|Physiographic regions of the U.S. Interior|
For purposes of description, the physical geography of the United States is split into several major physiographic divisions, three of which being the Rocky Mountain System, Intermontane Plateaus and the Pacific Mountain System (see subdivisions 16-25) lie in the western U.S. Please refer to the Geography of the United States for the other areas.
From the western border of the Great Plains to the Pacific coast, there is a vast elevated area, occupied by mountains, plateaus and intermontane plains. The intermontane plains range in altitudes from sea-level to 4000 feet. The plateaus range from 5000 to 10,000 ft. and the mountains from 8000 to 14,000 feet. The higher mountains are barren from the cold of altitude with the timber line in Colorado at roughly 11,000 to 12,000 ft.
The chief provinces of the Cordilleran region are:
There is also a province of plateaus between the central part of the Basin ranges and the southern part of the Rocky Mountains. An important geological characteristic of most of the Cordilleran region is that the Carboniferous strata, which in western Europe and the eastern United States contain many coal seams, are represented in the western United States by a marine limestone. That the important unconformity, which in Europe and the eastern United States separates the Palaeozoic and Mesozoic eras, does not occur in the western United States. There the formations over a great area follow in conformable sequence from early Palaeozoic through the Mesozoic.