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52-503: The Simi Hills are aligned east-west and run for 26 miles (42 km), and average around 7 mi (11 km) north-south. The Simi Hills are part of the central Transverse Ranges System. They lie almost entirely within southeastern Ventura County , with some southern and eastern foothills within western Los Angeles County . The Simi Hills are on the western edge of the San Fernando Valley . The Simi Valley lies to

104-631: A crucial wildlife corridor to the Santa Susanas, has been proposed for public open space parkland after the closed site's cleanup completion. The population of red-legged frogs is small and isolated, and was impacted by the Woolsey Fire swept through the area in November 2018. The Simi Hills were inhabited for over 8,000 years by Paleo-indians and Chumash-Venturaño Native Americans for settlements and hunting grounds. The Chumash had

156-640: A marked change to red beds of river systems of Oligocene age in the western and central segments. Limestones and dolomites of the marine Miocene Monterey Formation are found in the Santa Ynez Mountains and in the Coast Ranges to the north. The distinctive feature of the Transverse Ranges besides their anomalous orientation is that they are bounded by east-west trending faults. Most are left slip, strike slip faults . In

208-472: A modest 4,190 ft (1,277 m) above mean sea level ; this means that snow is less of a factor here than in the moderate to high mountain passes to the north like Donner Pass . Still sometimes, heavy snowfall can snarl traffic on Tejon and Cajon Pass, the higher two of the three freeway passes. I-5 and I-15 commonly experience heavy traffic over their mountainous route across these mountains. Salix lasiolepis Salix lasiolepis ( arroyo willow )

260-499: A result, there is high topographic relief throughout the range, and other than in marginal areas (e.g. the San Fernando Valley ) and a few river valleys (such as Lockwood Valley and Big Bear Valley), there are no large, flat basins within the ranges. The mountains are notable for being steep and difficult to traverse. There are few passes that are sufficiently low or wide enough to accommodate significant volumes of traffic. This has resulted in situations where major cities are linked to

312-546: Is a deciduous large shrub or small multi−trunked tree growing to 10 metres (33 ft) tall. The shoots are yellowish brown and densely hairy when young. The leaves are 3.5–12.5 centimetres (1.4–4.9 in) long and broadly lanceolate in shape. They are green above and glaucous green below. The undersides are covered with whitish or rusty hairs which gradually wear off during the summer. The flowers are arranged in yellow catkins 1.5–7 centimetres (0.59–2.76 in) long which are produced in early spring. The bloom period

364-484: Is a species of willow native to western North America . The core range of the arroyo willow includes most of California, including the California Coast Ranges , Arizona , Klamath Mountains , Peninsular Ranges , Sierra Nevada , and Transverse Ranges . It extends north into Washington , south into Baja California , and east into Idaho , Utah , Texas , and Coahuila (México). The plant

416-413: Is also an important member of the native plant habitat community here. The Simi Hills is the principal, and much wider, of only two terrestrial wildlife corridors linking the coastal Santa Monica Mountains with the inland Santa Susana Mountains , Topatopa Mountains , and San Gabriel Mountains , all of the transverse ranges fauna community. The Simi Hills are a critical wildlife corridor linkage for

468-488: Is commonly found growing in riparian zones in canyons and valleys, along pond shores, and in marshes and wetlands . It is found in many plant communities, including: chaparral , oak woodland , mixed evergreen forest , coast redwood forest, yellow pine forest , red fir forest, lodgepole pine forest, and grasslands . Arroyo Willow (Salix lasiolepis) is a severe allergen. Occurs in following seasons depending on latitude and elevation: Spring. Salix lasiolepis

520-631: The 2018 Southern California mudflows , may be cut off from timely road access to the rest of Southern California. Major peaks of the Transverse Ranges with at least 500 feet (150 m) of prominence, listed by height: This segment begins at Point Conception in Santa Barbara County, and include the Santa Ynez Mountains that run parallel to the coast behind Santa Barbara and the San Rafael Mountains on

572-610: The Mesozoic , mostly granites, can be found on Mount Pinos and generally in regions east of Tejon Pass . The youngest rocks are Cenozoic sedimentary and volcanic rocks that can be found throughout the ranges. The western segment is distinctive for the large thickness of Cretaceous and Cenozoic sedimentary rocks, estimated to be up to 10 kilometers. The thickest deposits of these are in the Santa Barbara Channel and Ventura basin. These are mostly marine in origin with

