Rocky Mountains, the eastern ranges of the great Cordilleran system in North America, which attains its greatest breadth within the United States (over 1000 miles between 38° and 42° N.). The mountain-chains forming the western boundary of the plateaus of this highland region are the Sierra Nevada and the Cascade Ranges (q.v.), and the eastern chains stretching continuously from the southern borders of the United States through Canada to the Arctic Ocean constitute the Rocky Mountains. 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. 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. 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 exposed to continued erosive action, and the region displays a labyrinth of naked crags and peaks rising from plateaus crossed by towering cliffs or deep canons, with here and there an isolated butte. The wonderful mesa or plateau region extends from southern Wyoming through western Colorado, eastern Utah, and south into New Mexico and Arizona. The country is divided by faults, flexures, and deep canons into numerous blocks or separate plateaus, and the 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; and in the continental divide here are found the 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 are higher, and as there are several elevated valleys known as 'Parks' enclosed between the parallel ranges, are known as the Park System. Its greatest development is in Colorado, where there are nearly forty peaks 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. In the Sawatch Range to the west are the Mount of the Holy Cross (14,176 feet) and Mount Harvard (14,375 feet). In the Sangre de Cristo Range, almost a continuation of the Sawatch, is Blanca Peak (14,463 feet), the highest point of the ' Rockies.' In the Parks rise the head-waters of the North and the South Platte, the Arkansas, the Grand, and the Rio Grande. The Uintah Mountains connect 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, and which serve as the connecting link between the northern and southern groups of this system. The Wind River Mountains in Wyoming are the highest of the ranges in the northern group, with Fremont's Peak (13,790 feet). The mountains of the northern group are wilder and less accessible than those of the southern chains, but not so high. Yellowstone Park (q.v.), in Wyoming, is famous for its wonderful scenery. Mount Hooker and Mount Brown are the most noted peaks beyond the Canadian line. The highland gradually descends northward to an elevation of about 800 feet near the Arctic Ocean.