The Fountain Formation is a Pennsylvanian bedrock unit consisting primarily of conglomerate,

sandstone Sandstone is a Clastic rock#Sedimentary clastic rocks, clastic sedimentary rock composed mainly of grain size, sand-sized (0.0625 to 2 mm) silicate mineral, silicate grains, Cementation (geology), cemented together by another mineral. Sand ...

, or arkose, in the states of

Colorado Colorado is a U.S. state, state in the Western United States. It is one of the Mountain states, sharing the Four Corners region with Arizona, New Mexico, and Utah. It is also bordered by Wyoming to the north, Nebraska to the northeast, Kansas ...

and

Wyoming Wyoming ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States, Western United States. It borders Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho t ...

in the United States, along the east side of the

Front Range The Front Range is a mountain range of the Southern Rocky Mountains of North America located in the central portion of the U.S. State of Colorado, and southeastern portion of the U.S. State of Wyoming. It is the first mountain range encounter ...

of the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in great-circle distance, straight-line distance from the northernmost part of Western Can ...

, and along the west edge of the Denver Basin.

Origin of name

The Fountain Formation was named by geologist C. W. Cross in 1894 for exposures along Fountain Creek in El Paso County, Colorado.

Stratigraphy

The Fountain Formation is found along the Front Range of Colorado. To the north, the Formation unconformably overlies

Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...

granite and gneiss. To the south, it overlies Mississippian, Ordovician and Devonian Limestones, as well as Cambrian sandstones. Outcrops of the formation typically dip steeply to the east.

Depositional environment

The formation was formed by the erosion of the Ancestral Rocky Mountains, and deposition by fluvial processes as

alluvial fan An alluvial fan is an accumulation of sediments that fans outwards from a concentrated source of sediments, such as a narrow canyon emerging from an escarpment. They are characteristic of mountainous terrain in arid to Semi-arid climate, semiar ...

s. The characteristic predominant red color and the composition of the Fountain reflect that of the

granite Granite ( ) is a coarse-grained (phanerite, phaneritic) intrusive rock, intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly coo ...

s and gneisses from which it was eroded.

Notable outcrops

* Flagstaff Mtn Park, near

Boulder, Colorado Boulder is a List of municipalities in Colorado#Home rule municipality, home rule city in Boulder County, Colorado, United States, and its county seat. With a population of 108,250 at the 2020 United States census, 2020 census, it is the most ...

* Flatirons, near

* Red Rocks Park and Amphitheater, Morrison, Colorado * Roxborough State Park, south of

Denver, Colorado Denver ( ) is a List of municipalities in Colorado#Consolidated city and county, consolidated city and county, the List of capitals in the United States, capital and List of municipalities in Colorado, most populous city of the U.S. state of ...

* Garden of the Gods in

Colorado Springs, Colorado Colorado Springs is the most populous city in El Paso County, Colorado, United States, and its county seat. The city had a population of 478,961 at the 2020 United States census, 2020 census, a 15.02% increase since 2010 United States Census, 2 ...

* Red Rock Canyon Open Space in

Red Canyon Park
near

Cañon City, Colorado Cañon City ( ) is a List of municipalities in Colorado#Home rule municipality, home rule city that is the county seat and the List of municipalities in Colorado, most populous municipality of Fremont County, Colorado, United States. The city p ...

Fossils

Marine invertebrates have been discovered in a limestone and shale member of the Fountain Formation, cropping out on a low hogback in Perry Park. Invertebrates include

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic animal, aquatic invertebrate animals, nearly all living in sedentary Colony (biology), colonies. Typically about long, they have a spe ...

ns,

brachiopod Brachiopods (), phylum (biology), phylum Brachiopoda, are a phylum of animals that have hard "valves" (shells) on the upper and lower surfaces, unlike the left and right arrangement in bivalve molluscs. Brachiopod valves are hinged at the rear e ...

s, crinoids,

echinoids Sea urchins or urchins () are echinoderms in the class (biology), class Echinoidea. About 950 species live on the seabed, inhabiting all oceans and depth zones from the intertidal zone to deep seas of . They typically have a globular body cove ...

, and

gastropods Gastropods (), commonly known as slugs and snails, belong to a large taxonomic class of invertebrates within the phylum Mollusca called Gastropoda (). This class comprises snails and slugs from saltwater, freshwater, and from the land. Ther ...

. Plant fossils have been discovered in Garden Park north of Cañon City, including ''Lepidophloios laricinus'', ''Sigillaria'', ''Syringodendron'' sp., ''Lepidophyloides'' sp., ''Lepidostrobus'' sp., ''Lepidostrobophyllum'' sp., ''Calamites'', ''Neuropterus'' sp., ''Cyclopteris'' sp., and ''Stigmaria ficoides''.Jennings, J.R., 1980, Fossil plants from the Fountain Formation (Pennsylvanian) of Colorado: Journal of Paleontology, v. 54, no. 1, p. 149-158.

Age

Rocks of the Fountain Formation are considered to be of Late Pennsylvanian age, and are between 290 and 340 million years old.

References

External links

Gravmag.com: Geology of the Fountain Formation
{{Chronostratigraphy of Colorado, Paleozoic state=expanded Pennsylvanian North America Geologic formations of Colorado Carboniferous Colorado Sandstone formations of the United States Conglomerate formations Fluvial deposits Carboniferous southern paleotropical deposits Shale formations of the United States Limestone formations of the United States