The Rocky Mountain region—which herein includes seven states in the mountain and basin provinces of the Rocky Mountain system, the western part of the Great Plains Province north of central New Mexico, the Colorado Plateaus, the eastern part of the Basin-and-Range Province, and the eastern part of the Columbia Plateaus—has produced oil from strata ranging in age from Cambrian to Oligocene; almost all the oil and gas fields are on anticlines and domes with surface structural closures ranging from 90 feet to 3,500 feet; lenticular sands on monoclines are yielding increasing amounts of oil; and minor amounts of oil are found in plunging anticlines and structural noses. The lowest altitude in an oil or gas field is about 2,167 feet in the Bowdoin gas field, Montana, and the highest is about 8,000 feet in the Wilson Creek, McCallum, and Gramp's (Price) oil fields, Colorado.

The late pre-Cambrian disturbances in the Grand Canyon area are the oldest clearly differentiated in the region. Epeirogenic and local orogenic episodes during the Paleozoic and Mesozoic culminated in the late Cretaceous and early Tertiary Laramide orogenies, which caused almost all the observable folding in the region, some of the Laramide folds being renewals of late Paleozoic folds.

The present dissected Rocky Mountains are largely the result of several post-Laramide epeirogenic movements in the Tertiary and Quaternary, separated by periods of erosion, the high altitudes of some ranges being caused largely by renewed movements along faults and by regional arching. Block faulting that began in the