Front Range Foothills

The Front Range foothills expose tilted and eroded layers of sandstone and shale resting on much older mountain rocks, with the removed layers having once totaled more than two miles in thickness at this location. These sediments were laid down as flat beds of sand and mud in streams, lakes, and shallow seas during a span beginning about 300 million years ago and ending about 70 million years ago, then were bent upward during the rise of the Front Range of the Rocky Mountains and later worn into their present form. Hard sandstone layers now form prominent ridges, while softer shale layers form valleys extending far north and south. The rock at this site, including that carved for Red Rocks Theater, belongs to the Fountain Formation and in places contains fossils of tiny sea animals from about 300 million years ago. Along the highway crossing the ridge in front are maroon and gray shales of the Morrison Formation, about 130 million years old, which have yielded remains of the largest dinosaurs. Above the Morrison Formation lies the hard Dakota sandstone of the Dakota Hogback, containing fossil clams and serving as a valuable source of artesian water and oil in the plains east of the hogback, while a few miles south it also contains uranium. Farther east, Mount Carbon has coal seams but consists mainly of clay used for bricks, and it also contains fossil seashells and dinosaur bones about 100 million years old. After these deposits were laid down, volcanic debris, partly preserved at Green Mountain, was deposited by streams, and gravel along the creeks was washed down during the ice age only a few thousand years ago, when mammoths roamed the Denver Plains.

Colorado · United States