7.4: The 11 Geomorphic Provinces
California's geomorphic provinces are naturally defined geologic regions that display a distinct landscape or landform. Earth scientists recognize eleven provinces in California. Each region displays unique, defining features based on geology, faults, topographic relief, and climate. These geomorphic provinces are remarkably diverse. They provide spectacular vistas and unique opportunities to learn about earth's geologic processes and history.
The Great (Central) Valley
The Great Valley, also known as the Central Valley, is an alluvial plain about 50 miles wide and 400 miles long in the central part of California. Its northern part is the Sacramento Valley, drained by the Sacramento River and its southern part is the San Joaquin Valley drained by the San Joaquin River. The Great Valley is a trough in which sediments have been deposited almost continuously since the Jurassic (about 160 million years ago). Great oil fields have been found in southernmost San Joaquin Valley and along anticlinal uplifts on its southwestern margin. In the Sacramento Valley, the Sutter Buttes, rise above the valley floor ad are remnants of an isolates Pliocene volcano.
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 for oil deposits 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.
Modoc Plateau
The Modoc Plateau is a volcanic table land (elevation 4,000- 6,000 feet above sea level) consisting of a thick accumulation of lava flows and tuff beds along with many small volcanic cones. Occasional lakes, marshes, and sluggishly flowing streams meander across the plateau. The plateau is cut by many north-south faults. The province is bound indefinitely by the Cascade Range on the west and the Basin and Range on the east and south.
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. The Surprise Valley fault zone within alluvial material, is still active. Despite 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. Lava tubes are natural tunnels or caves formed by flowing lava from a volcanic eruption.
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 collapsed pits and lava tunnels, turning to Tule Lake for food (subsequent drainage has reduced the lake shoreline).
The Klamath Mountains
The Klamath Mountains have rugged topography with prominent peaks and ridges reaching 6,000-8,000 feet above sea level. In the western Klamath, an irregular drainage is incised into an uplifted plateau called the Klamath peneplain. The uplift has left successive benches with gold-bearing gravels on the sides of the canyons. The Klamath River follows a circuitous course from the southern part of the Cascade Range through the Klamath Mountains. The Cascade Range province is a northern extension of the Sierra Nevada.
Massive bodies of granite, called plutons, were formed during the Jurassic and Cretaceous periods. These plutons created rock formations like the Ironside Mountain diorite, which stretches for 37 miles, starting near the Orleans Mountain lookout tower and continuing until the border between Humboldt and Siskiyou Counties. Another example is the Shasta Bally batholith, visible west of Redding at Buckhorn Summit. A few batholiths may date earlier, like the 400-million-year-old Mule Mountain. During the Pleistocene, the mountains had many glaciers and cirques. Glacially carved U-shaped valleys are remaining features from that period. Large boulders have often eroded out of moraine deposits, coming to rest a few miles away. Fast flowing rivers resulted in very little accumulation of alluvium, except for a rare 400-foot-thick deposit in Scott Valley, southwest of Yreka, California.
Sierra Nevada
The Sierra is a tilted fault block nearly 400 miles long. Its east face is steep and high, with multiple rigged scarps, contrasting with the gentle western slope (about 2°) that disappears under sediments of the Great Valley. Deep river canyons are cut into the western slope. Their upper courses, especially in massive granites of the higher Sierra, were modified by glacial sculpturing, forming such scenic features as Yosemite Valley. The high crest culminates in Mt. Whitney, the highest elevation in the 48 contiguous United States, with an elevation of 14,508 feet above sea level near the eastern scarp.
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" upon the coast of the proto-North American continent Laurentia. The Bullpen Lake sequence has bedded cherts and pillow lava that closely resembles rocks found on flat-topped offshore volcanic guyots, 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.
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 to 10,000 feet over the last three million years, creating a steep face of the Sierra, along the west of Owens Valley.
North of the San Joaquin River, the mountains have a tilted-block pattern caused by the Sierra Nevada fault, which is interpreted as being like 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 3.5 and 9.5 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 needed to dig through to get at placer gold, and many old buildings in the region are made of blocks of tuff.