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624-797: The Pacific Ocean and the northern Channel Islands . Onshore the Los Angeles Basin lies at the southern boundary of the western and central segments of the ranges. Major passes not along the San Andreas Fault include Gaviota Pass , San Marcos Pass , the Conejo Grade , Newhall Pass , and Cahuenga Pass . The Transverse Ranges manifest themselves as a series of roughly parallel ridges with an average height of 3,000–8,000 feet (910–2,440 m). The ranges are dissected by young, steep streams of relatively low flow rate; as

676-828: The Pinto , Eagle , and Orocopia Mountains are within the eastern segment. The Mojave Desert and California's low desert, including the Coachella Valley , are at the eastern end of the ranges. Ranges north of the western segment that are nearly transverse but are part of the California Coast Ranges include the San Rafael Mountains and the Sierra Madre Mountains . Likewise, the Tehachapi Mountains north of

728-584: The Rocketdyne Santa Susana Field Laboratory (SSFL) for open-air testing of rocket engines from 1947 to 1990, and the site of experimental nuclear reactor experiments with four nuclear accidents between 1959 and 1969; these projects and accidents were declassified in 1989. The first accident, in 1959, was the most serious with a full meltdown. The extent of the accident is unknown due to instrument limitations, other than that it released extensive radiation. More radiation

780-913: The San Gabriel Mountains and San Gabriel fault . The central segment includes mountains that range eastward to the San Andreas fault . The eastern segment extends from the Cajon Pass at the San Andreas fault eastward to the Colorado Desert. The central and eastern segments (near the San Andreas fault) have the highest elevations. Most of the ranges lie in the California chaparral and woodlands ecoregion. Lower elevations are dominated by chaparral and scrubland, while higher elevations support large conifer forests. Most of

832-640: The WWF -designated California montane chaparral and woodlands ecoregion , but the eastern points of the range touch two desert regions, the Mojave Desert and the Colorado Desert section of the Sonoran Desert . The Carrizo Plain adjoins the northern edge of the Transverse Range. Chaparral is a common feature of the Transverse Ranges. Common plant associates in chaparral , especially in

884-492: The evergreen coastal live oak ( Quercus agrifolia ), the deciduous valley oak ( Quercus lobata ), and the scrub oak ( Quercus dumosa ). Riparian zone plants include California sycamores ( Platanus racemosa ) and arroyo willows ( Salix lasiolepis ). Spring wildflowers include the redbush monkey flower ( Mimulus aurantiacus ), Plummer's mariposa lily ( Calochortus plummerae ), and canyon sunflower ( Encelia californica ). poison oak ( Toxicodendron diversilobum )

936-538: The most important oil-producing areas in the United States. The plant place of the Transverse ranges include coastal sage scrub , oak woodland and savanna , and pinyon-juniper woodland at lower elevations, and yellow pine forest , Lodgepole Pine , and subalpine forest at higher elevations. The Angeles and Los Padres National Forests cover portions of the Transverse ranges. The ranges are part of

988-493: The Conejo Valley. The Simi Hills further block these winds, which bring cool weather in both summer and winter from the San Fernando Valley . The southern lower hills are mostly covered in grasslands and oak savanna. The northern rocky hills area is primarily chaparral shrubland and oak woodlands . The Simi Hills are part of the California chaparral and woodlands ecoregion . The oaks ( Quercus spp. ) include:

1040-571: The Mojave Desert, although nearly transverse, are the southern end of the Sierra Nevada . Mountain ranges with peaks exceeding 4,500 feet (1,400 m), listed west to east, include: Examples of smaller mountain and hill ranges include: The climate in most of the range is Csb ( Warm-summer Mediterranean ) under the Köppen climate classification ; the upper reaches of San Gorgonio Mountain have an alpine tundra climate ( ET ), while

1092-546: The San Andreas Fault, which separates those segments from the Mojave Desert. The eastern segment bounds the southern Mojave Desert. Notable passes along the San Andreas fault include Tejon Pass , Cajon Pass , and San Gorgonio Pass . Components of Transverse Ranges to the north and east of the fault include the San Bernardino Mountains , Little San Bernardino Mountains , Pinto and Eagle Mountains . The western and southern boundaries are acknowledged to be

