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glacial troughs

Geography of the United States Pacific Mountain System


Physiographic regions of the U.S. Interior. (See legend)

For purposes of description, the physical geography of the United States is split into several major physiographic divisions, one being the Pacific Mountain System. Please refer to the Geography of the United States for the other areas.

The Pacific Ranges

Strong mountain ranges follow the trend of the Pacific coast, 150 or 200 miles inland. The Cascade Range enters from Canada, trending southward across the international boundary through Washington and Oregon to latitude 41°. The Sierra Nevada extends thence south-eastward through California to latitude 35°. The lower coast ranges, nearer the ocean, continue a little farther southward than the Sierra Nevada, before giving way to that part of the Basin Range province which reaches the Pacific in southernmost California.

Cascade Range

The Cascade Range is in essence a maturely dissected highland, composed in part of upwarped Colombian lavas, in part of older rocks, and crowned with several dissected volcanoes, of which the chief are (beginning in the north) Mount Baker (10,778 ft), Mount Rainier (14,410 ft), Mount Adams (12,276 ft.), Mount St. Helens (8,364 ft.), and Mount Hood (11,239 ft). The first four are in Washington and the last in northern Oregon. These bear snowfields and glaciers. The dissected highlands, with ridges of very irregular arrangement, are sculptured everywhere in a fashion that strongly suggests the work of numerous local Pleistocene glaciers as an important supplement to preglacial erosion. Lake Chelan, long and narrow, deep set between spurless ridges with hanging lateral valleys, and evidently of glacial origin, ornaments one of the eastern valleys. The range is squarely transected by the Columbia River, which bears every appearance of antecedent origin. The cascades in the river gorge are caused by a sub-recent landslide of great size from the mountain walls. The Klamath River, draining several lakes in the northwest part of the Basin Range province and traversing the Cascade Range to the Pacific, is apparently also an antecedent river.

The Cascade Mountains present a marked example of the effect of relief and aspect on rainfall. They rise across the path of the prevailing westerly winds not far inland from a great ocean. They receive an abundant rainfall (80 in. or more, annually) on the Westward or windward slope, and there they are heavily forested. The rainfall is light on the eastward slope and the piedmont district is dry so the forests thin out on that side of the range and treeless lava plains follow immediately eastward.

Sierra Nevada

Generally speaking, the Sierra Nevada is a great mountain block, largely composed of granite and deformed metamorphosed rocks, reduced to moderate relief in an earlier (Cretaceous and Tertiary) cycle of erosion. Sub-recently, it has been elevated with a slant to the west, and in this position sub-maturely dissected. The region was by no means a peneplain before its slanting uplift. Its surface then was hilly and in the south mountainous. In its central and still more in its northern part, it was overspread with lavas which flowed westward along the broad open valleys from many vents in the eastern part. Near the northern end of the range, eruptions have continued in the present cycle, forming many cones and young lava flows. The tilting of the mountain mass was presumably not a simple or a single movement. It was probably slow, for the Pitt River (headwaters of the Sacramento) traverses the northern part of the range in antecedent fashion. The tilting involved the subdivision of the great block into smaller ones, in the northern half of the range at least. Lake Tahoe (altitude 6225 ft) near the range crest is explained as occupying a depression between two block fragments. Farther north similar depressions now appear as aggraded highland meadows. The tilting of the great block resulted in presenting a strong slope to the east and a long moderate slope to the west. To the east, it faces the deserts of the Basin Range province and also is in large measure responsible for their aridity. The altitudes along the upraised edge of the block, or range crest, are approximately 5000 ft in the north and 11,000 ft in the south. The mountains in the southern part of the block, which had been reduced to subdued forms in the former cycle of erosion, were thus given a conspicuous height. There, they form the High Sierra and are greatly sharpened by revived erosion, both normal and glacial. In this way Mount Whitney (14,505 ft.) came to be the highest summit in the United States (excluding Alaska). In the new altitude of the mountain mass, its steep eastern face has been deeply carved with short canyons. On the western slope, an excellent beginning of dissection has been made in the erosion of many narrow valleys, whose greatest depth lies between their headwaters which still flow on the highland surface, and their mouths at the low western base of the range. The highlands and uplands between the chief valleys are but moderately dissected. Many small side streams still flow on the highland and descend by steeply incised gorges to the valleys of the larger rivers. Some of the chief valleys are not cut in the floors of the old valleys of the former cycle, because the rivers were displaced from their former courses by lava flows, which now stand up as table mountains. Glacial erosion has been potent in excavating great cirques and small rock-basins, especially among the higher southern surmounting summits, many of which have been thus somewhat reduced in, height while gaining an Alpine sharpness of form. Some of the short and steep canyons in the eastern slope have been converted into typical glacial troughs, and huge moraines have been laid on the desert floor below them. Some of the western valleys have also in part of their length been converted into U-shaped troughs. The famous Yosemite Valley, eroded in massive granite, with side cliffs 1000 or 2000 ft. in height, and the smaller Hetch Hetchy Valley not far away, are regarded by some observers as owing their peculiar forms to glacial modifications of normal preglacial valleys.

