With the exception of the Pleistocene terrace deposits, all of the sedimentary rocks exposed in the San Joaquin Hills are Tertiary in age. The Tertiary section has a thickness of about 5,000 feet and is correlated with rocks of Sespe, Vaqueros, Topanga, San Onofre, and Puente formations. The dip of strata is thought to be largely due to the action of normal faulting, rather than folding. Basic lavas are found intruded as dikes along or near the planes of many of the larger faults. While the age relationship between the faulting and the intrusion is not clear, it is thought that they are in part contemporaneous and that both are, at least, pre-Pleistocene in age. It is suggested that the present topography of the hills has resulted from a gentle northward tilting of the ar a occurring in Pleistocene and Recent time

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