California Coast Ranges The
California Coast Ranges extend from the Oregon line southward to the
Santa Ynez River. The mountains are drained by the
Russian,
Eel,
Mad and
Klamath rivers which tend to follow faults and folds. Per square mile of drainage, the Eel River has the highest suspended sediment load of any river in the US, exceeding the Colorado River and Mississippi River. The Franciscan subduction complex makes up the basement rock for part of the range, with a
mélange dominated by
graywacke first deposited in offshore deep ocean basins. These rocks are rich in plagioclase, quartz and chlorite mica (which gives them a greenish color) and range up to thick. In some places, they are interbedded with shale, limestone and
radiolarian chert. Examples of rock formations in the Franciscan subduction complex include the Calera limestone east of the San Andreas fault or the Laytonville limestone which extends north of San Francisco to Eureka. Franciscan rocks have a complex geological structure. In some places,
ophiolite sequences preserve remnant
oceanic crust rock on land, while other parts of the range are intruded with ultrabasic igneous rocks that have
serpentinized to
peridotite. In the southern part of the range, between the Nacimiento and San Andreas fault zones, the metamorphic rocks and granite plutons of the Salinian block make up the basement rock. These rocks were likely moved to their current location by the two major faults. Some Great Valley rocks are exposed in the Coastal Ranges as well, like the Jurassic Knoxville Formation shale. Sur Series schist, quartzite, marble, gneiss and granulite in the Salinian block were likely deposited in a marine shelf late in the
Paleozoic. Shelf and slope deposits would ultimately become Great Valley rocks, while the trench deposits became the Franciscan subduction complex. Major geologic changes began in the
Cenozoic with continued continental shelf deposition of shale, sandstone and clay as well as near-shore tropical coal deposits. During the Miocene, the Coast Ranges were flooded again, with underwater volcanic eruptions in the southern part of the range and deposition of fossil-rich shales like the Monterey formation. Uplift took place into the Pliocene and Pleistocene, slowly reducing sea levels inland in the Central Valley. The Coast Range is at even higher risk for damaging landslides than other parts of coastal California due to sheared serpentinite in Franciscan basement rocks.
Central Valley The
Central Valley includes the southern
San Joaquin Valley and the northern
Sacramento Valley. The
Great Valley Sequence is a thick formation at the western edge of the Central Valley that formed between the
Jurassic and
Cretaceous, overlying the
Franciscan Assemblage and granite rocks associated with the
Sierra Nevada to the east. It represents an ancient
forearc basin that took shape as oceanic crust subducted under the west edge of the continent. During the Jurassic, many sediments shed into the region from the rising proto-Sierra Nevada. For much of the
Cenozoic the region was filled with lakes and brackish swamps. Thick
Miocene sediments formed in narrow seaways from the Pacific. The Stockton fault and White Wolf fault by Bakersfield are both major tectonic features. The 7.6 magnitude
1952 Kern County earthquake was one of the most powerful in California in the 20th century. The subsurface is well known from oil wells and oil fields are bounded in the east by the Kern Front fault. A small andesitic dome near Marysville is the only example of volcanic rocks in the valley exposed near the surface.
Coronado Island The distinctive Coronado Island in San Diego County helps to create the large sheltered harbor of San Diego. It formed as a sandspit filled in to the north from sediment deposited by the Tijuana River and sheltered from wind and waves by the Cretaceous uplands of Point Loma. Two small islands to the north: North Island and South Island have been connected with fill to create the Naval Air Station. Without continuous dredging, San Diego Bay would fill with sediment and become dry land with deposition from the San Diego, Sweetwater and Otay rivers.
