimate source of the oil in this field also. The migration of the oil 
probably took place along joint cracks in the shale, as was the case 
with the asphaltum in the Santa Maria and other fields. The oil, on 
reaching the upper limit of the shale passed across the plane of uncon¬ 
formity and accumulated beneath an impervious shale in the porous 
sands at the base of the Pismo. Where this porous layer approaches 
the surface the more volatile parts of the oil have escaped arid there 
remains nothing but the bitumen, while the more deeply covered 
sands retain the oil in its lighter and liquid state. The migration of 
the oil, as in every similar case coming under the notice of the writers, 
has been accompanied by a loss of its volatile constituents and a con¬ 
sequent lowering of the gravity. This is evidenced by the fact that 
although the gravity of the oil from the Monterey formation in the 
Santa Maria field averages about 25°, that from the Pismo in the 
Arroyo Grande field is only 14°. 
CONCLUSIONS REGARDING FUTURE DEVELOPMENT. 
It seems almost certain that considerable portions of the Pismo 
formation toward the middle of the area northwest and north of 
Arroyo Grande will be found to be oil producing. This conclusion is 
based on the assumption that the Pismo of this region is underlain 
