San Nicolas Island is the outermost of a group of eight islands off southern California. Point Mugu is the nearest point on the mainland, about 63 miles north; Los Angeles City Hall is approximately 90 miles northeast.

A geologic study of San Nicolas Island was begun by personnel of the U. S. Geological Survey in March, 1955, at the request of the director of Naval Petroleum Reserves.

About 3,550 feet of alternating sandstone, siltstone, and conglomerate constitute the exposed Tertiary section on the island. Foraminiferal studies indicate that these rocks are of late Eocene age. Several small igneous dikes that may be Miocene cut the sedimentary rocks in the southeastern part of the island. Quaternary dune sand and marine terrace deposits cover much of the central and western parts of the island.

Structurally, San Nicolas Island is a broad complexly faulted anticline with a southeast plunge. The axis roughly parallels the long dimension of the island and lies near the southwest shoreline. Two sets of intersecting faults which trend approximately north-south and east-west appear to have formed almost simultaneously. Most of these faults are high-angle normal faults, the largest having maximum apparent vertical displacement of about 400 feet.

Geologic diving operations were conducted off the west end of the island in an attempt to trace the seaward extent of the anticline mapped on shore. Self-contained underwater breathing apparatus was used by the divers in the study of approximately 5 square miles of sea floor at depths ranging from 30 to 120 feet.

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