Klamath Mountains The Klamath Mountains formed as small island arc terranes accreted against the coast of North America before and during the
Devonian and overlain by newer rocks, deposited from the
Cretaceous to modern times. Geologists have subdivided the four terranes in the mountain range, referring to them as "plates." The Klamath Mountains have several areas of preserved oceanic crust. The Josephine ophiolite forms the basement of the western Klamath Mountains, with extensive
peridotite from the boundary between the oceanic crust and the mantle, as well as chromite and nickel deposits. It is exposed in the Smith River drainage to the north of
US Highway 199 in
Del Norte County. By contrast, the Trinity ophiolite is found in the east, known for its numerous ultramafic rocks and being one of the most extensive ophiolites in the US. The Western Jurassic Plate stretches for on the western edge of the mountains. The thick Galice formation
slate and metamorphosed
greywacke overlie the Josephine ophiolite and likely deposited in a deep ocean environment offshore of the island arc. It also interfingers with up to of volcanic rocks in the Rogue Formation. Some rocks in the Western Jurassic Plate seem to have been subducted, reaching
blueschist grade metamorphism in the sequence of
metamorphic facies in the South Fork Mountain schist. By contrast, the Eastern Klamath Plate is made up of limestone, chert and pyroclastic rocks, divided in half by the Trinity ophiolite. A thrust fault, known as the Trinity or Bully Choop thrust, separates the Eastern Klamath Plate from the Central Metamorphic Plate. This region of the mountains was likely underthrusted by the adjoining plates, producing multiple schists around 380 to 400 million years ago in the
Ordovician. Many of these schists, like the Condrey Mountain schist are more resistant to erosion than other rocks close by. The Western Paleozoic and Triassic Plate is the most common unit of the Klamath Mountains and is up to wide at the Oregon state line. Geologists have struggled to define its
structural geology with complex sequences of deep ocean crust, upper mantle rock and tectonic melanges. Granite plutons emplaced during the
Jurassic and
Cretaceous, forming rock units like the Ironside Mountain diorite, which outcrops for from the Orleans Mountain lookout tower to the line between Humboldt County and Siskiyou County or the Shasta Bally batholith at Buckhorn Summit west of Redding. A few may date earlier, like the 400 million year old Mule Mountain. During the
Pleistocene, the mountains had a large number of glaciers and
cirques and glacially-carved U-shaped valleys are remaining features from that time period. Large boulders have often eroded out of moraine deposits, coming to rest a few miles away. Fast flowing rivers meant that the region has accumulated very little alluvium, except for a rare thick deposit in Scott Valley, southwest of
Yreka, California.
Modoc Plateau The Modoc Plateau is an undulating expanse of Miocene to Holocene basalt law flows at the southwest edge of the Columbia Plateau, covering . It is drained by the Pit River, which ultimately reaches Shasta Lake and the Sacramento River. The Warner Basalt is the most common rock in the plateau, bordered by the Surprise Valley fault zone that first became active in the Miocene 15 million years ago. Based on faulting in recent alluvial material, the Surprise Valley fault zone is still active. In spite of low rainfall, the numerous lava tubes and volcanic fractures in the plateau produce the Fall River Springs, one of the largest springs in the US. At least 300 lava tubes are known in
Lava Beds National Monument, some of which preserve ice year round. The rugged terrain of the Modoc Plateau played an important role in the
Modoc War. In 1873, 53 Modoc men held off 650 US troops, killing 70 of them in heavy weather in the lava plateau to the south of Tule Lake. The Modoc exploited collapse pits and lava tunnels, turning to Tule Lake for food (subsequent drainage has reduced the lake shoreline).
Mojave Desert The
Mojave Desert extends into areas of
Basin and Range terrain and includes some of the oldest rocks exposed at the surface in California. The movement of the San Andreas Fault and
Garlock Fault helped to induce arid conditions during the
Cenozoic. Unlike most of California, the Mojave region has numerous
Precambrian Proterozoic rocks, such as granitic gneiss or marble intruded with
porphyry in the
Ord Mountains or schist in the
Old Woman Mountains. The Marble Mountain
Cambrian quartzite lies unconformably atop Proterozoic granites. During the
Jurassic high sea levels retreated, switching to the deposition of the terrestrial Aztec sandstone. Volcanic rocks erupted around Barstow in the
Triassic. Lake beds interbedded with volcanic ash are typical of the
Cenozoic. Volcanism continued through the
Holocene, forming the 300 foot thick basalts of the Cima Volcanic Field or the Barstow-Amboy axis of volcanic craters, which have protected underlying granite from erosion. Unique geology in the region formed
bastnaesite mined at the Mountain Pass rare earth mine. Additionally,
borate deposited during arid conditions, with mining after 1926.
Newport Bay At the edge of the Los Angeles coastal plain and just west of the San Joaquin Hills, Newport Bay is a major hub of boating. Geologists have interpreted the bay as the drowned channel of the Santa Ana River. Flow in and out of the estuary has been controlled with a man-made structure since heavy silting in 1915. Virtually all of the islands in the bay are man-made from dredged sediments. However, a bulge in the bay's sand spit records offshore presence of the Newport submarine canyon. In the 19th century, a railroad pier was built out into the canyon to avoid waves closer in to shore when loading and unloading ships.