Transverse Ranges
Transverse Ranges are an east-west trending series of steep mountain ranges and valleys. The east-west structure of the Transverse Ranges is oblique to the normal northwest trend of coastal California, hence the name "Transverse." The province extends offshore to include San Miguel, Santa Rosa, and Santa Cruz islands. Its eastern extension, the San Bernardino Mountains, has been displaced to the south along the San Andreas Fault. Intense north-south compression is squeezing the Transverse Ranges. As a result, this is one of the most rapidly rising regions on earth. Great thicknesses of Cenozoic petroleum-rich sedimentary rocks have been folded and faulted, making this one of the important oil- producing areas in the United States.
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. The California State Rock, serpentine, is part of this complex—having a deep-sea origin. These are Jurassic and Cretaceous age greywacke, chert, basalt, ultrabasic rocks, and serpentinite from modified oceanic crust. Subsequently, sand and mud that were deposited when the region was underwater, became sandstones and shale. Up to 10,000 feet 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 one mile deep formed in the Miocene and filled with thick sediments, volcanic eruptions near the San Andreas Fault led to rhyolite and basalt eruptions. All these rocks were uplifted in the Pleistocene at the same time as the Coast Range orogeny, 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 14,000 feet 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 during the Pleistocene, with some marine terraces raised 1000 feet.
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 15,000 feet of sediment which is most of the rock exposed on the Channel Islands. Pillow lava structures from rapid cooling water indicate underwater volcanism that produced basalt, andesite, and diabase flows during the Miocene. In fact, volcanic rocks are 10,000 feet 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 then 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.
Cascade Range
The Cascade Range, a chain of volcanic cones, extends through Washington and Oregon into California. In California, it is dominated by Mt. Shasta, a glacier-mantled volcanic cone, rising 14,162 feet above sea level. The southern termination is Lassen Peak, which last erupted in the early 1900s. The Cascade Range is transected by deep canyons of the Pit River. The river flows through the range between these two major volcanic cones, after winding across interior Modoc Plateau on its way to the Sacramento River.
The Cascades are part of the Pacific Ocean's Ring of Fire, the ring of volcanoes and associated mountains around the Pacific Ocean. All the eruptions in the contiguous United States over the last 200 years have been from Cascade volcanoes. The two most recent were Lassen Peak from 1914 to 1921 and a major eruption of Mount St. Helens in 1980. Minor eruptions of Mount St. Helens have also occurred since, most recently from 2004 to 2008. The Cascade Range is a part of the American Cordillera, a nearly continuous chain of mountain ranges (cordillera) that form the western "backbone" of North, Central, and South America.
Coast Ranges
The Coast Ranges are northwest-trending mountain ranges (2,000 to 4,000, occasionally 6,000 feet elevation above sea level), and valleys. The ranges and valleys trend northwest, subparallel to the San Andreas Fault. In fact, the California State gem is found in San Benito County, called Benitoite. Strata dip beneath alluvium of the Great Valley. To the west is the Pacific Ocean. The coastline is uplifted, terraced and wavecut. The Coast Ranges are composed of thick Mesozoic and Cenozoic sedimentary strata. The northern and southern ranges are separated by a depression containing the San Francisco Bay. The northern Coast Ranges are dominated by irregular, knobby, landslide-topography of the Franciscan Complex. The eastern border is characterized by strike-ridges and valleys in Upper Mesozoic strata. In several areas, Franciscan rocks are overlain by volcanic cones and flows of the Quien Sabe, Sonoma and Clear Lake volcanic fields. The Coast Ranges are subparallel to the active San Andreas Fault. The San Andreas is more than 600 miles long, extending from Pt. Arena to the Gulf of California. West of the San Andreas is the Salinian Block, a granitic core extending from the southern extremity of the Coast Ranges to the north of the Farallon Islands.
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 by draining 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.
Peninsular Ranges
A series of ranges is separated by northwest trending valleys, subparallel to faults branching from the San Andreas Fault. The trend of topography is like the Coast Ranges, but the geology is more like the Sierra Nevada, with granitic rock intruding the older metamorphic rocks. The Peninsular Ranges extend into lower California and are bound on the east by the Colorado Desert. The Los Angeles Basin and the island group (Santa Catalina, Santa Barbara, and the distinctly terraced San Clemente and San Nicolas islands), together with the surrounding continental shelf, which was cut by deep submarine fault troughs, are included in this province.
