Since 1965 the U.S. Geological Survey has been systematically mapping the geology of a strip about 40 mi wide and 500 mi long along the San Andreas fault. The geology is being compiled and mapped on seven 80-mi-long segments, one after the other, on a topographic base prepared at a scale of 1:125,000 (1 in. = 2 mi). This involves field checking of available geology and mapping areas inadequately mapped or unmapped.

Geologic mapping has been completed for the first segment, which extends from Cholame to Elkhorn Hills, and includes the Pyramid Hills, Reef Ridge, Orchard Peak, Temblor, Caliente, La Panza and Sierra Madre Ranges, Carrizo Plain, and Cuyama Valley.

Regional mapping of this segment resulted in slight revision of the stratigraphy. The Pliocene marine section along the west border of the San Joaquin Valley is mapped as the Etchegoin Formation and is divided into three members where divisible. In the Sierra Madre and La Panza Ranges, the enormously thick marine clastic section of Late Cretaceous(?) age is now believed to be of Paleocene and Eocene ages. East of the San Andreas fault the thick Cretaceous marine section above the Franciscan Formation is divided into three formations.

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