Peninsular Ranges The
Peninsular Ranges are a group of mountain ranges that extend from the Los Angeles basin and Transverse Ranges southward the entire length of Baja California. The eastern ranges, including the Santa Rosa Mountains are typically over high, with San Jacinto Peak reaching , whereas the western ranges like the Santa Ana, Agua Tibia and Laguna mountains are lower. Like the Sierra Nevada, the Peninsular Ranges have gentle western slopes and steep eastern faces. The mountains are drained by the Santa Margarita, San Luis Rey, San Diego and San Dieguito rivers, while San Felipe Creek in the east drains into the Salton Sea. The oldest "roof" rocks in the Peninsular Ranges date to the
Paleozoic, such as limestone deposits near Riverside quarried for the concrete industry. The oldest rocks are found in the San Jacinto and Santa Rosa mountains, with
schist and
gneiss that may be up to thick. With the exception of some metasedimentary and metavolcanic rocks, most rocks in the Peninsular Ranges are igneous and
Jurassic in age, dating to the time of the
Nevadan orogeny. Most intrusive igneous rocks are tonalite, granodiorite, quartz diorite or gabbro. Geologists group the plutons of the Peninsular Ranges as the Southern California batholith, which includes several large individual plutons like the San Marcos Gabbro, Woodson Mountain Granodiorite or Bonsall Tonalite. Compared with the Sierra Nevada, rocks from this time period tend be more calcic than silicic. The rocks likely formed much further away from the Sierras and were relocated by the dramatic movements of faults. A thick sequence of mostly terrestrial sedimentary rocks including the Rosario, Ladd, Trabuco and Williams formations is exposed on the western slope of the Santa Ana Mountains and Santa Ana Canyon, stretching southward to Camp Pendleton, San Onofre, and smaller exposures in Encinitas, Leucadia, Point Loma and La Jolla. These rocks date to the Cretaceous, with a lower sequence of conglomerate overlain by sandstone and shale, with huge
ammonite fossils. In the early
Cenozoic, widespread erosion of crystalline rocks inland produced huge quantities of sediment which deposited on the Cretaceous rocks of the Peninsular Ranges. Examples include the Silverado Formation with of
Paleocene terrestrial sedimentary rock in the Santa Anas or the
Eocene Poway Formation near San Diego. The Poway Formation has rounded metavolcanic pebbles with no known source nearby, suggesting an original source somewhere in Sonora before a major offset to the north by faults. Much more recent
Pliocene terrestrial sedimentary rocks are also common in the northern Peninsular Ranges like the San Timoteo Canyon and Mount Eden conglomerate, sandstone and siltstone which are up to thick. Fossils are common in the marine rocks of the Pacific Beach and Mission Bay formation around San Diego. The Peninsular Ranges were never glaciated during the
Pleistocene. In the nineteenth century, mining began in the vicinity of Julian, extracting nickel and gold, in the Julian Schist. Hot springs including San Jacinto, Eden, Saboba and Gilman are active in the San Jacinto Mountains due to the presence of the San Jacinto fault zone and Elsinore was founded on its hot springs.
San Francisco Bay, San Pablo Bay and Suisun Bay The three interconnected Suisun, San Pablo and San Francisco bays occupy a structural depression dating to the
Pliocene which flooded several times due to Pleistocene glaciations. Underlying these bays is the
Merced Formation, with of marine rocks overlain by of terrestrial sedimentary rocks and capped with more modern sediments from the
Sacramento River. The bay system was the second place in the US to be mapped in 1826 by Edward Belcher and Alex Collie, a surveyor and surgeon respectively on the British survey vessel
HMS Blossom.
