A Geological Tour of Denver, Golden, and Colorado's Front Range

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Geologic Time Period

The Fountain Formation

Fountain Formation at Red Rocks Park

Fountain Formation at Red Rocks Park, with Mount Morrison in the background, from Mount Falcon Park, Morrison, Colorado.

Erosion of the Ancestral Rockies...

What is known as the Ancestral Rockies Orogeny, i.e., the formation of the Ancestral Rockies, began around 320 million years ago (M.Y.A.). Divided into Frontrangia and Uncompahgria, the uplift of the Ancestral Rockies is believed to be connected to a collision between the African and North American plates during the formation of the supercontinent Pangea (Pangaea). Almost as soon the mountains formed they began to erode. Streams carried away boulders and rocks of granite and quartz. The rock was deposited as sand in streambeds further down.  The accumulation of sand and gravel would continue for thirty-five million years.[1]

Time and pressure would turn the sand into a 1,200-foot-thick layer of sandstone.  Over time, groundwater carrying dissolved hematite, an iron oxide, percolated through the formation, turning it dark red.  The city of Denver would be built on top of the sandstone, but the formation would lie at a depth of some 12,000 feet below the city.  What is known as the Fountain Formation would be forced to the surface along the Front Range in the uplift which accompanied the formation of the Rocky Mountains.  It can be seen at places such as Garden of the Gods Park near Colorado Springs, at Roxborough State Park and Red Rocks Park, and at the Boulder Flatirons.[2]

The red sandstone of the Fountain Formation contains few fossils. The sand is abrasive and its chemical composition corrosively hostile to the long-term preservation of remains. That there were plants and animals living near the rivers and streams can only be seen in fossils found in other layers and places.  The roots of 100-foot scale trees are found in Colorado Springs. Remains of conifers and horsetails are found near Vail and Minturn, Colorado, and fossilized insects, such as cockroaches and dragonflies, are found in eastern Kansas.[3]


(1) Halka Chronic and Felicie Williams, "Roadside Geology of Colorado, 2nd ed.," Mountain Press Publishing Company, (Missoula, MT 2005), p. 13;
Kirk R. Johnson and Robert G. Raynolds, "Ancient Denvers: Scenes from the Past 300 Million Years of the Colorado Front Range," Denver Museum of Nature & Science, (Denver 2003), p. 6.
(2) Johnson and Raynolds, "Ancient Denvers," p. 6;
Chronic and Felicie Williams, "Roadside Geology of Colorado," p. 26.
(3) Johnson and Raynolds, "Ancient Denvers," p. 6.