A widespread volcanic ash layer, 2 to 10 cm thick, in the “Pliocene” Pico Formation (Wheelerian Microfaunal Stage) has been dated radiometrically. Eight mineral separations from two separate localities give a mean age of 8.69 ± 0.5 m.y. Minerals dated include microphenocrysts of biotite, sanidine, and plagioclase, and microcrystalline aggregates of feldspar; all give concordant ages. The minerals are unaltered and show euhedral grain boundaries ; some feldspar crystals have rims of glass. The glass matrix, comprising 95 percent of the sample, is isotropic and apparently unaltered. But glass ages of 1.1, <0.2, and < 0.4 m.y. are discordant with one another and with the mineral ages, indicating apparent argon leakage; therefore, the glass is useless in radiometric dating despite its unaltered appearance.

The ash layer is near the base of the Santa Barbara Zone, which is defined by a molluscan assemblage resembling that living today in Pacific Northwest waters. The temperature shift from a warm-water molluscan assemblage in underlying beds to the coldwater Santa Barbara fauna was previously correlated with the onset of Pleistocene glaciation and, because beds containing the cold-water mollusks are folded, the age of folding (“Pasadenan” orogeny) has traditionally been considered intra-Pleistocene. The new radiometric age suggests that both the molluscan temperature shift and “Pasadenan” orogeny occurred in Pliocene time.