Age of the Martinez Formation of Pacheco Syncline, Contra Costa County, California

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An excellent section of Eocene strata is exposed on the southwest limb of Pacheco syncline, south of the town of Martinez, California. The lower Eocene beds there represent the type Martinez formation, a name proposed by J. D. Whitney, in Gabb, 1869, p. xiii, (see bibliography). The line of section discussed in the present report runs through Muir Station on the Santa Fe Railway, parallel to and one mile distant from Gabb's (1865, p. 14) section. The exact locality is shown on the United States Geological Survey map of the Concord Quadrangle, and on the accompanying map, Figure 1. The purpose of this report is to present evidence to prove that the upper part of the type Martinez formation is of middle Eocene age. The species found in the strata on the southwest limb of the Pacheco syncline are recorded on the accompanying columnar section (Figure 2).