| Transverse Ranges | |
|---|---|
 The ellipse outlines the region of the California Transverse Ranges | |
| Highest point | |
| Peak | San Gorgonio Mountain |
| Elevation | 11,503 ft (3,506 m) |
| Coordinates | 34°05′57″N116°49′29″W / 34.099162°N 116.824853°W /34.099162; -116.824853 |
| Dimensions | |
| Length | 300 mi (480 km) |
| Geography | |
 | |
| Country | United States |
| Parent range | Pacific Coast Ranges |
TheTransverse Ranges are agroup of mountain ranges ofSouthern California, in thePacific Coast Ranges physiographic region in North America. The Transverse Ranges begin at the southern end of theCalifornia Coast Ranges and lie within Santa Barbara, Ventura, Los Angeles, San Bernardino, Riverside and Kern counties. ThePeninsular Ranges lie to the south. The name is due to the ranges' east–west orientation, making themtransverse to the general northwest–southeast orientation of most of California's coastal mountains.[1][2][3][4]
The ranges extend from west ofPoint Conception eastward approximately 500 kilometers into theMojave andColorado 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 theSan Gabriel Mountains andSan Gabriel fault. The central segment includes mountains that range eastward to theSan Andreas fault. The eastern segment extends from theCajon Pass at the San Andreas fault eastward to the Colorado Desert.[1] The central and eastern segments (near the San Andreas fault) have the highest elevations.
Most of the ranges lie in theCalifornia chaparral and woodlands ecoregion. Lower elevations are dominated by chaparral and scrubland, while higher elevations support large conifer forests. Most of the ranges in the system arefault blocks, and were uplifted by tectonic movements late in theCenozoic Era. West ofTejon 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.[1]
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The western and central segments of the Transverse Ranges are bounded to the north and east by the San Andreas Fault, which separates those segments from the Mojave Desert.[1][2] The eastern segment bounds the southern Mojave Desert. Notable passes along the San Andreas fault includeTejon Pass,Cajon Pass, andSan Gorgonio Pass. Components of Transverse Ranges to the north and east of the fault include theSan Bernardino Mountains,Little San Bernardino Mountains,Pinto andEagle Mountains. The western and southern boundaries are acknowledged to be thePacific Ocean and the northernChannel Islands. Onshore theLos Angeles Basin lies at the southern boundary of the western and central segments of the ranges. Major passes not along the San Andreas Fault includeGaviota Pass,San Marcos Pass, theConejo Grade,Newhall Pass, andCahuenga Pass.
The Transverse Ranges manifest themselves as a series of roughly parallel ridges with an average height of 3,000–8,000 feet (900–2,400 m). The ranges are dissected by young, steep streams of relatively low flow rate; as a result, there is high topographic relief throughout the range, and other than in marginal areas (e.g. theSan Fernando Valley) and a few river valleys (such asLockwood 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 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 asSanta Barbara during the2005 La Conchita landslide and the2018 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 theSanta Ynez Mountains that run parallel to the coast behindSanta Barbara and theSan Rafael Mountains on the other side of theSanta Ynez Valley. The western Transverse Ranges also include theTopatopa Mountains and theSanta Susana Mountains of Ventura County and Los Angeles County, theSimi Hills, theSanta Monica Mountains that run along the Pacific coast behindMalibu, and whose eastern portion are known as theHollywood Hills, and theChalk Hills. The northernChannel Islands of California are also part of the Transverse Ranges;San Miguel,Santa Rosa,Santa Cruz andAnacapa Islands are a westward extension of the Santa Monica Mountains.
The Ranges include the steepSan Gabriel Mountains northeast ofLos Angeles, theSierra Pelona Mountains just south of the San Andreas fault, the Tehachapi Mountains southeast of Bakersfield, theVerdugo Mountains, the Liebre-Sawmill Mountains, theSan Rafael Hills,Puente Hills,San Jose Hills, andChino Hills.
TheSan Bernardino Mountains,Little San Bernardino Mountains, and thePinto,Eagle, andOrocopia Mountains are within the eastern segment. The Mojave Desert and California's low desert, including theCoachella 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 theSan Rafael Mountains and theSierra Madre Mountains. Likewise, theTehachapi Mountains north of the Mojave Desert, although nearly transverse, are the southern end of theSierra 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 isCsb (Warm-summerMediterranean) under theKöppen climate classification; the upper reaches of San Gorgonio Mountain have an alpine tundra climate (ET), while 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).[5]
Snow falls above 6,000 feet (1,800 m) most winters, and above 3,000 feet (900 m) every few years. It is rare for elevations above 8,000 feet (2,000 m) to go multiple winters without snow, even during severe droughts. Due to relatively low humidity, the regionalsnow line lies at about 14,000–16,000 feet (4,000–5,000 m), above the highest elevation of the range; as such, snow does not persist year-round except in the form ofsnow patches.