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1144-681: The Santa Barbara Channel and Ventura Basin. Much oil has pooled in the Monterey Formation, which is produced onshore and offshore. In the eastern Santa Barbara Channel oil is held in younger sedimentary rocks. The Los Angeles Basin south of the western segment, formed during rotation of that segment away from the Peninsular Ranges. It is also a site of prolific oil production. The California Geologic Survey states: Great thicknesses of Cenozoic petroleum-rich sedimentary rocks have been folded and faulted, making California one of

1196-810: The Santa Monica Mountains to these and other Transverse Ranges further east. The undeveloped native habitat provides routes that protect larger land wildlife of the Santa Monicas from genetic isolation. The Wallis Annenberg Wildlife Crossing is a proposed vegetated overpass spanning the Ventura Freeway at Liberty Canyon in Agoura Hills, California . Large sections of the Simi Hills are protected by parks and open space preserves. The Santa Susana Field Laboratory property,

1248-667: The Simi Hills typically experience rainy , mild winters. Snow is rare in the Simi Hills, even in the highest areas. Summers are warm and dry and wildfires do occur here. Cool winds from the Pacific Ocean come from the Oxnard Plain and blow into the inland areas through the Santa Clara River Valley and the Conejo Valley , though some low hills, such as Conejo Mountain , block these winds from

1300-464: The Simi Hills. In the first half of the 20th century, there were four large movie ranches in the Simi Hills for filming motion pictures on location. The gated community of Bell Canyon began development of geographic Bell Canyon in the 1968. To the north of U.S. 101 , east of Thousand Oaks , and west of Simi Valley the early 1960s suburban expansion of metropolitan Los Angeles brought the development of small to significantly sized parcels of land in

1352-562: The Simi Hills. Hillside subdivisions regraded natural contours into terraced lots, changing the Hills viewshed, drainage patterns, and habitats in those areas. The extensive planned new community projects at Jordan Ranch and Ahmanson Ranch were eventually stopped by local citizens and the Santa Monica Mountains Conservancy keeping extensive Hills acreage natural in open space parks. The Simi Hills were home of

1404-563: The Transverse Ranges and the Peninsular Ranges to the south. There are a number of important freeways that cross the Transverse Ranges, such as (from west to east) US-101, I-5 at Tejon Pass , SR 14 at Soledad Pass , and I-15 at Cajon Pass . These highways link Southern California with places to the north and northeast like San Francisco and Las Vegas , respectively. With the exception of several high passes on less-traveled SR 33 , SR 2 , SR 330 . SR 18 and SR 38 , none of these passes are at high elevations, with Cajon Pass being at

1456-902: The Transverse Ranges; San Miguel , Santa Rosa , Santa Cruz and Anacapa Islands are a westward extension of the Santa Monica Mountains. The Ranges include the steep San Gabriel Mountains northeast of Los Angeles , the Sierra Pelona Mountains just south of the San Andreas fault, the Tehachapi Mountains southeast of Bakersfield, the Verdugo Mountains , the Liebre-Sawmill Mountains, the San Rafael Hills , Puente Hills , San Jose Hills , and Chino Hills . The San Bernardino Mountains , Little San Bernardino Mountains , and

1508-725: The balance of the natural world. Upstream were healing springs and are rock outcrop 'grinding stones.' The Burro Flats Painted Cave , an example of the Rock art of the Chumash people , is nearby. The Juan Bautista de Anza expedition passed through the area in 1769, being the first European sighting of the Simi Hills. The U.S. National Park Service administers the Juan Bautista de Anza National Historic Trail which enters at Moore Canyon in El Escorpión Park and crosses across

1560-612: The bounding faults are left slip Santa Ynez and Pine Mountain Faults. The San Gabriel Fault and San Andreas bound the central segment. The eastern segment is bounded by the Pinto Mountain Fault in the north and the Salton Creek Fault in the south. The large thickness of marine sediments in the western segment have made it a habitat for oil. Several dozen fields are found onshore and offshore, particularly in

