Continental Divide, the "backbone" of a continent. In North America, from N Alaska to New Mexico, it moves along the crest of the Rocky Mts., which separates streams with outlets to the west of the divide from those with outlets to the east. In SW New Mexico the divide crosses an area of low relief; it becomes more distinct in N Mexico, where it follows the Sierra Madre Occidental. In the United States it has been called the Great Divide, a name also occasionally used to designate the whole Rocky Mt. system, especially the southern section, where the high, rugged ranges presented an almost impenetrable barrier to westbound explorers and settlers. Glacier, Yellowstone, and Rocky Mountain national parks lie on the Continental Divide, and the Continental Divide National Scenic Trail runs along it (see National Parks and Monuments, table). There also are other, lower and less extensive divides in North America, such as the eastern one formed mainly by the Appalachian Mts.