Along a small portion of the Caballero Trail the Upper Jurassic Santa Monica Formation is exposed. This rock unit began in the Jurassic as a marine, deep-water black shale, but was moderately metamorphosed during the Upper Cretaceous by intrusion of plutonic ignous rocks. The rock is now a dark gray, severely fractured, low-grade slate. The contact between the Santa Monica Formation and the overlying Trabuco Formation is an unconformity that represents several tens of millions of years of uplift and erosion of the Santa Monica Formation before deposition of the Trabuco Formation.
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