1612-612: The central and eastern segments, including the San Gabriel and San Bernardino Mountains. The oldest basement (deepest) rocks are of Proterozoic age, and are found in the San Gabriel Mountains and the San Bernardino Mountains. The Jurassic - Cretaceous Franciscan Assemblage is found in the western section of the ranges and is the presumed basement in this segment. Exposed plutonic rocks from

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1664-456: The established village of Hu'wam in Cañon del Escorpión (Bell Canyon). It was a multicultural 'crossroads' destination, where Chumash, Tongva , and Tataviam peoples traded and lived beside Bell Creek below Escorpión Peak , at the present day Bell Canyon Park . This peak in the Simi Hills (aka: Castle Peak) is one of nine alignment points in Chumash territory and is essential to maintaining

1716-518: The fault moved east to open the Gulf of California . The crust within the Pacific Plate south of the ranges can not easily make the left turn westward as the entire plate moves northwestward, forcing pieces of the crust to compress and lift. Prior to this shift of the fault to create the left bend, northwest–southeast trending rock belts in all of the Transverse Ranges began to rotate clockwise in

1768-418: The faults outside that province. Between the western segment and the Peninsular Ranges to the south is the complex Malibu Coast—Santa Monica— Hollywood fault , which exists as the border between these two mostly geologically unitary provinces. These faults are part of the same thrust fault system south of the northern Channel Islands of San Miguel, Santa Rosa, and Anacapa Islands. North of the western segment

1820-622: The headwaters of the Los Angeles River , by name its beginning with their confluence in nearby Canoga Park . 90% of the Santa Susana Field Lab property drains into the Los Angeles River via tributaries. Peaks in this region include Simi Peak , 2,403 ft (732 m), Chatsworth Peak , 2,314 ft (700 m), and Escorpión Peak (aka: Castle Peak), 1,475 ft (450 m). Because of its low elevation,

1872-591: The interaction of the Pacific Plate and the North American Plate along the dextral (right slip) San Andreas Fault system. Their orientation along an east–west axis as opposed to the general northwest–southeast trend of most California ranges results from a pronounced left step in the San Andreas Fault that occurred in the Pliocene ( c.  4 million years ago) when southern reaches of

1924-451: The lower northern slopes of the range have a desert ( BW ) or steppe ( BS ) climate; most of the nearby lowlands to the south and west have a Hot-summer Mediterranean climate ( Csa ). Snow falls above 6,000 feet (1,800 m) most winters, and above 3,000 feet (910 m) every few years. It is rare for elevations above 8,000 feet (2,400 m) to go multiple winters without snow, even during severe droughts. Due to relatively low humidity,

1976-636: The mountain ranges, including the Oxnard Plain of coastal Ventura County , the Santa Clarita Valley north of Los Angeles, the San Fernando Valley , which is mostly included in the City of Los Angeles. The Los Angeles Basin, which includes the portion of Los Angeles County south of the Santa Monica Mountains and most of Orange County , and the Inland Empire basin, which includes the cities of San Bernardino and Riverside , lie between

2028-822: The north) are in the hills and adjacent valleys within Ventura County. The two nearby mountain ranges are: the higher Santa Susana Mountains adjacent on the northeast across Santa Susana Pass ; and the Santa Monica Mountains running nearby along the south. The hills provide the complete or partial watersheds for several year-round creeks and numerous seasonal streams. They include Las Virgenes Creek (tributary of Malibu Creek ), Moore's Canyon Creek, Bell Creek , Dayton Creek, Woolsey Canyon Creek, Brandeis Creek, Runkle Canyon Creek, Arroyo Simi , Palo Comado Creek, Cheeseboro Creek, and Arroyo Calabasas (northern fork). Bell Creek and Arroyo Calabasas are

2080-562: The north, and the Conejo Valley lies to the southwest. The San Fernando Valley communities of Chatsworth , West Hills , and Woodland Hills are in the eastern hills and adjacent valley floor in Los Angeles city and county . The cities of Agoura Hills and Westlake Village are also located in Los Angeles County, generally southwest of the Simi Hills. The cities of Thousand Oaks (to the west) and Simi Valley city (to

2132-874: The other side of the Santa Ynez Valley . The western Transverse Ranges also include the Topatopa Mountains and the Santa Susana Mountains of Ventura County and Los Angeles County, the Simi Hills , the Santa Monica Mountains that run along the Pacific coast behind Malibu , and whose eastern portion are known as the Hollywood Hills , and the Chalk Hills . The northern Channel Islands of California are also part of