The western slope of the Sierra Nevada bears fine forests similar to those of the Cascade Range and of the Coast Range, but of more open growth, and with the redwood exchanged for groves of big trees (Sequoia gigantea) of which the tallest examples reach 325 ft. The higher summits in the south are above the tree line and expose great areas of bare rock. Mountaineering is here a delightful summer recreation, with camps in the highland forests and ascents to the lofty peaks. Gold occurs in quartz veins traversing various formations (some as young as Jurassic), and also in gravels, which were for the most part deposited previous to the uplift of the Sierra block. Some of the gravels then occurred as piedmont deposits along the western border of the old mountains. These gravels are now more or less dissected by new-cut valleys. Other auriferous gravels are buried under the upland lava flows, and are now reached by tunnels driven in beneath the rim of the table mountains.

Other

The northernmost part of the coast ranges, in Washington, is often given independent rank as the Olympic Mountains (Mount Olympus, 7,965 ft.). It is a picturesque mountain group, bearing snowfields and glaciers, and suggestive of the dome-like uplift of a previously worn-down mass. Farther south, through Oregon and northern California, many members of the coast ranges resemble the Cascades and the Sierra in offering well-attested examples of the uplift of masses of disordered structure, that had been reduced to a tame surface by the erosion of an earlier cycle, and that are now again more or less dissected.

Several of the ranges ascend abruptly from the sea. Their base is cut back in high cliffs. The Santa Cruz Mountains, south of San Francisco, is a range of this kind. There are moderate re-entrants between the ranges that have a continuous concave seaward beaches such as Monterey Bay. On still other parts of the coast a recent small elevatory movement has exposed part of the former ea bottom in a narrow coastal plain, of which some typical arborless examples are found in Oregon. Most of the recent movements appear to have been upward, for the coast presents few embayments such as would result from the depression and partial submergence of a dissected mountain range. here are three important exceptions must be made to this rule. In the north, the Strait of Juan de Fuca and the intricately branching waterways of Puget Sound between the Cascade and the Olympic ranges occupy trough-like depressions which were filled by extensive glaciers in Pleistocene times and mark the beginning of the great stretch of fjorded coast which extends northward to Alaska. The second important embayment is the estuary of the Columbia river. Finally, the more important one is San Francisco Bay, situated about midway on the Pacific coast of the United States, the result of a moderate depression whereby a transverse valley, formerly followed by Sacramento river through the outermost of the Coast ranges, has been converted into a narrow strait the Golden Gate and a wider intermontane longitudinal valley has been flooded, forming the expansion of the inner bay.

The Coast Range is heavily forested in the north, where rainfall is abundant in all seasons. However, its lower ranges and valleys have a scanty tree growth in the south, where the rainfall is very light. Here, Coast redwoods and Coast live oaks (Quercus agrifolia) grow. The chief metalliferous deposits of the range are of mercury at New Almaden, not far south of San Francisco. The open valleys between the spaced ranges offer many tempting sites for settlement, but in the south irrigation is needed for cultivation.

The belt of relative depression between the inner Pacific ranges and the Coast range is divided by the fine volcano Mount Shasta (14,380 ft) in northern California into unlike portions. To the north, the floor of the depression is for the most part above baselevel, and hence is dissected by open valleys, partly longitudinal, partly transverse, among hills of moderate relief. This district was originally for the most part forested, but is now coming to be cleared and farmed.

South of Mt Shasta, the California Central Valley is an admirable example of an aggraded intermontane depression, about 400 miles long and from 30 to 70 miles wide. The floor of this depression being below baselevel, it has necessarily come to be the seat of the mountain waste brought down by the many streams from the newly uplifted Sierra Nevada on the east and the coast ranges on the west. Each stream forms an alluvial fan of very gentle slope. The fans all become laterally confluent and incline very gently forward to meet in a nearly level axial belt. There the trunk rivers the Sacramento from the north and the San Joaquin from the southeast wander in braided courses with their waters entering San Francisco Bay. Kings River, rising in the high southern Sieria near Mt Whitney, has built its fan rather actively A little north of the center of the valley rise the Marysville Buttes, the remains of a maturely dissected volcano (2128 ft). Elsewhere the floor of the valley is a featureless, treeless plain. (W. M. D.)

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