Rocky Mountains, a major mountain system in the western United States and Canada. The mountains rise abruptly from the Great Plains and extend from northern New Mexico to the Arctic Ocean in northwestern Alaska. The Rockies cross parts of Colorado, Wyoming, Utah, Idaho, Montana, and Washington in the United States and parts of Alberta, British Columbia, the Yukon Territory, and the Northwest Territories in Canada. The length of the system is about 3,300 miles (5,300 km); the width is as much as 400 miles (640 km).
The Continental Divide runs along the crest of the Rocky Mountains throughout most of their course. Rivers drain eastward to the Atlantic Ocean or Gulf of Mexico, westward to the Pacific Ocean, or northward to the Arctic Ocean. Among the rivers rising in the Rockies are the Rio Grande, Arkansas, Colorado, Snake, Missouri, Columbia, Fraser, Saskatchewan, Athabasca, Peace, Mackenzie, and Yukon.
Formation of the Rockies has taken millions of years. At the beginning a vast sea covered the area now occupied by the mountains. Sediments thousands of feet thick were laid down on the sea's floor. Successive periods followed in which the land was uplifted to great heights and then worn down again by water, wind, and ice. The present mountains are the result of an uplift in which there was great folding, faulting, and thrusting of the land. In some areas, volcanic activity aided the mountain building. During the last Ice Age, great glaciers helped carve and shape the mountains.