 The Temblor Range and surrounding region contains extensive outcrops of the Monterey Formation (Miocene age, about 20 to 9 million years).Rocks from the Monterey formation consist mostly of silicate shales and porcellanite (silica derived from fossil plankton in an intermediate to deep-water marine setting).Fossils and sediments from the Monterey Formation show that the Carrizo Plain region was a marine basin with shallow to intermediate depths (marine waters covered the southern San Joaquin Valley region).