|Physiographic regions of the U.S. Interior|
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 on northern Mexico, where the axial crystalline rocks rise to 12,000 ft (3,700 m) 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 (500 km). In Colorado alone, there are 54 summits over 14,000 ft (4,267 m) in altitude, though none rise any higher than Mount Elbert at 14,433 ft (4,399 m). Turning more to the northwest through Wyoming, the ranges decrease in breadth and height. In Montana, their breadth is not more than 150 miles (240 km), and only seven summits exceed 11,000 feet (3350 m) with one reaching 12,834 ft (3,912 m).
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 Front Range in Colorado, 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 80 miles (130 km) farther south, where Pike's Peak (14,108 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, 3,775 ft, 1,151 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 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, 4,153 m), 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 to 6000 feet (1,500 to 1,800 m) 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. Elbert, 14,433 ft (4,399 m), 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, 4,190 m). 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:
San Luis Valley is an oval basin about 60 miles (100 km) long near the southern end of the mountain system in New Mexico and Colorado. Its level, treeless floor, at an altitude of 7000 feet (2,100 m), 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 or 3,000 m), 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 (420 km) east-west by 200 miles (320 km) 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.