Under the auspices of the Scripps Institution of Oceanography three visits have been made since April, 1955, to areas on the west coast of Baja California, 300-350 miles south of San Diego. Reconnaissance geologic maps have been prepared of the San Benito Islands and the nearby northwest part of the Vizcaino Peninsula.

Laboratory study of the rocks and fossils is not yet complete, but systematic examination of the areas shows the following.

1. Probably more than 10,000 feet of pre-Miocene chloritic sandstones, grits, and conglomerates occur in two series separated by faulting. A variety of intrusive and extrusive igneous rocks is confined to the older series, and probable Middle Cretaceous ammonites occur sparingly in the younger series.

2. These are unconformably overlain by more than 1,700 feet of highly fossiliferous middle Miocene siltstones, sandstones, and shales, locally of Monterey facies.

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3. Unconformably on the Miocene are estimated 500+ feet of extremely fossiliferous Pliocene silts and sandstones.

4. Fossiliferous marine Pleistocene terrace deposits are faulted and gently tilted on the mainland. Emerged wave-cut benches are prominent on the islands.

5. The dominant structural pattern is a series of large northwest-trending faults separating the region into strips of older and younger rocks. Some faults have shear zones hundreds of feet wide containing a remarkable assemblage of more or less serpentinized basement rocks, most of which are not found in outcrops elsewhere. Four episodes of deformation can be differentiated; the earliest is pre-Miocene and the latest post-Pleistocene.

6. The San Benito Islands share in the northwest structural grain but lack the Miocene and younger rocks. Glaucophane schists, red cherts, graywackes, altered basic volcanics, and serpentine are highly sheared and suggest that these islands may be entirely within a large northwest-trending fault zone.

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