California's Coastal Mountains

The following materials are exerpts from the California Coastal Commission's California Coastal Resource Guide, which can be ordered from University of California Press by calling 1-800-822-6657.

California's coastal mountains trace a sinuous 800-mile course from the northwest corner of Del Norte County south to the Mexican border. Except for a break in the chain at the Golden Gate, they form a continuous series of ranges and valleys, separating the coast from the Great Central Valley and the deserts of the interior. This mountainous barrier has a dramatic effect on California's climate: storms originating over the Pacific Ocean bring rain to the western slopes, while the eastern slopes remain relatively dry. Many of California's industries flourish in the climatic conditions created by the coastal mountains--the evergreen trees that support the north coast timber industry thrive on the increased rainfall and frequent fog of the region; coastal fog cools hot inland valleys just east of the coast mountains where wine grapes are cultivated; and fruit and nut trees and cool weather vegetables are grown in coastal areas from San Mateo County to San Diego.

The geologic history of California's coastal mountains begins several hundred million years ago when, according to current geologic theory, movement of the earth's crust set in motion the processes that created the coastal ranges. The geologic theory of plate tectonics describes the system of loosely interlocking plates, floating upon an underling mantle of less solid material, that cover the earth's surface. The North American Plate supports the continent of North America, and the Pacific Plate lies beneath the Pacific Ocean. About 250 million years ago these two plates, which had been gradually moving towards each other, collided; the sea floor crust of the Pacific Plate slipped beneath the continent, heating and melting as it reached the earth's interior. Between 150 and 140 million years ago this molten rock, or magma, began to push upward, forming the Klamath and Peninsular ranges.

About 30 million years ago the relative movements of the North American Plate and the Pacific Plate changed from a head-on contact to a lateral slipping against each other. This zone of slippage, extending nearly the length of the state, is called the San Andreas Fault. Along this zone, folding of the sea floor along the margin of the North American Plate resulted in the creation of the Coast and Transverse ranges, which are composed of the crushed, crumpled, and folded sea floor sediments.

The coastal mountains constitute four geomorphic provinces or geologic regions within California. The northern-most is the Klamath Mountains province, which lies near the coast in northwestern Del Norte County and extends north into Oregon. The northwest-trending Coast Ranges, the largest of the state's geomorphic provinces, rises abruptly from the shore in northern Humboldt County and extend 400 miles south to the Santa Ynez River in Santa Barbara County. The Transverse Ranges lie along an east- west axis, from the Santa Barbara coast to the Mojave Desert, creating a natural barrier between Central and Southern California. The massive Peninsular Ranges complete the coastal mountain system, extending south from the Los Angeles Basin to the tip of the Baja Peninsula.

In Northern California, the Klamath Mountains are composed of metamorphic and granitic rock--formed as a result of extreme changes in temperature, pressure, and chemical composition that occurred when molten material from below the earth's crust was pushed to the surface. South of the Klamath Mountains, the Coast Ranges lie close to the continent's edge, from Humboldt County to San Francisco Bay, forming a series of low mountains paralleling the coast. south of the bay, which separates the Coast Ranges into northern and southern ranges, are the Diablo, Gabilan, Santa Cruz, and Santa Lucia mountains, the highest of which reach to 4,000 feet. The sea floor sediment--sandstones and shales--that make up the Coast Rages were crumpled so completely that it is difficult to discern individual layers of sedimentation. Visible in the sea cliffs along the Northern California coast are massive and steeply dipping rock layers, called the Franciscan Formation; a spectacular example of this geology can be seen along the cliffs at Devil's Slide in San Mateo County.

The Transverse Ranges include the Santa Monica Mountains, which extend offshore to form the Northern Channel Islands of Santa Cruz, Santa Rosa, and San Miguel off the coast of Santa Barbara County. Three hundred miles east, the Transverse Ranges terminate abruptly in the San Gabriel and San Bernadino mountains, dropping off into the Mojave and Colorado deserts. Severely folded, twisted , and uplifted, these mountains exhibit extreme differences in geologic age. Sedimentary rocks are most common on the western slopes such as the Santa Ynez and Santa Monica mountains, which hug the coast from Santa Barbara County south to Los Angeles County; in the rugged eastern mountains, granitic and metamorphic rock dominate. To the south, the Peninsular Ranges-- steep, narrow , and northwest trending--include in the southeast the rugged San Jacinto, Santa Rosa, Agua Tibia, and Laguna mountains that plunge into the Coachella and Imperial valleys. To the west the rolling slopes of the Santa Ana Mountains gradually descend onto broad marine terraces that front the ocean; these mountains submerge westward, forming the Southern Channel Islands of Santa Barbara, San Nicolas, and San Clemente. The highest of the coastal mountains rages, the Transverse and Peninsular Ranges both contain peaks of over 10,000 feet.

Dramatic changes in elevation and a variety of climatic zones contribute e to a diversity of plant life in California's coastal mountains. Conifers--redwood and Douglas-fir--cloak the windward slops of the Klamath Mountains and the northern Coast Ranges. Heavy winter rainfall, summer fog, and moderate temperatures have produced redwood groves where 2,000 year-old trees tower more than 300 feet above the forest floor. South of San Francisco Bay, the slopes of the Santa Cruz Mountains are covered with stands of redwood while the drier regions of the southern Coast Ranges are vegetated with oaks, pines, and chaparral. As precipitation decreases southward , in the central and southern Coast Ranges, stands of hardwoods including tanbark oak, coast live oak, big- leaf maple, and madrone begin to outnumber conifers. On steeper slopes and exposed ridges where thin soils lose moisture rapidly, drought-resistant chaparral species such as chamise, manzanita, sage, and scrub oak take hold.

In the semi-arid Transverse and Peninsular ranges, chaparral is abundant on windward and southwest-facing slopes, whereas mixed- coniferous forest--Ponderosa pine, sugar pine and whit fir--grows in isolated stands in protected areas. Hardy, drought-resistant digger and knobcone pines grow on the dry, rocky slopes.

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