Hydraulic mining associated with the
California Gold Rush greatly accelerated the silting up of the bays until the practice was prohibited in 1884. The majority of bay waters are now less than deep,
Sierra Nevada The basement rocks of the Sierra Nevada date to the
Paleozoic and include rocks in the Shoo Fly complex and Grizzly Formation. These deposited as part of a series of small island arcs, "rafted" against the coast of the proto-North American continent
Laurentia. In the Shoo Fly complex, the Bullpen Lake sequence has bedded cherts and cooled pillow lava that closely resembles the geology of
guyots, flat-topped undersea volcanoes, suggesting a deep-sea origin for the base of the Sierras sometime before the
Devonian. In the
Triassic, the Paleozoic rocks of the Calaveras complex were heaved under the Shoo Fly complex by the
Sonoma orogeny, which is preserved in roof pendants and country rock inclusions. During the
Nevadan orogeny in the
Jurassic, extensive folding and faulting altered the rocks and huge granite
batholiths erupted. The Foothill Metamorphic Belt likely came ashore as an island arc terrane, colliding with the edge of North America to the west of the current Melones fault zone. This added metavolcanic and metasedimentary rocks to the slowly building Sierras. Geologists debate how high the early Sierra Nevada were. of Jurassic marine sediments in the Sacramento Valley do not necessarily correspond exactly to the height of the mountains, but fist-sized cobbles in conglomerates from the
Cretaceous suggest steep conditions and potentially higher altitudes than today. Most erosion of the Sierra Nevada was finished by the
Eocene and faulting may have reactivated the roots of the mountains to create the current mountain range. The geomorphology of the Sierra Nevada is comparatively recent, dating to as recently as the
Quaternary. Movement along the Sierra Nevada fault helped to create the new Muir Crest and parts of the mountain range rose up to over the last three million years, creating a steep face to the west of Owens Valley. North of the San Joaquin River, the mountains have a tilted-block pattern caused the Sierra Nevada fault, which is interpreted as being similar to the more common
Basin and Range terrain to the east. However, the Greenhorn Fault system is more active to the south, extending to the Tehachapi Mountains and causing more plateau-like landforms. Volcanic eruptions in the
Miocene between 9.5 and 3.5 million years ago filled old eroded canyons in parts of the Sierras with lava flows. The
Kern River and
San Joaquin River both run in valleys "refilled" with lava. Rhyolite volcanic ash buried other streams during the
Oligocene forming
tuff that early miners need to dig through to get at placer gold, and many old buildings in the region are made of blocks of tuff.
Transverse Ranges The
Transverse Ranges extend from Point Arguello to
Joshua Tree National Park, bounded by the San Andreas Fault to the north. It is the only coastal mountain range in the US—and potentially North America—with rocks older than the
Phanerozoic. The
Santa Ynez Mountains extend up the coast of
Santa Barbara County and contain Franciscan basement rocks (also referred to as the Franciscan basement complex) like the Coast Ranges. These are
Jurassic and
Cretaceous age greywacke, chert, basalt, ultrabasic rocks and
serpentinite from modified oceanic crust. Subsequently, sandstone and shale deposited while the region was still underwater. Up to of sandstone and red shale deposited during the
Eocene, after which sea levels dropped in the
Oligocene preserved the sand, gravel and silt of the Sespe Formation. Before the end of the Oligocene, sea levels rose again leaving behind the Vaqueros Formation and then the
Miocene deep water silt and clay of the Rincon Formation. As deep basins up to deep formed in the Miocene and filled with thick sediments, volcanic eruptions related to the San Andreas Fault led to rhyolite and basalt eruptions. All of these rocks were uplifted in the
Pleistocene at the same time as the Coast Range orogeny and formed an anticlinal arch or tilted block against the Santa Ynez fault in the north. Because the mountain range is young only a very narrow coastal plain has developed around Santa Barbara. Similar rock formations and patterns are found throughout Ventura County, in the Topatopa Mountains and Pine Mountains. Thick
Pliocene marine rocks up to thick are found on the edges of the Santa Clara River Valley and parts of the San Fernando Valley, stretching to Fillmore. Major uplift was going on in Pleistocene, with some marine terraces raised . The
Santa Monica Mountains and the Channel Islands are different than the mountains to the north because they have granitic and metamorphic basement rocks more like the Sierra Nevada. The oldest rock in this part of the range is the Santa Monica Slate. Thick sequences of marine rocks from the
Late Cretaceous through the
Paleocene and
Eocene are common in the mountains and on San Miguel Island. During the Miocene, a deep marine channel filled with up to of sediment which is most of the rock exposed on the Channel Islands. Pillow structures from rapid cooling water indicate underwater volcanism that produced basalt, andesite and diabase flows during the Miocene. In fact volcanic rocks are thick in the western part of the Santa Monica Mountains. The San Onofre Breccia also formed during this period with distinct
glaucophane schist, gabbro, limestone and greenschist. The Transverse Ranges are poorly drained by streams, but subject to periodic intense flooding, typically every 20 to 25 years. In 1815, the Los Angeles River, which at the time drained to Long Beach, flooded so substantially that it changed course, joining Ballona Creek and flowing to Santa Monica Bay, before another flood in 1825 redirected it back to Long Beach. Serious flooding in 1862 created a lake in the Santa Ana River, while serious flooding in 1938 inundated the San Fernando Valley and parts of the Los Angeles coastal plain, damaging 100 bridges and killing 43 people. ==Natural resource geology==