The nonmarine redbeds of the Sespe Formation and the overlying gray marine sandstones of the Vaqueros Formation are widespread in southern California, and in most places yield Eocene and Oligocene mammals and molluscs. However, in the Santa Ana Mountains of northern Orange County, California, these same formations yield early Miocene (early Hemingfordian) mammals. Many of these fossils are also found in classic early Hemingfordian faunas of the High Plains, such as the Marsland fauna of Nebraska, the Flint Hill fauna of South Dakota, and the Martin Canyon A fauna of Colorado. A 3500-foot-thick section was taken along recent excavations of the Orange County Transportation Corridor south of Jamboree Road. The normal and reversed directions, held both in magnetite and hematite, passed a reversal test. The pattern of magnetic polarity zones most likely correlates with Chrons C5Cr to C5Er (17.0-19.0 Ma), consistent with the fission-track date of 19.2 + 0.5 Ma beneath the Marsland Formation in Nebraska.