| 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 Rocky Mountain System. Please refer to the Geography of the United States for the other areas.
The Rocky Mountains begin in northern Mexico, where the axial crystalline
rocks rise to 12,000 ft. between the horizontal structures of the plains
on the east and the plateaus on the west. The upturned stratified formations
wrap around the mountain flanks of the range, with ridges and valleys formed
on their eroded edges and drained southward by the Pecos river to the
Rio Grande and the Gulf of Mexico. The mountains rapidly grow wider and
higher northward, taking on new complications of structure and
including large basins between the axes of uplift. In northern Colorado and
Utah, the mountains become a complex of ranges with a breadth of 300 miles.
In Colorado alone, there are 40 summits over 14,000 ft. in altitude, though
none rises to 14,500. Tturning more to the northwest through Wyoming,
the ranges decrease in breadth and height. In Montana, their breadth is
not more than 150 miles, and only seven summits exceed 11,000 feet with
one reaching 12,834.
As far north as the gorge of the Missouri river in Montana, the Front Range,
facing the Great Plains, is a rather simple uplift, usually formed by
upturning the flanking strata, less often by a fracture. Along the eastern
side of the Colorado Front Range, most of the upturned stratified
formations have been so well worn down that with exception of a few low
piedmont ridges, their even surface may now be included with that of the
plains, and the crystalline core of the range is exposed almost to the
mountain base. Here, the streams that drain the higher areas descend to
the plains through narrow canyons in the mountain border. A well-known
example is the gorge of Clear Creek. The crystalline highlands thereabouts,
at altitudes of 8000 to 10,000 ft., are of so moderate a relief as to
suggest that the mass had stood much lower in a former cycle of erosion
and had then been worn down to rounded hills. Since uplift to the present
altitude, the revived streams of the current cycle of erosion have not
entrenched themselves deep enough to develop strong relief. This idea is
confirmed 80 miles farther south, where Pike's Peak (14,108 ft.), a
conspicuous landmark far out on the plains, has every appearance of
being a huge monadnock surmounting a rough peneplain of 10,000 feet.
The idea is still better confirmed farther north in Wyoming, where the
Laramie Mountains, flanked with upturned strata on the east and west, are
for the most part a broad upland at altitudes of 7000 or 8000 feet with
no strong surmounting summits and yet no deep carved valleys. Here the
first of the Pacific railways chose its pass. From the summit, there is
very little relief of the upland surface. This low range turns westward
in a curve through the Rattlesnake Mountains towards the high
Wind River Mountains (Gannett Peak, 3,775 ft.). This is an anticlinal range
within the body of the mountain system with flanking strata rising well on
the slopes. Flanking strata are even better exhibited in the
Bighorn Mountains, the front range of northern Wyoming. They are crescent
in outline and convex to the northeast, like the Laramie Range, but much
higher. Here, heavy sheets of limestone arch far up towards the range crest
and are deeply notched where consequent streams have cut down their gorges.
Farther north in Montana, beyond the gorge of the Missouri river, the
structure of the Front Range is altogether different. It is the carved
residual of a great mass of moderately bent Palaeozoic strata. These strata
have overthrust eastward upon the Mesozoic strata of the plains. Instead
of exposing the oldest rocks along the axis and the youngest rocks low
down on the flanks, the younger rocks of the northern range follow its axis,
while the oldest rocks outcrop along its eastern flanks. There, they override
the much younger strata of the plains. The harder strata, instead of
lapping on the mountain flanks in great slab-like masses, as in the Bighorns,
form out-facing scarps, which retreat into the mountain interior where
they are cut down by outflowing streams.
The structure of the inner ranges is so variable as to elude simple
description. The Uinta range is a broad anticlinal structure in
northeast Utah with east-west trend corresponding to the east-west
Rattlesnake Mountains mentioned earlier. The Wasatch Range, trending
north-south in central Utah, is peculiar in possessing large east-west
folds which are seen in cross-section in the dissected western face of
the range. It is visible because the whole mass is squarely cut off by a great
north-south fault with down-throw to the Basin Range province with
the fault face being elaborately carved.
Volcanic action has been limited in the Rocky Mountains proper.
