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Between Newhall and Castaic in the eastern part of the Ventura
basin, the San Gabriel fault, one of the major faults of southern Califor-
nia, trends northwest across the basin and marks a significant strati-
graphic break between the western and easternmost portions of the basin.
This fault marks the eastern limit of both oil production and the thick
section of marine sediments. Except for minor amounts of marine
sediments, the easternmost Ventura basin is underlain by a thick series
of nonmarine sediments and interlayered volcanic rocks. To replace
the cumbersome term, easternmost Ventura basin, Bailey and Jahns
(1954) have suggested the name Soledad basin, which implies a sharper
distinction between the areas on opposite sides of the San Gabriel fault.
This study is mainly concerned with the northern and eastern two-
thirds of the Soledad basin (pl. 1).
Members of the various railroad, geological and geographical
surveys of the middle 19th Century passed through this region and made
many scattered geological observations. The fir st attempts at system-
atic analysis of this region were made by Hershey ( 1902a; 19O2b), who
briefly described the major rock units. He named the Pelona schist,
and this designation since has been extended to other areas at some
· distance from the type area in the Sierra Pelona. He also named
several other units, but these designations have been revised by later
investigators. Hershey's oldest Tertiary unit, the Escondido series,
lies directly on his probable pre-Cambrian Gneiss and Pelona Schist
series. His next younger Tertiary unit, the Mellenia series, crops out