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"Looking across fields to rocky mountains behind, ca. 1900-1909. Courtesy of the Brigham Young University - Harold B. Lee Library via Mountain West Digital Library.

Mountain Formation

The Rocky Mountains and other subranges like the Uinta Mountains in Utah and Wyoming took shape during a period of intense plate tectonic activity that formed much of the rugged landscape of the western United States. Three major mountain building episodes reshaped the west from about 170 to 30 million years ago. The last mountain building event, the Laramide orogeny about 70 to 40 million years ago, is responsible for the raising of the Rocky Mountains. The growth of the Rocky Mountains, far from a subduction zone, may be due to the angle of the subducting oceanic plate being flattened, moving the focus of mountain building farther inland. This increased the friction and other interactions with the continental landmass above it. Tremendous thrusts piled sheets of crust on top of each other, building the broad, high Rocky Mountain range.

The east-west orientation of the Uintas may relate to changing stress patterns and rotation of the Colorado Plateau. Both the Rockies and Uintas have been extensively glaciated during the last ice age. Glaciers can still be viewed in Glacier National Park in Montana, but most disappear totally during the summer months due to global environmental changes. The Henry Mountains in Utah formed when molten material from the core of the Earth forced itself upward into the existing layers of crust. Erosion has exposed the hardened molten material which forms the major peaks and the domed and arched sedimentary rocks seen on the flanks of the mountains.