The Sespe Formation is the first to appear above the "Great Unconformity" and is marked by interbedded mudstones, sandstones and pebbly sandstones. This bed sequence indicates an alluvial fan, meandering stream or braided stream origin. Upward from the Sespe Formation toward the Vaqueros, the grains become finer and the beds become thinner; indicating a transition to a shallow marine environment. The Vaqueros Formation is marked by two sandstone, siltstone and shale units. There are also characteristic mollusk fossils that indicate the area was dominately shallow marine.