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

sandstone Sandstone is a clastic sedimentary rock composed mainly of sand-sized (0.0625 to 2 mm) silicate grains. Sandstones comprise about 20–25% of all sedimentary rocks. Most sandstone is composed of quartz or feldspar (both silicates ...

, or arkose, in the states of

Colorado Colorado (, other variants) is a state in the Mountain states, Mountain West subregion of the Western United States. It encompasses most of the Southern Rocky Mountains, as well as the northeastern portion of the Colorado Plateau and the wes ...

and

Wyoming Wyoming () is a state in the Mountain West subregion of the Western United States. It is bordered by Montana to the north and northwest, South Dakota and Nebraska to the east, Idaho to the west, Utah to the southwest, and Colorado to the s ...

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 encountered ...

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 straight-line distance from the northernmost part of western Canada, to New Mexico ...

, and along the west edge of the

Denver Basin The Denver Basin, variously referred to as the Julesburg Basin, Denver-Julesburg Basin (after Julesburg, Colorado), or the D-J Basin, is a geologic structural basin centered in eastern Colorado in the United States, but extending into southeast W ...

Origin of name

The Fountain Formation was named by geologist W. C. Cross in 1894 for exposures along Fountain Creek in

El Paso County, Colorado El Paso County is a county located in the U.S. state of Colorado. The 2020 Census recorded its population as 730,395. The Census Bureau's 2020 result indicates it is the most populous county in Colorado, surpassing the City and County of De ...

Stratigraphy

The Fountain Formation is found along the Front Range of Colorado. To the north, the Formation unconformably overlies Precambrian 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 The geology of the Rocky Mountains is that of a discontinuous series of mountain ranges with distinct geological origins. Collectively these make up the Rocky Mountains, a mountain system that stretches from Northern British Columbia through cen ...

, 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 semiarid climates, but a ...

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

granite Granite () is a coarse-grained ( phaneritic) 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 cools and solidifies under ...

s and

gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...

es from which it was eroded.

Notable outcrops

Flatirons The Flatirons are rock formations in the western United States, near Boulder, Colorado, consisting of flatirons. There are five large, numbered Flatirons ranging from north to south (First through Fifth, respectively) along the east slope of ...

, near Boulder, Colorado *

Red Rocks Park Red Rocks Park is a mountain park in Jefferson County, Colorado, owned and maintained by the city of Denver as part of the Denver Mountain Parks system. The park is known for its very large red sandstone outcrops. Many of these rock formations wi ...

and Amphitheater,

Golden, Colorado Golden is a home rule city that is the county seat of Jefferson County, Colorado, United States. The city population was 20,399 at the 2020 United States Census. Golden lies along Clear Creek at the base of the Front Range of the Rocky Mountai ...

Roxborough State Park Roxborough State Park is a state park of Colorado, United States, known for dramatic red sandstone formations. Located in Douglas County south of Denver, Colorado, the park was established in 1975. In 1980 it was recognized as a National Na ...

, south of

Denver, Colorado Denver () is a consolidated city and county, the capital, and most populous city of the U.S. state of Colorado. Its population was 715,522 at the 2020 census, a 19.22% increase since 2010. It is the 19th-most populous city in the Unit ...

Garden of the Gods Garden of the Gods (Arapaho: ''Ho3o’uu Niitko’usi’i'') is a public park located in Colorado Springs, Colorado, United States. It was designated a National Natural Landmark in 1971. Name The area now known as Garden of the Gods was f ...

in Colorado Springs, Colorado * Red Rock Canyon Open Space in Colorado Springs, Colorado

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 invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

ns,

brachiopod Brachiopods (), phylum Brachiopoda, are a phylum of trochozoan 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 end, w ...

crinoid Crinoids are marine animals that make up the class Crinoidea. Crinoids that are attached to the sea bottom by a stalk in their adult form are commonly called sea lilies, while the unstalked forms are called feather stars or comatulids, which are ...

s, echinoids, and

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

. Plant fossils have been discovered in Garden Park north of

Cañon City A canyon (from ; archaic British English spelling: ''cañon''), or gorge, is a deep cleft between escarpments or cliffs resulting from weathering and the erosive activity of a river over geologic time scales. Rivers have a natural tendency to cu ...

, 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