Note: This paper is dedicated to Aaron and Elizabeth Waters on the occasion of Dr. Waters' retirement.
The Pinnacles Volcanic Formation and the Neenach Volcanic Formation originated from a series of common eruptive centers during the early Miocene. The once contiguous units were subsequently split and separated 314 km from each other by movement along the San Andreas fault. The actual site of the eruptions has important implications for the age of the big bend segment of the San Andreas fault. If the eruptions occurred in southern California with the Pinnacles Volcanic Formation moving northward, then the big bend segment must have been active during early Miocene time. Alternatively, if the eruptions occurred in northern California with the Neenach Volcanic Formation moving southward, the big bend segment of the fault need be no older than Pliocene-Pleistocene. Early Miocene paleogeographic reconstructions which do not place the Pinnacles and Neenach areas adjacent to each other are probably not valid.