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Geography of the United States Rocky Mountain System
(詳細はRocky Mountains begin in northern New Mexico, where the axial crystalline rocks rise to 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 . In Colorado alone, there are 54 summits over in altitude, though none rise any higher than Mount Elbert at . Turning more to the northwest through Wyoming, the ranges decrease in breadth and height. In Montana, their breadth is not more than , and only seven summits exceed 11,000 feet (3350 m) with one reaching .
In general, the peaks in the Rocky Mountains tend to be fairly gentle and rounded relative to those found in other mountain ranges of comparable scale.
==Front Range==
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 fault. Along the eastern side of the Rocky Mountains 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 (2,400 to 3,000 m), 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 by Pike's Peak (14,115 ft, 4,300 m), a conspicuous landmark far out on the plains, has every appearance of being a huge monadnock surmounting a rough peneplain of 10,000 feet (3,000 m). 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 to 8000 feet (2,100 to 2,400 m) 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, 13,804 ft, 4,1207 m). 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 folded 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.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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