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 22,000 feet thick.
The Peninsular Ranges are primarily igneous in origin, during the Jurassic period, roughly 145 to 201 million years ago. There are also pockets of metasedimentary and metavolcanic rock. The Pala District in San Diego County has minerals like tourmaline and lepidolite, often found within pegmatites, a coarse-grained igneous rock. Geologists have classified the larger igneous bodies, or plutons, of the Peninsular Ranges as the Southern California batholith. This massive batholith encompasses several distinct and impressive plutons, including the San Marcos Gabbro, Woodson Mountain Granodiorite, and Bonsall Tonalite.
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 1400 feet of Paleocene sedimentary rock in the Santa Anas. The Poway Formation, which is also found on the Channel Islands, 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. 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 7000 feet 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.
Basin & Range
The Basin and Range is the westernmost part of the Great Basin. The province is characterized by interior drainage with lakes and playas, and the typical horst and graben structure (subparallel, fault-bounded ranges separated by down dropped basins). Death Valley, the lowest area in the United States (280 feet below sea level at Badwater), is one of these grabens. Another graben, Owens Valley, lies between the bold eastern fault scarp of the Sierra Nevada and Inyo Mountains. The northern Basin and Range Province includes the Honey Lake Basin.
Mojave Desert
The Mojave is a broad interior region of isolated mountain ranges separated by expanses of desert plains. It has an interior enclosed drainage and many playas. There are two important fault trends that control topography a prominent NW-SE trend and a secondary east-west trend (apparent alignment with Transverse Ranges is significant). The Mojave province is wedged in a sharp angle between the Garlock Fault (southern boundary Sierra Nevada) and the San Andreas Fault, where it bends east from its northwest trend. The northern boundary of the Mojave is separated from the prominent Basin and Range by the eastern extension of the Garlock Fault.
The movement of the San Andreas Fault (sliding right) and the Garlock Fault (sliding left) during the Cenozoic Era played a role in creating the arid conditions. This squeezing of the Earth's crust near Gorman may have contributed to a rain shadow effect, blocking moisture from reaching the region. Precambrian and Proterozoic rocks, formed over 2.5 billion years ago, dominate the Mojave landscape. Examples include banded rock (granitic gneiss) and marble found in the Ord Mountains, and layered rock (schist) in the Old Woman Mountains. Some of these ancient formations were even intruded by younger igneous rock called porphyry.
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, which is used in cellphone technology, mined at the Mountain Pass rare earth mine. Additionally, borate was deposited during arid conditions, with mining after 1926.
Extremes in temperatures throughout the seasons characterize the climate of the Mojave Desert. Freezing temperatures as well as strong winds are not uncommon in the winter, as well as precipitation such as rain and snow in the mountains. In contrast, temperatures above 100 °F (38 °C) are not uncommon during the summer months. There is an annual average precipitation of 2 to 6 inches (51 to 152 mm), although regions at high altitudes such as the portion of the Mojave Desert in the San Gabriel mountains.
Colorado Desert
Most of the Colorado Desert lies at a relatively low elevation, below 1,000 feet (305 m), with the lowest point of the desert floor at 275 feet (84 m) below sea level, at the Salton Sea. Although the highest peaks of the Peninsular Ranges reach elevations of nearly 10,000 feet (3,048 m), most of the region's mountains do not exceed 3,000 feet (914 m).
In this region, the geology is dominated by the transition of the tectonic plate boundary from rift to fault. The southernmost strands of the San Andreas Fault connect to the northernmost extensions of the East Pacific Rise. Consequently, the region is subject to earthquakes, and the crust is being stretched, which will result in a sinking of the terrain over time. The region is characterized by the ancient beach lines, silt deposits of extinct Lake Cahuilla as well as the mud pots. Mud pots are a unique way for geothermal energy to reveal itself on the surface, offering a window into the Earth's dynamic processes. The most famous examples are found near the Salton Sea, a remnant of a prehistoric lake. Interestingly, despite the harsh environment, some extremophile microorganisms even manage to thrive in the hot, sulfurous mud of these pots.