The stratigraphy of the northern part of Santa Rosa Island is somewhat similar to that of the Santa Barbara Coastal district. These two areas represent, respectively, the southern and northern margins of the Ventura Basin. The Island area mapped offered a section from Pleistocene to Eocene, with Pliocene evidently absent. Formations recognized include marine terraces, Santa Margarita sandstone, Monterey shale, Rincon shale, Vaqueros sandstone, Sespe, Cozy Dell(?) shale, and Matilija(?) sandstone. An interesting time equivalence between the Vaqueros and upper Sespe formations is indicated.

The Santa Rosa fault is the dominant structural feature of the Island. It trends east-west and divides the Island in half. A horizontal displacement of nearly five miles is indicated. Other significant but smaller faults are the Sandy Point, Garanon, and Arlington. Important folds are the Garanon, Tecolote, and Soledad anticlines and the West End and Becher's Bay synclines.

Five wells have been drilled and abandoned on the Island and a sixth is now being drilled.

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