Geology of the Hollister Field Office area is extraordinarily diverse. Most of the geology in the region owes its diversity and complexity to the formation of the California Coast Range accretionary wedge.
Several hundred million years ago, the Pacific plate was sliding beneath the North American continental plate in a process called subduction. As the Pacific plate slid beneath the North American plate, rocks on its surface were scraped onto the edge of the North American plate, resulting in a chaotic jumble of different rock types called an accretionary wedge. Rock types included a wide variety of marine sedimentary rocks of different ages and at different stages of metamorphism (mudstone, sandstone, conglomerate, shale, schist, chert, limestone, as well as igneous rocks (basalt, granite, peridotite, serpentinite).
As the subduction fault transitioned to the San Andreas strike slip fault, compression caused the accretionary wedge to rise, creating the California Coast Range that we see today. Evidence that the earth is continuing to move along the San Andreas fault can be seen at several points along State Highway 25 where the pavement has been cracked and laterally sheared. Highway 25 follows the San Andreas fault zone for most of the highway’s length.
Read about the geology of the California Coastal National Monument.
Serpentine is a general name for ultramafic (high magnesium and iron-containing) rocks including dunite, lherzolite, harzburgite, peridotite, and serpentinite. It can be readily identified by its shiny blue-green luster. Serpentine is a relatively rare rock type, created when oceanic mantle is emplaced on land, typically at fault zones near continental margins. Serpentine rock types are favorite locations to visit for geologists and rockhounds due to the diverse suite of minerals associated with them. In 1965 serpentine was named the State Rock of California.
Serpentine of the Clear Creek Management Area (CCMA) is pervasively sheared and hydrothermally altered. It contains a wide variety of minerals, several of which were industrially mined including cinnabar, magnesite, chromite, and chrysotile asbestos. The New Idria mines complex (private land adjacent to the CCMA) was the second largest mercury mine in the U.S. and operated from about 1854 to 1972. The KCAC Mine was the last operating chrysotile asbestos mine in the U.S., having ceased operations in 2002. The Gem Mine (private land) is the only location for gem-grade benitoite in the world and one of the few known localities for neptunite.
Exposed serpentine outcrop eroding into serpentine soil
Serpentine soils provide a very rare habitat, occupying less than 1% of the land surface area in California. Serpentine (ultramafic) soils are extremely stressful for plant growth due to low nutrient levels and high levels of magnesium and heavy metals. Plants that grow on serpentine soils often contain special adaptations to tolerate their extreme environment. Some plant species, termed “serpentine endemics,” grow exclusively on serpentine soils. Many serpentine endemic plant species have very limited distributions and as a result are rare, threatened, or endangered, such as the San Benito evening primrose.
Serpentine soils often create varied vegetation patterns. Perhaps nowhere else in California is the change in vegetation on serpentine soils more dramatic than at the CCMA where dense chaparral grows immediately adjacent to stark, moonscape barrens absolutely devoid of vegetation. At higher elevations, serpentine chaparral and barrens alternate with conifer forest. The serpentine barrens of the Clear Creek Management Area are largely natural - plant growth on the barrens is extremely slow. Serpentine soils of the CCMA have also been impacted by mining, logging, and OHV (Off-Highway Vehicle) recreation resulting in localized vegetation loss and soil erosion in heavily-used areas.
Sand dunes are a rare ecosystem in the California South Coast Range. Dunes occur in areas where the soil is predominantly sandy, plant cover is sparse, and high winds are frequent. Most sand dunes in the South Coast Range are restricted to coastal environments, however, sand dunes also occur to the interior of the South Coast Range on Monocline Ridge in the Ciervo Hills of Fresno County. Monocline Ridge is underlain by poorly cemented marine sandstone and the high ridge is very exposed - frequently experiencing high winds. This creates small sand dunes at most of the hilltops along the ridge. The series of sand dunes along Monocline Ridge are called the Monvero Residual Dunes in the Movero Soil Series classification and is a recognized BLM Research Natural Area.
View from the Movero Residual Dunes overlooking the San Joaquin Valley
The Kettleman Hills North Dome and Coalinga area have a long history of oil production, first discovered in the region around 1870. Most of the oil in the area is located within the large-scale Kreyenhagen-Temblor petroleum system, a formation of organic-rich Eocene aged shales. The Pliocene-Miocene aged Etchegoin-Temblor formation has also yielded large volumes of oil. The R.C. Baker Museum in Coalinga displays numerous local historical oil industry photos and oil extraction equipment. The museum also has a large collection of local fossils (mostly seashells) and regional minerals.
Westside Oil Fields, Coalinga 1912 (Image Courtesy R.C. Baker Museum)
Selenite gypsum (hydrated calcium sulfate) is common in the Moreno Shale of the Panoche Hills and Tumey Hills. It forms large, transparent plates resembling shattered windowpane glass. Gypsum is formed as an evaporate mineral. The San Joaquin Valley was a shallow sea 65 million years ago. As the land was gradually uplifted and the sea dried, the gypsum crystallized within the shale layers.