The Cudahy Camp Formation, located in the El Paso basin, east-central California, consists of a 350-m-thick volcanic and sedimentary sequence of early Miocene age. Previous investigators determined from paleocurrent indicators that the source for these rocks was to the south and southeast, but failed to suggest a specific location for that source. Restoration of 64 km of left slip on the Garlock fault places Cudahy Camp rocks north and northwest of the early Miocene Eagle Crags volcanic field. The unique geochemical nature of basalts found in both locations, coincident ages of volcanic rocks, identical clast content of volcanic tuffs, and identical stratigraphic sequence indicate that the source for the Cudahy Camp Formation was the Eagle Crags volcanic field. This is the first documented occurrence of Miocene rocks offset by the Garlock fault. Because the Cudahy Camp rocks and their sources appear to be offset by the full displacement on the Garlock fault, we conclude that movement on the Garlock started after earliest middle Miocene time. Further constraining initiation of movement on the Garlock fault adds significantly to fully understanding the evolution of the western North American plate boundary in the middle to late Cenozoic.