United States, Mountain West

The mountain west region encompasses four states: Colorado, Wyoming, Montana, and Idaho.

These states represent the northern, middle, and southern Rocky Mountain physiographic regions in the United States. This area is one of the most diverse regions in the United States because it includes eight different physiographic provinces: The Great Plains, the northern Rockies, the middle Rockies, the southern Rockies, the Wyoming Basin, the Columbia Plateau, the Basin and Range, and the Colorado Plateau. This diversity in geology and geomorphology also translates into differences in climate, vegetation, and wildlife habitat. In addition, the natural resources in this area attracted the early pioneers and the different amenities are attracting a new and expanding western population.

This portion of the United States occupies over 432,538 square miles (1.12 million square kilometers) and stretches approximately 860 miles (1,400 kilometers) along the crests of the Rocky Mountains. The northern border of Idaho and Montana follows latitude 49 degrees north separating the United States from Canada, while the southern border is Colorado at latitude 37 degrees north. The eastern limit of this region is longitude 102 degrees west—the eastern border of Colorado—and the western edge is longitude 117 degrees west—the western border of Idaho.

The Mountain West is a region dominated by the Rocky Mountains, a complex of mountains, valleys, and basins formed during the Cretaceous Period (140–65 million years ago). However, portions of the southern Rockies were uplifted more than 3.9 billion years ago during the Precambrian Period. The backbone of the Rockies was created by a combination of igneous and metamorphic rock and the edges are tilted sedimentary layers forming long ridges or hogbacks. The movements of the earth's crust have created a series of folded and faulted mountains. The Rockies have also experienced several volcanic episodes and numerous intrusions, lava flows, and other magmatic features are evident throughout the four states. The various uplifts, folds, and faults have occurred over more than three billion years and the mountains have experienced a multitude of erosional periods creating several flat basins filled with sedimentary material, the most notable of which is the Wyoming high basin. Finally, the three ice and intermittent ice advances have scarred the mountains with spectacular [Page 1842]glacial features and remnants still present today in Glacier, Rocky Mountain, Grand Teton, and Yellowstone National Parks.

Adjacent to the Rocky Mountains to the east is the Great Plains, an area of low relief except for outlier uplifted mountains. The majority of the geologic formations were created during the Mesozoic and Cenozoic periods (225–70 million years ago). The surface material is mainly the erosional deposits from the Rocky Mountains. In addition, the northern portions display the large impacts of the continental glaciers, and in western Montana, both continental glacial and the long reach of alpine glacial features.

The western side of the Rocky Mountains border several physiographic provinces. The northern Rockies are adjacent to the Columbia Plateau, an area dominated by volcanic materials of the Miocene-Pliocene periods (25–2 million years ago). Some of the lava flows extend over 100 miles (160 kilometers) and experienced folding and faulting creating ridges and steep hillsides. To the west of the middle Rocky Mountains is the great expanse of the Basin and Range Province, a large area occupying over 297,800 square miles (771,300 square kilometers) of recently faulted mountains and valleys. This province consists of a series of parallel mountain ranges with wide valleys of low relief. The southern Rocky Mountains in Colorado blend into the Colorado Plateau—a long, high area above 4,920 feet (1,500 meters) of mainly horizontal sedimentary rock eroded into steep-walled valleys, exposing folded and faulted rock formations. Scattered throughout the area are igneous structures, including large shield and conic volcano mountains, lavacapped mesas and tables, and lava flows.

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