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.
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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.
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 (10,827 ft), Mount Rainier (14,363 ft), Mount Adams (12,470 ft.) and Mount Hood (11,225 ft). The first three 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.
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 (14,502 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.
The northernmost part of the coast ranges, in Washington, is often given
independent rank as the Olympic Mountains (Mount Olympus, 8150 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
sea bottom in a narrow coastal plain, of which some typical harborless
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.
There 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 forded
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 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.)The Pacific Ranges
Cascade Range
Sierra Nevada
Other