The Rincon Formation is a sedimentary geologic unit of Lower Miocene age, abundant in the coastal portions of southern Santa Barbara County, California eastward into Ventura County. Consisting of massive to poorly bedded shale, mudstone, and siltstone, it weathers readily to a rounded hilly topography with clayey, loamy soils in which landslides and slumps are frequent. It is recognizable on the south slopes of the Santa Ynez Mountains as the band at the base of the mountains which supports grasses rather than chaparral. Outcrops of the unit are infrequent, with the best exposures on the coastal bluffs near Naples, in the San Marcos Foothills, at the Tajiguas Landfill, and in road cuts. The geologic unit is notorious as a source of radon gas related to its high uranium content, released by radioactive decay.
Weathered outcrop of the Rincon Formation, Sycamore Canyon, Santa Barbara, California.
Close-up of the Rincon Shale in a road cut, showing weathered, highly fractured surface.
The Monterey Formation is an extensive Miocene oil-rich geological sedimentary formation in California, with outcrops of the formation in parts of the California Coast Ranges, Peninsular Ranges, and on some of California's off-shore islands. The type locality is near the city of Monterey, California.
The Monterey Formation is the major source-rock for 37 to 38 billion barrels of oil in conventional traps such as sandstones. This is most of California's known oil resources. The Monterey has been extensively investigated and mapped for petroleum potential, and is of major importance for understanding the complex geological history of California. Its rocks are mostly highly siliceous strata that vary greatly in composition, stratigraphy, and tectono-stratigraphic history.
Outcrop of Monterey Formation, Gaviota State Park, California
Upper Miocene Migeulito member exposed at Hazard Reef, Montana de Oro State Park. This is the first onshore outcrop of the Monterey Formation south of the Monterey Peninsula. Large exposures continue south along the coast, often associated with important oilfields.
Tar "volcano" in the old Carpinteria Asphalt mine. Heavy oil exudes from joint cracks in the upturned Monterey shale forming the floor of mine. 1906 photo, U.S. Geological Survey Bulletin 321
Fold in Monterey Formation