The tree line lies at about 11,000 feet (3,000 m); San Gorgonio Mountain is the only peak with an alpine environment.
| Climate data for Big Bear Lake, California, (1991–2020 normals, extremes 1914–present) | |||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Month | Jan | Feb | Mar | Apr | May | Jun | Jul | Aug | Sep | Oct | Nov | Dec | Year |
| Record high °F (°C) | 71 (22) | 72 (22) | 80 (27) | 82 (28) | 87 (31) | 98 (37) | 94 (34) | 93 (34) | 90 (32) | 85 (29) | 74 (23) | 70 (21) | 98 (37) |
| Mean maximum °F (°C) | 60.1 (15.6) | 60.3 (15.7) | 65.1 (18.4) | 72.2 (22.3) | 78.4 (25.8) | 85.8 (29.9) | 88.7 (31.5) | 86.5 (30.3) | 82.1 (27.8) | 75.5 (24.2) | 68.3 (20.2) | 61.8 (16.6) | 89.8 (32.1) |
| Mean daily maximum °F (°C) | 46.1 (7.8) | 45.7 (7.6) | 51.2 (10.7) | 57.3 (14.1) | 65.8 (18.8) | 75.1 (23.9) | 79.7 (26.5) | 78.7 (25.9) | 73.0 (22.8) | 63.4 (17.4) | 53.5 (11.9) | 46.2 (7.9) | 61.3 (16.3) |
| Daily mean °F (°C) | 34.3 (1.3) | 34.3 (1.3) | 38.9 (3.8) | 44.1 (6.7) | 51.3 (10.7) | 59.3 (15.2) | 65.1 (18.4) | 64.1 (17.8) | 58.5 (14.7) | 49.0 (9.4) | 40.4 (4.7) | 34.3 (1.3) | 47.8 (8.8) |
| Mean daily minimum °F (°C) | 22.7 (−5.2) | 22.9 (−5.1) | 26.5 (−3.1) | 30.8 (−0.7) | 36.8 (2.7) | 43.5 (6.4) | 50.5 (10.3) | 49.5 (9.7) | 44.0 (6.7) | 34.6 (1.4) | 27.4 (−2.6) | 22.6 (−5.2) | 34.3 (1.3) |
| Mean minimum °F (°C) | 8.8 (−12.9) | 9.8 (−12.3) | 14.1 (−9.9) | 20.1 (−6.6) | 27.0 (−2.8) | 33.4 (0.8) | 40.8 (4.9) | 40.0 (4.4) | 32.5 (0.3) | 24.5 (−4.2) | 15.4 (−9.2) | 10.1 (−12.2) | 4.9 (−15.1) |
| Record low °F (°C) | −25 (−32) | −16 (−27) | −12 (−24) | −7 (−22) | 15 (−9) | 22 (−6) | 28 (−2) | 28 (−2) | 19 (−7) | 10 (−12) | −15 (−26) | −14 (−26) | −25 (−32) |
| Averageprecipitation inches (mm) | 4.51 (115) | 4.39 (112) | 2.37 (60) | 0.78 (20) | 0.42 (11) | 0.15 (3.8) | 0.69 (18) | 0.82 (21) | 0.40 (10) | 0.79 (20) | 1.35 (34) | 3.31 (84) | 19.98 (508.8) |
| Average snowfall inches (cm) | 13.4 (34) | 14.3 (36) | 11.7 (30) | 3.0 (7.6) | 0.6 (1.5) | 0.0 (0.0) | 0.0 (0.0) | 0.0 (0.0) | 0.0 (0.0) | 1.2 (3.0) | 3.4 (8.6) | 11.0 (28) | 58.6 (148.7) |
| Average precipitation days(≥ 0.01 in) | 6.6 | 6.8 | 5.5 | 3.7 | 2.0 | 0.7 | 2.6 | 2.8 | 2.0 | 2.2 | 2.9 | 5.5 | 43.3 |
| Average snowy days(≥ 0.1 in) | 3.5 | 3.5 | 2.7 | 1.2 | 0.3 | 0.1 | 0.0 | 0.0 | 0.0 | 0.3 | 1.1 | 3.3 | 16.0 |
| Source: NOAA[6][7][8] | |||||||||||||
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The Transverse Ranges result from a complex of tectonic forces andfaulting stemming from the interaction of thePacific Plate and theNorth 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 thePliocene (c. 4 million years ago) when southern reaches of the fault moved east to open theGulf of California.[9] 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 the right shear of Pacific Plate – North American Plate motion.[10][11][12] This tectonic rotation began in Early Miocene Time and continues today.[13] The total rotation is about 90° in the Western Transverse Ranges and less (about 40°) in the eastern ranges.[14]Catalina Island shows the most rotation: almost 120°.[11]
A mechanism proposed for the rotation event is capture of the subducting Monterey plate by 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.[15]
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 the central and eastern segments, including the San Gabriel and San Bernardino Mountains.