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2184-497: The outboard Pacific plate. Because the Monterey plate was then beneath southern California, the capture resulted in pulling of the overlying crust out and northward from the rest of California. Rocks of the Transverse Ranges exhibit extreme differences in geologic age and composition, varying from sedimentary rocks in the western Santa Ynez and volcanic rocks in the Santa Monica Mountains to primarily granitic and metamorphic rock in

2236-807: The range. The Rim of the Valley Trail Corridor is in the planning stages and includes the Simi Hills open space parklands and proposed new areas. Transverse Ranges The Transverse Ranges are a group of mountain ranges of southern California, in the Pacific Coast Ranges physiographic region in North America. The Transverse Ranges begin at the southern end of the California Coast Ranges and lie within Santa Barbara, Ventura, Los Angeles, San Bernardino, Riverside and Kern counties. The Peninsular Ranges lie to

2288-480: The ranges in the system are fault blocks , and were uplifted by tectonic movements late in the Cenozoic Era . West of Tejon Pass , the primary rock types are varied, with a mix of sedimentary, volcanic, and metamorphic rocks, while regions east of the pass are dominated by plutonic granitic and metasedimentary rocks. The western and central segments of the Transverse Ranges are bounded to the north and east by

2340-420: The regional snow line lies at about 14,000–16,000 feet (4,300–4,900 m), above the highest elevation of the range; as such, snow does not persist year-round except in the form of snow patches . The tree line lies at about 11,000 feet (3,400 m); San Gorgonio Mountain is the only peak with an alpine environment. The Transverse Ranges result from a complex of tectonic forces and faulting stemming from

2392-531: The rest of the state by relatively few roads; for example, the vast majority of traffic between the Central Valley and the Los Angeles area is routed through Tejon Pass. This results in significant traffic issues throughout Southern California when a pass has to be shut down due to heavy snow or construction. Occasionally, major cities, such as Santa Barbara during the 2005 La Conchita landslide and

2444-547: The right shear of Pacific Plate – North American Plate motion. This tectonic rotation began in Early Miocene Time and continues today. The total rotation is about 90° in the Western Transverse Ranges and less (about 40°) in the eastern ranges. Catalina Island shows the most rotation: almost 120°. A mechanism proposed for the rotation event is capture of the subducting Monterey plate by

2496-480: The south. The name is due to the ranges' east–west orientation, making them transverse to the general northwest–southeast orientation of most of California's coastal mountains. The ranges extend from west of Point Conception eastward approximately 500 kilometers into the Mojave and Colorado Desert . The geology and topography of the ranges express three distinct segments that have contrasting elevations, rock types, and vegetation. The western segment extends to

2548-424: The southern Hills through Upper Las Virgenes Canyon Open Space Preserve and Cheeseboro/Palo Comado Canyon Park to the Conejo Valley . In 1845 the land grant for Rancho El Escorpión , beside the Peak and named for it, was issued to one Chumash and two Tongva people and a rare instance of Native Americans being grantees, by Mexican Governor Pío Pico . The Rancho El Conejo was to the west, and included that end of

2600-506: The transition between coastal chaparral and coastal sage scrub, include California sagebrush and Toyon , the latter shrub having its southern distribution limit defined by the Transverse Ranges. Pinyon-juniper woodland is common on the north slopes of the San Gabriel and San Bernardino Mountains. This ecosystem is easily visible in the well-travelled Soledad Pass region, between Pearblossom and Santa Clarita . A number of densely populated coastal plains and interior valleys lie between

2652-450: The western and central segments many of the faults are thrust faults . Faults in the Coast Ranges and Peninsular Ranges trend northwest-southeast and butt into the east-west trending faults of the Transverse Ranges. Because all of these faults are considered active and seismic but they do to cut each other, the only geometry that satisfies that observation is if the east-west faults and Transverse Ranges are rotating clockwise with respect to

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2704-410: Was released in the 1959 event than in the Three Mile Island accident 20 years later. The groundwater under portions of the Simi Hills, contaminated with toxins and radionuclides that were also historically used at SSFL, has been and is a key concern in new development decisions and the SSFL property's future limited to parkland use after an impending cleanup, initiating preservation of more open space in

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