Rincon Formation is massive to poorly bedded, and consists of predominantly argillaceous to silty shale and mudstone, with occasional dolomite. Two layers of siliceous shale are found in the middle of the unit, and they outcrop noticeably in the region west of Goleta. The base of the unit, which rests conformably on the shallower-water Vaqueros Formation, contains a glauconitic layer. Several thin layers of bentonite occur within the unit, and the unit is capped by one as well, which forms the base of the conformably overlying Monterey Formation. North of the Santa Ynez River, the Rincon is typically capped not by the Monterey but by the Temblor Formation, a sandstone similar to the underlying Vaqueros; additionally, north of the river the Rincon Formation grades into this sandstone to the east. Northeast of Blue Canyon and the Little Pine Fault, in the backcountry of easternmost Santa Barbara County and Ventura County, the Rincon Formation disappears, and the Temblor Formation rests unconformably on the much older Cozy Dell Shale. The Rincon Shale was deposited on the deep sea floor during the time at which the Miocene sea reached its greatest depth. The faunal assemblage found as fossils and fragmentary fossils indicates that the sea was tropical to subtropical at this time. During this period the sea deepened due to tectonic forces, as the area of deposition was at the collision boundary of the North American and Pacific Plates, shortly after the Farallon Plate had been completely buried beneath the American plate. At that time only clay- and mud-sized grains could reach the deposition location, since it was far from the shore, and sediment accumulated slowly. The unit represents a period of around 7.5 million years – from 24 Ma (million years before present) to 17.5 – covering, with the Vaqueros Formation, the entirety of the lower Miocene. The Rincon Formation weathers to a distinctive loamy, black soil with a high expansion-contraction potential, presenting a risk to structures built on soils derived from the formation. Landslides and slumps are frequent on soils produced from the Rincon, although they usually are not large, limited to about 15 feet in depth. As is common with shale units, the formation has a low permeability, and therefore where oil and gas reservoirs occur in the area it serves as the impermeable cap keeping the hydrocarbons in place. Some of these reservoirs include the Ellwood Oil Field, Gaviota