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 the south. The name is due to the east–west orientation of the ranges, 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 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.

Snow falls above most winters, and above every few years. It is rare for elevations above to go multiple winters without snow, even during severe droughts. Due to relatively low humidity, the regional snow line lies at about , 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 ; San Gorgonio Mountain is the only peak with an alpine environment.

Geology

thumb|View west of the eastern portion of the San Bernardino and San Gabriel Mountains, with the [[Mojave Desert on the right and Silverwood Lake near the boundary. The San Andreas Fault runs straight up the middle toward the horizon.]]

thumb|The San Andreas Fault trends more east-west where it cuts through the Transverse Ranges.

Tectonics

The Transverse Ranges result from a complex of tectonic forces and faulting stemming from 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 ( million years ago) when southern reaches of 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 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°.

Rocks

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 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 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. These faults are part of the same thrust fault system south of the northern Channel Islands of San Miguel, Santa Rosa, and Anacapa Islands.</blockquote>

Ecology

thumb|Savannah-like oak woodlands are typical of the westernmost ranges; shown here is the north slope of the [[Santa Ynez Mountains]]

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 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 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.

Urban impact

A number of densely populated coastal plains and interior valleys lie between 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 the Transverse Ranges and the Peninsular Ranges to the south.

Transportation

thumb| View looking westward at the western portion of the San Bernardino and San Gabriel Mountains, with the [[San Bernardino Valley to the left, with San Bernardino itself in the foreground. The Foothill Freeway (I-210) runs toward the horizon, while I-15 runs through Cajon Pass to the right.]]

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 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.

References

External links

Plio-Pleistocene Tectonic Shortening of the Transverse Ranges , block model animation by Tanya Atwater