Rocky Mountains, a name formerly somewhat loosely applied to all the mountains of North America between the Great Plains and the Pacific Ocean, is a term now used to designate only the eastern ranges of the great Cordilleran system. This vast mountain-system acquires its greatest breadth within the limits of the United States, where between the parallels 38° and 42° N. lat. it attains a width of more than 1000 miles. Toward the north and the south the plateaus of this highland gradually diminish in breadth, but they are enclosed on the east and on the west by high mountain-chains. Those forming the western boundary are the Sierra Nevada and the Cascade Ranges (q.v.), and the eastern chains stretching with uninterrupted continuity from the southern borders of the United States through the Dominion of Canada to the Arctic Ocean constitute the Rocky Mountains. Between these eastern and western boundaries the plateau region is greatly diversified by chains which, as a rule, trend in the same general direction as the border ranges.
The name 'Rocky Mountains' is peculiarly appropriate, as there probably exists nowhere else such an extensive region of naked rock almost entirely devoid of vegetation. The geological structure is complex, but the greater part of the rocks exposed are Mesozoic intermingled with Tertiary and Quaternary deposits. As this system is consequently of much more recent origin than the Appalachians, it is naturally higher, and it presents also a sharper and more rugged outline. Its remarkably barren aspect is due also to other geological peculiarities and to climatic causes. In comparatively recent ages this whole region has been the scene of vast volcanic eruptions, and the lava overflows which have covered the stratified rocks in many places to a depth of thousands of feet have augmented the expanse of sterile surface. By resisting the erosion of the streams and of the atmosphere, these lava beds have also greatly aided in producing the precipitous and deeply furrowed watercourses by which this wonderful plateau region is traversed. The high mountain barrier at the western boundary of the highland robs the winds which sweep across the Pacific of much of their moisture, and the great aridity of this region thus prevents the growth of vegetation. The surface is consequently exposed to continued erosive action, which is specially rapid at such great elevations. The denudation is the more complete as the sand and smaller disintegrated fragments are swept away by the winds, and no opportunity is afforded for the accumulation of a soil. On account of these various causes the erosion of the surface is uneven, and the region displays a labyrinth of naked crags and peaks arising from plateaus crossed by towering cliffs or deep cañons, with here and there an isolated butte. The scenery of the wonderful mesa or plateau region which lies between the eastern and western ranges of the Rocky Mountains, and extends from southern Wyoming through western Colorado, eastern Utah, and south into New Mexico and Arizona, is unequalled by that of any other portion of the globe. The country is divided by faults, flexures, and deep cañons into numerous blocks or separate plateaus, and the wonderful carving of the rocks and the brilliant colouring of the exposed strata almost surpass belief.
A high plateau region in Wyoming, over which passes the Union Pacific Railroad, marks a separation of the Rocky Mountains into a northern and a southern group, each of which has its characteristic features. The continental divide which extends north and south with the ranges of the Rocky Mountains culminates in this plateau, where are found the extreme head-waters of the three great river-systems of the United States—the Mississippi, the Columbia, and the Colorado. The ranges of the southern group have a general north and south trend, and are higher than those of the northern group. As there are several elevated valleys known as 'Parks' enclosed between the parallel ranges, this group is sometimes known as the Park System. It extends southward from the Laramie Plains across central Colorado into New Mexico. Its greatest development is in Colorado, where there are nearly forty peaks each over 14,000 feet in height. The Medicine Bow Range and the Colorado or Front Range form the eastern edge of the Rocky Mountain System, and rise abruptly from the gentler slope of the Plains. In this range are the well-known landmarks, Long's Peak (14,271 feet) and Pike's Peak (14,134 feet), as well as Gray's Peak (14,341 feet), its highest point, which is too far west to be visible from the Plains. This range forms the eastern wall of North, Middle, and South Parks, and the Park Range constitutes their western boundary. To the west of the southern end of the Park Range lies the Sawatch Range, with the famous Mount of the Holy Cross (14,176 feet) and Mount Harvard (14,375 feet). Farther south are the San Juan Mountains, which constitute the western boundary of San Luis Park. To the north and west of this range lies a high broken country merging into the mesa region of western Colorado. Uncompaligre Peak (14,408 feet) is the culminating point of this section. The eastern border of San Luis Park is formed by the Sangre de Cristo Range, which is almost a continuation of the Sawatch. Its loftiest summit, Blanca Peak (14,463 feet), is the highest point of the 'Rockies.' The Elk Mountains, a series of short parallel ranges with sharp volcanic peaks, lie to the west of the Sawatch Range. In the Parks rise the head-waters of the North and the South Platte, the Arkansas, the Grand, and the Rio Grande. Beside these large parks there are among these ranges many smaller but beautiful valleys. West of the Park Range are the Uintah Mountains, composed of a broad fold of thick strata, of which the Upper Tertiary and Cretaceous layers have been eroded to the depth of more than 3 miles, exposing the underlying Carboniferous rocks. This range has an east and west trend, and connects the eastern and western ranges of the Rocky Mountain System. The most important of the western ranges are the Wahsatch Mountains, which form a part of the eastern rim of the Great Basin (q.v.), and which serve as the connecting link between the northern and southern groups of this system. The greatest development of the northern group is in Wyoming. The Wind River Mountains are the highest of the ranges, with Fremont's Peak (13,790 feet) as the culminating point. To the west are the Tetons, Mount Hayden (13,691 feet), and the Snake River Mountains. The mountains of the northern group are wilder and less accessible than those of the southern chains, but not so high. They also present scenery which is less varied; they are not so definitely marked by regular ranges, and there are but few prominent peaks except in the groups already mentioned and in the geyser region of the Yellowstone. In Idaho and Montana there are numerous enclosed mountain valleys, which are called 'Parks' or 'Prairies,' but they are not so high as the 'Parks' of Colorado. The Bitter Root Mountains form the divide between the head-waters of the Missouri and those of the Columbia, and also between the tributaries of Clarke's Fork and of the Snake River. The Lapwai and Cœur d'Alene ranges, which lie to the west and northwest, connect the Rocky Mountains with the Blue Mountains, and between these groups and the Cascade Range are the Great Plains of the Columbia River. Yellowstone Park (q.v.), in the north-western part of Wyoming, is famous for its hot springs, geysers, mud volcanoes, and its wonderful scenery. The disposition of the mountains toward the east is peculiar, as they occur in more or less detached and isolated groups, among which are the Crazy Mountains, Judith Mountains, and the Big Horn Mountains. Still farther east are the Black Hills, completely detached from the main system, and noted for their mineral wealth. Beyond the Canadian line too little is known of the Rocky Mountains to warrant a detailed description. Mount Hooker and Mount Brown seem not to be 15,000 feet but little over 9000 feet high. The highland gradually descends towards the north, reaching an elevation of about 800 feet in the vicinity of the Arctic Ocean, and the northern ranges form the divide between the head-waters of the Mackenzie and the Yukon Rivers.