Physical features of western North America.
To the west of the coasts of Oregon and Washington the Pacific Plate is spreading along the Gorda and Juan de Fuca oceanic ridges. The Juan de Fuca Plate, east of this spreading center, is subducting under the North American Plate. The molten mantle rock produced by this subduction is responsible for the major volcanoes in the Cascade Range. All the Cascade composite cones are of the explosive type, their molten rock being high in silica. Until the eruption of Mount St. Helens in 1980, Lassen Peak had been the most recently active volcano (1914-17) in the contiguous 48 United States. The Mount St. Helens eruption, which blew off the top of its cone, was of greater magnitude than any other eruption in the region since the eruption and collapse of Mount Mazama 6,600 years ago, which formed the caldera now occupied by Crater Lake.
Because the subduction process is not continuous--it is impeded as huge
quantities of materials accumulate on plate boundaries--enormous stresses develop. The release of these stresses
creates shock waves of a larger magnitude than any other known earthquakes (i.e., greater than 9 on the Richter scale). It is thought that for the past 4,000 years
such subduction-related earthquakes have occurred at intervals of 300 to 1,000 years.
The arrangement of the ranges in the system can be visualized as being in
the shape of an extremely elongated H with a closed base. From north to south the west side of the H is made up
of the ranges of the Queen Charlotte Islands and Vancouver Island, the Olympic Mountains, and the Washington, Oregon,
and California Coast Ranges. From north to south the east side of the H consists of the Canadian Coast Mountains,
the Cascade Range, and the Sierra Nevada. The Klamath Mountains of southern Oregon and northern California make
up the east-west cross in the center of the H, while the Transverse Ranges bend eastward from the California Coast
Ranges to form the closed base of the H.
(Editor's Note: you cross these last mountains, The Transverse Ranges, driving Interstate 5, The Ridge Route, from Los Angeles to Bakersfield.)
Inside the H north of the Klamath Mountains are the drowned inside passage of British Columbia, the Puget Sound Lowland of Washington, and the Willamette Valley of Oregon. Inside the H south of the Klamath Mountains is the Central Valley of California.
Coastal plains are either narrow or nonexistent along the entire north-south extent of the coastal ranges. Offshore a narrow continental shelf drops abruptly into ocean depths. In places waves have cut notches and terraces. More resistant igneous rocks stand as sea cliffs with undercut notches. Softer sedimentary rocks have been eroded to form embayments. There is evidence of both the rise and fall of the coast, the result of tectonic activity.
The ranges of Vancouver Island and the Queen Charlotte Islands have been heavily glaciated. Stream valleys have been deepened by glaciers to produce a fjordlike coast, with relatively short streams draining the interior. Southward, across the Strait of Juan de Fuca, the Olympic Mountains rise to almost 8,000 feet (2,440 meters). They consist of folded sedimentary and metamorphic rock and also have been heavily glaciated. Drainage is radial from the highest peaks; among the major streams are the Hoh, Quinault, and Elwha.
The Canadian Coast Mountains and North Cascades differ structurally from the Middle and South Cascades. These northern ranges consist of a dissected upland of late Paleozoic rock (i.e., about 300 million years old) that has been folded, metamorphosed, and intruded by granites. Ridges in the North Cascades rise to elevations between 6,000 and 8,000 feet; above these ridges stand the composite volcanic cones of Glacier Peak and Mount Baker. The Coast Mountains of British Columbia are considerably lower, with the highest elevations reaching 3,000 to 4,000 feet in the south. The higher peaks, however, often are glacier-covered. All the ranges have been heavily dissected by running water both before and after the Pleistocene Epoch (1,600,000 to 10,000 years ago). During the Pleistocene they were covered by a cordilleran ice sheet, the glaciers of which occupied and deepened many existing stream valleys. On the east side of the North Cascades, Lake Chelan is in a glacially formed valley, and its deepest points are more than 1,500 feet below the surface. In the Coast Mountains glacial action has produced a spectacular fjord coast. Farther south in southern British Columbia and Washington are deep glacial valleys opening out onto the Fraser River delta and the Puget Sound Lowland.
The Middle Cascades, which extend southward from west-central Washington into Oregon, are an uplifted and faulted region consisting of volcanics of Tertiary and Quaternary age (i.e., from the past 66.4 million years). These volcanics consist of successive layers of tuffs, breccias, and mudflows, covered by basaltic flows. The range can be divided into eastern and western sections, the western being the oldest. Capping the higher, eastern part of the range is a more recent layer of Tertiary and Quaternary andesites and basalts. Elevations reach 4,000 to 6,000 feet, with a number of volcanic peaks--such as Mounts Rainier and Hood--standing high above the general surface relief; Rainier, at 14,410 feet (4,392 meters), is the highest peak in the Pacific mountain system.
The Columbia River cuts through the Middle Cascades in a magnificent gorge. On the southern (Oregon) side are numerous hanging valleys with streams that plunge in spectacular waterfalls into the gorge. Multnomah Falls, at 620 feet, is second in height in the United States only to Yosemite Falls in California. During the latter part of the Pleistocene (10,000-12,000 years ago), a large lake (Lake Missoula) was impounded by an ice dam in western Montana. On several occasions the dam gave way and released enormous quantities of water, which then rapidly drained to the sea. These floods deepened and widened the existing Columbia River valley and are responsible for the present profile of the gorge.
