Geology of Red Rocks, Colorado

Front Range Hogbacks

The steeply angled, deeply colored strata of Red Rocks Park, near the town of Morrison (roughly 20 miles west of Denver), are a prime geologic display. In addition, they make up a natural, acoustically-pleasing amphitheater that serves as a breathtaking concert venue for major bands, from The Beatles to the Grateful Dead. 

Fountain Formation

The red rocks of Red Rocks belong to the Fountain Formation, a set of coarse-grained conglomerate and sandstone beds that is also well exposed in the Garden of the Gods, the Boulder Flatirons and Red Rock Canyon elsewhere in Colorado. These rocks, which are nearly 300 million years old, formed as an early version of the Rocky Mountains, known as the Ancestral Rockies, rose and shed their gravelly sediment in the oxygen-rich atmosphere of Pennsylvanian times. 

There are a couple of clues that point to this sediment being deposited close to its initial source, meaning that Red Rocks must not have been very far away from the Ancestral Rocky Mountains: 

• The sediments are coarse-grained, meaning they did not break down much during transport. Large pebbles and rocks, which can not travel far downstream before being deposited, can be seen within the sandstone and conglomerate.
• The sandstone contains a large amount of feldspar. In mature sandstones that have traveled great distances, feldspar is usually weathered into clay, leaving only quartz. 

Over time, this loose sediment was buried and lithified into horizontal sheets of rock. 

Uplift and Tilt

Around 75 million years ago, the Laramide orogeny took place, uplifting the entire region and forming the most recent version of the Rocky Mountains. The tectonic source of this orogeny is not clearly understood, but some point to shallow subduction ~1,000 miles to the west on the edge of the North American tectonic plate. Whatever the cause, this uplift tilted the sheets of horizontal rock at Red Rocks like the raising of a draw bridge. Some rock formations at the park have slopes nearing 90 degrees. 

Millions of years of erosion carved the softer rock away and left impressive monoliths, like Ship Rock, Creation Rock, and Stage Rock. Today, the Fountain Formation is around 1350 meters thick. 

Iron oxides and pink feldspar grains give the stone its color. In many places, the Fountain Formation lies directly upon Precambrian granite, aged at approximately 1.7 billion years old. 

Past the red rocks at Red Rocks, younger strata of the Front Range appear in hogbacks, the continuation of Dinosaur Ridge. All these rocks have the same tilt.

Ship Rock

Thick and thin beds in Ship Rock are respectively conglomerate and sandstone of the Fountain Formation. They resemble nearshore turbidites.

Fountain Formation North of Red Rocks

More subdued outcrops of the Fountain Formation north of Red Rocks are still distinctive. Behind rises Mount Morrison's 1.7-billion-year-old gneiss and granite.

Red Rocks Unconformity

The plaque marks the unconformity between the Fountain Formation and Proterozoic gneiss, 1.4 billion years older. All evidence of the vast time between is gone.

Fountain Formation Arkosic Conglomerate

A gravelly sandstone is called conglomerate. The prevalence of pink alkali feldspar along with the quartz in this conglomerate makes it an arkose.

Precambrian Gneiss

Uplift exposed this ancient gneiss to erosion, and its large pink feldspar and whitish quartz grains yielded the arkosic gravel of the Fountain Formation.