Central San Clemente Island is underlain primarily by nonmarine volcanic rocks. A 1,200-ft core hole, drilled near sea level on the west coast of the island, penetrated a homogeneous sequence of basaltic andesite flows varying in thickness from 11 to 169 ft. Whole-rock potassium-argon dates of samples taken near the top and bottom of the core hole indicate that the cored sequence was extruded in less than 1 m. y. during Miocene time. Unconformably overlying the andesitic flows are dacitic flows reaching a total thickness of about 300 ft. A distinctive volcanic breccia, which is in part of sedimentary origin, is commonly present at the base of the dacites. Miocene sediments and Quaternary beach sands overlie the volcanic rocks.
The Tertiary rocks are folded into a northwest-trending anticline. The axis of the anticline corresponds approximately to the topographic crest of the island located about .5 mi inland from the eastern shoreline. North-northeast to northeast-trending faults cut the Miocene rocks but do not displace the prominent surf-cut terraces. Striations within well-exposed fault zones indicate that most movement has been oblique, with a principally horizontal component. The faults may be left-lateral, secondarily related to major right-slip movement along the northwest-trending San Clemente fault.