The oldest basement (deepest) rocks are ofProterozoic age, and are found in the San Gabriel Mountains and the San Bernardino Mountains. TheJurassic-CretaceousFranciscan Assemblage is found in the western section of the ranges and is the presumed basement in this segment. Exposed plutonic rocks from theMesozoic, mostly granites, can be found onMount Pinos and generally in regions east ofTejon Pass. The youngest rocks areCenozoic 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.[1] The thickest deposits of these are in the Santa Barbara Channel and Ventura basin. These are mostly marine in origin with a marked change tored beds of river systems ofOligocene age in the western and central segments.
Limestones anddolomites of the marine MioceneMonterey 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 the western and central segments many of the faults arethrust faults.[1][2] 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 the faults outside that province.[10]
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.[16] These faults are part of the same thrust fault system south of the northern Channel Islands of San Miguel, Santa Rosa, and Anacapa Islands.[1] North of the western segment the bounding faults are left slipSanta Ynez and Pine Mountain Faults. TheSan 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 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. TheLos Angeles Basin south of the western segment, formed during rotation of that segment away from the Peninsular Ranges.[10] It is also a site of prolific oil production. The California Geologic Survey states:
Great thicknesses ofCenozoic petroleum-rich sedimentary rocks have been folded and faulted, making California one of themost important oil-producing areas in the United States.[17]
The plant place of the Transverse ranges includecoastal sage scrub,oak woodland and savanna, andpinyon-juniper woodland at lower elevations, andyellow pine forest,Lodgepole Pine, andsubalpine forest at higher elevations.[18] TheAngeles andLos Padres National Forests cover portions of the Transverse ranges. The ranges are part of theWWF-designatedCalifornia montane chaparral and woodlandsecoregion, but the eastern points of the range touch two desert regions, the Mojave Desert and the Colorado Desert section of theSonoran Desert. TheCarrizo Plain adjoins the northern edge of the Transverse Range.[citation needed]
Chaparral is a common feature of the Transverse Ranges. Common plant associates inchaparral, especially in the transition between coastal chaparral and coastal sage scrub, includeCalifornia sagebrush andToyon, the latter shrub having its southern distribution limit defined by the Transverse Ranges.[19]
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-travelledSoledad Pass region, betweenPearblossom andSanta Clarita.[citation needed]
A number of densely populated coastal plains and interior valleys lie between the mountain ranges, including theOxnard Plain of coastalVentura County, theSanta Clarita Valley north of Los Angeles, theSan Fernando Valley, which is mostly included in the City of Los Angeles. The Los Angeles Basin, which includes the portion ofLos Angeles County south of the Santa Monica Mountains and most ofOrange County, and theInland Empire basin, which includes the cities ofSan Bernardino andRiverside, lie between the Transverse Ranges and the Peninsular Ranges to the south.[citation needed]
There are a number of important freeways that cross the Transverse Ranges, such as (from west to east) US-101,I-5 atTejon Pass,SR 14 atSoledad Pass, andI-15 atCajon Pass. These highways linkSouthern California with places to the north and northeast likeSan Francisco andLas Vegas, respectively. With the exception of several high passes on less-traveledSR 33,SR 2,SR 330.SR 18 andSR 38, none of these passes are at high elevations, with Cajon Pass being at a modest 4,190 ft (1,277 m)above mean sea level; this means thatsnow is less of a factor here than in the moderate to high mountain passes to the north likeDonner 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.[citation needed]
