Recent work by graduates of California State University, Northridge, serves as the basis for constructing six preliminary middle-Tertiary paleogeographic maps of the area represented by the two-degree Los Angeles map sheet of California. The

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maps are made partially palinspastic by a limited restoration of rocks along the San Gabriel and Big Pine faults to the positions occupied during the time represented by each map. The maps show that the area was divided into two depositional basins by the northwest-trending San Rafael highland. Fluvial deposition occurred in both basins during the Oligocene. In the northeastern Cuyama-Soledad basin, Oligocene-Miocene marine deposits transgressed eastward over a large delta. In the southwestern Ventura basin, marine transgression was from the southwest. In the late early Miocene the ocean breached the San Rafael highland and created a strait between the remaining San Rafael peninsula and the newly formed Ynez island. During the medial Miocene, marine transgression continued, further co necting the two basins into one and shrinking the size of Ynez island. Slight marine regression in the southeast at this time was caused by tectonic uplift in the region of the present-day Simi Hills. During the latest middle Miocene, movement occurred on the San Gabriel fault, thus isolating the Soledad basin and creating an inland lake. With continued fault movement, this lake moved southward during late Miocene and merged with the ocean creating a large estuary. Marine transgression continued in the southeast during the late Miocene, while marine regression occurred in the northwest.

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