The South Cascades, extending from southern Oregon into northern California, differ from the Middle Cascades in that they were not uplifted. Even so, two of the major volcanoes in the western United States, Lassen Peak and Mount Shasta, surmount the range. The Pit River provides a low-elevation passage across these mountains.
The Klamath Mountains are the oldest of the Pacific coastal mountains, dating to the early Paleozoic Era (i.e., about 500 million years ago). They are extremely complex, probably resulting from the collision of tectonic plates in the Early Triassic Period (245-240 million years ago). Later they were intruded by granite batholiths. The Klamath Mountains have been glaciated in their higher elevations and have been heavily dissected by streams; the major watercourse crossing them is the Rogue River.
Both the Coast and the Transverse ranges were formed by plate collisions. The Washington and Oregon Coast Ranges consist of folded gray mudstones and siltstones oriented in a north-south direction. The major streams are antecedent to the uplift and have been drowned in their lower courses, producing estuaries. In addition to the Columbia these include the Umpqua and Siuslaw rivers. The California Coast Ranges also are made up of folded and faulted sedimentary rocks. The major faults trend northwest-southeast, however, and the rivers tend to follow these lines of weakness. The San Andreas Fault, passing through the southern California ranges, more or less bisects them before heading offshore near San Francisco. North of San Francisco Bay are the Napa, Russian, Eel, and Klamath rivers, while the Salinas River is the major coastal stream south of the bay. The eastern section of the Transverse Ranges consists of granites and metamorphic rocks, while the western portion resembles the sedimentary structure of the Coast Ranges; streams draining them include the Santa Clara and Santa Ana rivers.
In the Pacific mountain system are found all of the 11 orders of soils, of which three are most abundant. Inceptisols dominate in the coastal ranges from the Queen Charlotte Islands south to San Francisco Bay and in the Cascade Range. They have weakly differentiated horizons (layers), are little altered from their parent material, and occur where summers are cool. The California Coast Ranges south of San Francisco and the Transverse Ranges have dark-colored mollisols rich in organic material. Much of the Klamath Mountains area is characterized by ultisols, leached reddish soils that develop where winters are mild and moist and summers are warm and dry.
The orientation of the Pacific mountains has a profound effect on the climate of the western United States and Canada. Regionally, they act as a barrier to storms from the Pacific Ocean, which especially in winter bring large quantities of precipitation to the western slopes of the ranges. For example, the highest annual precipitation levels in the 48 contiguous states (more than 150 inches [3,800 millimeters]) occur on the southwestern slope of the Olympic Mountains (Hoh Rainforest), while the greatest annual precipitation total in Canada (more than 200 inches) occurs along the British Columbia coast north of Vancouver Island. Inland precipitation decreases on the east side of the coastal ranges and increases again on the western slopes of the Cascades, in some places exceeding 100 inches; much of this is in the form of snow. Immediately east of the Cascades the annual precipitation decreases drastically to only 8 inches at Yakima, Wash. In California more than 50 inches falls on the windward side of the Coast Ranges, decreasing to 30 inches in the higher parts of the Transverse Ranges.
Climate is the major influence on vegetation type. Conifers predominate and can grow to enormous size, especially on the moister, western slopes. The Sitka spruce is dominant along the coast from southern British Columbia to northern California. The largest standing mid-latitude rain forest in the United States is on the west side of the Olympic Mountains. Inland and up into the Cascades, Douglas fir and western hemlock dominate. They give way at high elevations to trees such as Pacific silver fir and mountain hemlock. On the eastern slopes of the Cascades, ponderosa pine is the major tree.
Along the coast from southern Oregon to the Monterey Peninsula of California, the redwood is dominant, occurring with Sitka spruce, Douglas fir, and hardwoods such as alder. Farther from the coast, the Coast Ranges are characterized by mixed forests of bigleaf maple, madrone, various oaks, and pines and other conifers. On the eastern slopes is an oak-grassland association. In the drier Transverse Ranges is found the bigcone Douglas fir, as well as pines and oaks.
The anadromous (river-spawning) salmon is the most distinctive creature of the coastal ranges. Five species--pink, chum, coho, sockeye, and chinook--are found in the streams draining the mountains, each with its own distinctive range and environmental conditions. The pink and chum spawn in coastal streams near the ocean, while the sockeye usually spawns in upstream lakes. The chinook (or king) favors large rivers such as the Columbia and Sacramento and travels hundreds of miles inland. The coho also favors larger rivers.
The American shad, another anadromous fish and originally native to the Atlantic coast, was introduced in the late 19th century and has adapted to streams of the Pacific coast. Also inhabiting coastal waters are harbor seals, northern fur seals, northern elephant seals, sea otters, and northern and California sea lions. The pelts of sea otters were the first furs traded in the Pacific coastal region, obtained from the Indians of British Columbia and sold in China.
The larger land mammals include Roosevelt elk in the coastal ranges from British Columbia to northern California and black bears in the coastal ranges and Cascades. Three species of deer are found: mule deer on the eastern slopes of the Cascades, black-tailed deer in the coastal and Cascade ranges, and, locally in the Coast Ranges of Oregon, white-tailed deer. Three members of the cat family--lynx, bobcat, and mountain lion--are found throughout the Pacific mountain system. The beaver, a mainstay of the 18th- and 19th-century fur trade, is found as far south as northern California. Brought to the brink of extinction in the 19th century, it is now protected in most areas.
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