West Spanish Peak (13,626 ft.), in the Front Range of southern Colorado,
may be mentioned as a fine example of a deeply dissected volcano,
originally of greater height, with many unusually strong radiating
dike-ridges near its denuded flanks. In north-western Wyoming, there are
extensive and heavy lava sheets, uplifted and dissected, and crowned
with a few dissected volcanoes. Associated with this is a remarkable
group of geysers and hot springs in Yellowstone National Park from which the
Yellowstone River, a branch of the Missouri, flows northeastward,
and the Snake River, a branch of the Columbia, flows southwestward.
The central and southern ranges of the Rocky Mountains are not like the
abnormally sharpened peaks found in the ice-sculptured Alps. Many of these
ranges are characterized by the rounded tops and the rather evenly slanting,
waste-covered slopes which normally result from the long-continued action of
the ordinary agencies of erosion. They bear little snow in summer and
few if any glaciers. The forests are often scanty on the middle and lower
slopes. The general impression of great altitude is much weakened because
the mountains are seen from a base which itself is 5000 or 6000 feet above
sea level. Travelling along the range from south to north reveals most
strikingly a gradual increase in the share of sculpture due to Pleistocene
glaciers. In New Mexico, if glaciers were formed at all in the high valleys,
they were so small as not to greatly to modify the more normal forms.
In central Colorado and Wyoming, where the mountains are higher and
the Pleistocene glaciers were larger, the valley heads were hollowed
out in well-formed cirques, often holding small lakes. The mountain valleys
were enlarged into U-shaped troughs as far down as the ice reached, with
hanging lateral valleys on the way. Different stages of cirque development,
with accompanying transformations of mountain shape, are finely illustrated
in several ranges around the headwaters of the Arkansas River in central
Colorado, where the highest summit of the Rocky Mountains is found
(Mt Massive, 14,424 ft., in the Sawatch Mountains). Perhaps even better illustrated
in the Bighorn range of Wyoming. In this central region,
it is the exception rather than the rule that the cirques were enlarged
enough by retrogressive glacial erosion as to sharpen the preglacial
dome-like summits into acute peaks. In no case did glacial action here
extend down to the plains at the eastern base of the mountains. However,
the widened, trough-like glaciated valleys frequently descend to the level
of the elevated intermontane basins, where moraines were deployed forward on
the basin floor. The finest examples of this kind are the moraines about
Jackson Lake on the basin floor east of the Teton Mountains
(Grand Teton, 13,747 ft.). This superb north-south range lies close to the
meridional boundary line between Wyoming and Idaho. Farther north in Montana,
in spite of a decrease of height, there are today a few small glaciers with
snowfields of good size. Here the effects of sculpture by the much larger
Pleistocene glaciers are seen in forms almost as exaggerated as the Alps.
The intermontane basins which so strongly characterize the Rocky Mountain
system are areas which have been less uplifted than the enclosing ranges.
They have usually become the depositories of waste from the surrounding
mountains.
Some of the most important basins are:
Rocky Mountains
San Luis Valley is an oval basin about 60 miles long near the southern
end of the mountain system in New Mexico and Colorado. Its level,
treeless floor, at an altitude of 7000 feet, is as yet hardly trenched
by the Rio Grande, which escapes through an impassable canyon
southward on its way to the Gulf of Mexico. The much smaller basin of
the upper Arkansas river in Colorado is well known because the Royal
Gorge, a very narrow cleft by which the river escapes through the
Front Range to the plains, is followed by a railroad at river level.
South Park, directly west of Pike's Peak, is one of the highest basins
(nearly 10,000 ft.), and gains its name from the scattered, park-like
growth of large pine trees. It is drained chiefly by the South Platte River (Missouri-Mississippi system), through a deep gorge in the
dissected mass of the plateau-like Front Range. The Laramie Plains and
the Green River basin, essentially a single structural basin between
the east-west ranges of the Rattlesnake Mountains on the north and the
Uinta Range on the south, measuring roughly 260 miles east-west by 200
miles north-south, make up the largest intermontane basin. It is well known
from being traversed through most of its length by the Union Pacific Railroad. Its eastern part is drained northeastward through a gorge
that separates the Laramie and Rattlesnake (Front) ranges by the North Platte River to the Missouri-Mississippi. Its western part, where the
basin floor is much dissected, is drained southward by the Green River
through a deep canyon in the Uinta Range to the Colorado River and
then to the Pacific Ocean. The Bighorn basin has a moderately dissected
floor, drained northeastward by Bighorn River through a deep canyon in
the range of the same name to the Missouri. Several smaller basins
occur in Montana, all somewhat dissected and drained through narrow
gorges and canyons by members of the Missouri system.