Boulder Red Rocks Are Part Of The "Ancestral Rocky Mountains"

Boulder + Miscellaneous,
originally uploaded by kenoatman.

This a west Boulder landmark, near the terminus of Pearl Street in the mouth of Boulder Canyon.

Wikipedia says:

The Fountain Formation is a conglomerate sandstone or arkose, red or reddish-gray in color, that is found in various locations in the state of Colorado in the USA, along the east side of the Front Range of the Rocky Mountains, along the west edge of the Denver Basin. Especially notable examples of this formation are the Flatirons, the Garden of the Gods, Roxborough State Park, and Red Rocks Park and Amphitheater (and the sandstone shown in the photo).

The Fountain Formation is named for the city of Fountain, Colorado in El Paso County, where an exemplary example of this formation may be found.

Rocks of the Fountain Formation are considered to be of Pennsylvanian age, and are between 290 and 296 million years old. They were formed by the erosion of the Ancestral Rocky Mountains, and their color and composition reflects those of the granites and gneisses from which they were eroded.