Rocky Mountains

Topography

Topographically, the Rockies are usually divided into five sections: the Southern Rockies, Middle Rockies, Northern Rockies (all in the United States), the Rocky Mountain system of Canada, and Brooks Range in Alaska. The Wyoming Basin, the system's principal topographic break, is sometimes considered a sixth section.

The Southern Rockies, in New Mexico, Colorado, and S Wyoming, are dominated by two north-south belts of folded mountains that have been eroded to expose cores of Precambrian rocks rimmed by younger sedimentary rocks. The eastern belt comprises the Laramie, Medicine Bow, and Wet Mts. and the Front Range. The principal ranges of the western belt are the Park, Gore, Mosquito, Sawatch, and Sangre de Cristo. Between the two belts are three basins known as the North, South, and Middle "parks." To the southwest are the San Juan Mts., a nonlinear group of uplands composed mainly of volcanic rocks. The Southern Rockies are the system's highest section and include many peaks above 14,000 ft (4,267 m), among them Mt. Elbert and Mt. Massive (14,418 ft/4,395 m), both in the Sawatch Mts.

The Middle Rockies, chiefly in NE Utah and W Wyoming, lie N of the Southern Rockies and are separated from them by the Wyoming Basin. The ranges of this section are generally lower and less continuous than those to the south. The principal parts are the Wasatch and Teton ranges (which are both great tilted fault blocks), the Yellowstone Plateau and Absaroka Range (both developed on volcanic rocks), the Bighorn, Beartooth, Owl Creek, and Uinta Mts., and the Wind River Range (all broad folded mountains). All of these component sections have been eroded down to their Precambrian cores and are rimmed by Paleozoic and Mesozoic sedimentary rocks. The highest peaks of the Middle Rockies are Gannet Peak (13,785 ft/4,202 m) in the Wind River Range and Grand Teton (13,766 ft/4,196 m) in the Teton Range.

The Northern Rockies, in NE Washington, N and central Idaho, NW Wyoming, and W Montana extend N from Yellowstone National Park to the U.S.-Canadian border. They are composed of the Clearwater and Salmon River Mts., the Sawtooth and Lost River ranges (all of which developed in the batholith of central Idaho), and the Bitterroot Range along the Idaho-Mont. line. In the east are the Front Ranges of Montana. A series of north-south trending ranges separated by narrow trenches and valleys occupies most of N Montana and the Idaho panhandle. Two especially distinctive trenches are the Rocky Mountain Trench, which extends NW from Flathead Lake, and the Purcell Trench, which extends N from Coeur d'Alene Lake. The Okanagan Highlands, in NE Washington, form the western edge of the Northern Rockies. The peaks of the Northern Rockies are generally lower than those to the south; among the highest are Borah Peak (12,655 ft/3,857 m) and Leatherman Peak (12,230 ft/3,728 m) in the Lost River Range.

The Rocky Mt. system of Canada is composed of two major sections: the high rugged peaks of the Canadian Rockies proper, to the east, and the Columbia Mts. group on the west. The Canadian Rockies are located along the British Columbia–Alberta border and include Mt. Robson (12,972 ft/3,954 m; highest peak of the Rocky Mts. in Canada), Mt. Columbia (12,295 ft/3,748 m), and Mt. Forbes (11,902 ft/3,628 m). The prominent, wide-floored Rocky Mountain Trench, west of the crest line, continues c.800 mi (1,290 km) into Canada from Montana and is drained by the headwaters of the Peace River and by sections of the Fraser, Columbia, and Kootenay rivers. The Purcell Trench to the west also crosses into Canada and joins the Rocky Mountain Trench c.200 mi (320 km) north of the border. Farther to the west is the Columbia Mts. group, which includes the Selkirk, Purcell, Monashee, and Cariboo Mts. The Rockies continue into Yukon and the Northwest Territories as the Mackenzie, Richardson, and Franklin Mts. In N Alaska, the Brooks Range, a cold and treeless region rising to Mt. Chamberlin (9,020 ft/2,749 m), forms the northernmost section of the Rocky Mts.

The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved.

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