Transverse Ranges

San Gabriel Mountains

San Bernardino Mountains, with Mt. San Gorgonio in the foreground and Big Bear Lake in the background.

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 between Santa Barbara and San Diego counties. They derive the name Transverse Ranges due to their East-West orientation, making them transverse to the general North-South orientation of most of California's coastal mountains.

Geology

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.

The Transverse Ranges represent a complex of tectonic forces and faulting stemming from the interaction of the Pacific Plate and the North American Plate along the San Andreas Fault system. Their orientation along an east-west axis as opposed to the general southeast-northwest trend of most California ranges results from a pronounced step in the San Andreas Fault, the cause of which is a subject of intensive ongoing study. The San Andreas Fault is a dextral strike-slip fault with a right step, causing the mountains. Their elevation is somewhat better understood as a consequence of this step. The crust atop the Pacific Plate south of the ranges does not easily make the turn westward as the entire plate moves northwestward, forcing pieces of the crust to compress and lift.

The crust which comprises the Transverse Ranges is part of what is known as the Salinian Block, originally a piece of the North American Plate which was broken off what is now northwestern Mexico as the Gulf of California rifted open.

Physiographically, the Tranverse Ranges are a distinct physiographic section of the larger Pacific Border province, which in turn is part of the larger Pacific Mountain System. They exhibit extreme differences in geologic age and composition, varying from sedimentary rocks in the western Santa Ynez and Santa Monica mountains to primarily granitic and metamorphic rock in the eastern regions, where they terminate abruptly in the San Gabriel and San Bernardino mountains.

Geography

Ranges

San Bernardino is a foothill community, shown above in the Shandin Hills Mountain Range.

The Transverse ranges run predominantly east-west, while the other coast ranges tend north-south, including the Central Coast Ranges to the north and the Peninsular Ranges to the south. They begin at Point Conception in Santa Barbara County, and include the Santa Ynez Mountains that run parallel to the coast behind Santa Barbara. Also in Santa Barbara County, they include the San Rafael Mountains and the Sierra Madre Mountains, both of which extend approximately to the Ventura County line. The 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 Ranges include the steep San Gabriel Mountains northeast of Los Angeles, the San Rafael Hills, Puente Hills, San Jose Hills, and Chino Hills, and the San Bernardino Mountains. To the north of the Transverse ranges are the Central Coast Ranges, The Central Valley, and the Tehachapi Mountains, which separate the Central Valley from the Mojave Desert to the east, and link the Transverse ranges to the Sierra Nevada. The Mojave Desert and California's low desert, including the Coachella Valley, are at the eastern end of the ranges. The northern Channel Islands of California are also part of the Transverse Ranges; San Miguel, Santa Rosa, Santa Cruz and Anacapa Islands are a westward extension of the Santa Monica Mountains.

Mountain and hill ranges in the Transverse Ranges:

• Santa Ynez Mountains
• San Rafael Mountains
• Sierra Madre Mountains
• Topatopa Mountains
• Santa Susana Mountains
• Simi Hills
• Chalk Hills
• Santa Monica Mountains
• San Gabriel Mountains

• San Rafael Hills
• Puente Hills
• San Jose Hills
• Shandin Hills
• San Bernardino Mountains
• Little San Bernardino Mountains
• Tehachapi Mountains
• Sierra Pelona Mountains
• San Emigdio Mountains

Peaks

Mountains in the Transverse Ranges, listed by height:

1. Mount San Gorgonio, 11,499 feet (3,505 m), San Bernardino Mountains.
2. San Bernardino Peak, 10,649 feet (3,246 m), San Bernardino Mountains.
3. Mount San Antonio (Old Baldy), 10,064 feet (3,068 m), San Gabriel Mountains.
4. Mount Pinos, 8,847 feet (2,697 m) San Emigdio Mountains.
5. Frazier Mountain, 8,017 feet (2,444 m) San Emigdio Mountains.
6. Reyes Peak, 7,510 feet (2,290 m), Pine Mountain, San Emigdio Mountains.
7. Mount Wilson,5,710 feet (1,740 m), San Gabriel Mountains.
8. Oat Mountain, 3,747 feet (1,142 m), Santa Susana Mountains.

Transportation

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 Expressway (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, like (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.

Ecology

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 forest, 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 California 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.

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.

See also

• Southern California faults
• Transverse Ranges topics index

Further reading

• U.S. Geological Survey. (1987). Recent reverse faulting in the Transverse Ranges, California [U.S. Geological Survey Professional Paper 1339]. Washington, D.C.: U.S. Government Printing Office.