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The Rocky Mountains, Natural Features
Cretaceous Period, geologic time, earth's crust, Rio Grande, Great Plains
The Rocky Mountains were formed relatively recently in geologic time. During the Cretaceous Period, from approximately 135 million to 65 million years ago, a saltwater extension of the Gulf of Mexico covered most of this region, as well as the Great Plains. A deep layer of sediment, perhaps as much as 6,000 m (20,000 ft) thick, covered the granitic rocks that now form the Rocky Mountains. Finally, a series of crustal uplifts, accompanied by large-scale erosion, created the mountainous terrain that exists in the region today. .
These block-faulted mountains are young compared with many of the world’s other mountains. They are characterized by a complex system of troughs and ridges that are a result of tectonic uplift and subsidence of adjacent blocks of the earth's crust. Other features of the area’s landscape have been shaped through glaciation. Many of the major rivers of the western United States, including the Columbia, Fraser, Missouri, Colorado, Arkansas, Rio Grande, and Snake, originate in the upper elevations of the Rocky Mountains.
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