Rainbow Basin is a geological formation in the Calico Peaks range, located approximately north of Barstow in the

Mojave Desert The Mojave Desert (; ; ) is a desert in the rain shadow of the southern Sierra Nevada mountains and Transverse Ranges in the Southwestern United States. Named for the Indigenous peoples of the Americas, indigenous Mohave people, it is located pr ...

San Bernardino County San Bernardino County ( ), officially the County of San Bernardino and sometimes abbreviated as S.B. County, is a county located in the southern portion of the U.S. state of California, and is located within the Inland Empire area. As of th ...

California California () is a U.S. state, state in the Western United States that lies on the West Coast of the United States, Pacific Coast. It borders Oregon to the north, Nevada and Arizona to the east, and shares Mexico–United States border, an ...

. The Rainbow Basin has been designated a

National Natural Landmark The National Natural Landmarks (NNL) Program recognizes and encourages the conservation of outstanding examples of the natural history of the United States. It is the only national natural areas program that identifies and recognizes the best e ...

and is in the

Bureau of Land Management The Bureau of Land Management (BLM) is an agency within the United States Department of the Interior responsible for administering federal lands, U.S. federal lands. Headquartered in Washington, D.C., the BLM oversees more than of land, or one ...

managed Rainbow Basin Natural Area. Rainbow Basin is a mixture of private and public land, but it is managed by the

. It is accessible to the public via Irwin Road from Barstow to an unpaved loop road through the colorful basin. The basin is notable for the fantastic and beautiful shapes of its rock formations as well as its

fossil A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserve ...

beds, which have provided scientists with valuable information about life during the middle

Miocene The Miocene ( ) is the first epoch (geology), geological epoch of the Neogene Period and extends from about (Ma). The Miocene was named by Scottish geologist Charles Lyell; the name comes from the Greek words (', "less") and (', "new") and mea ...

epoch, between 12 and 16 million years ago; and to the northeast the Calico Early Man Site.

Geology

Underneath Rainbow Basin is the massive

batholith A batholith () is a large mass of intrusive rock, intrusive igneous rock (also called plutonic rock), larger than in area, that forms from cooled magma deep in the Earth's crust. Batholiths are almost always made mostly of felsic or intermediate ...

that lies below much of the western Mojave (Dibblee 1968:15). Made from a type of rock called quartz monzonite, this batholith dates to either the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 143.1 to 66 mya (unit), million years ago (Mya). It is the third and final period of the Mesozoic Era (geology), Era, as well as the longest. At around 77.1 million years, it is the ...

, or possibly the late

Jurassic The Jurassic ( ) is a Geological period, geologic period and System (stratigraphy), stratigraphic system that spanned from the end of the Triassic Period million years ago (Mya) to the beginning of the Cretaceous Period, approximately 143.1 Mya. ...

period (Dibblee 1967:43). Early in the

Cenozoic The Cenozoic Era ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterized by the dominance of mammals, insects, birds and angiosperms (flowering plants). It is the latest of three g ...

Era this batholith was exposed in the area surrounding Rainbow Basin and bent downward as it underwent compression, to form a basin (Bureau of Land Management 1992:30). Sediments deposited in this basin became the

sedimentary rocks Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedim ...

that are most visible in Rainbow Basin today (Dibblee 1968:56-57). Further compression, uplift, and finally extension left these

sedimentary Sedimentary rocks are types of rock formed by the cementation of sediments—i.e. particles made of minerals (geological detritus) or organic matter (biological detritus)—that have been accumulated or deposited at Earth's surface. Sedime ...

formations deeply folded, the most prominent fold being the Barstow Syncline. These same stresses also produced several faults in the Rainbow Basin area (Bureau of Land Management 1992:30). The thick

layers can be divided into three distinct formations. The lowest is called the Jackhammer Formation, and it is composed of layers of

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 ...

, siltstone,

limestone Limestone is a type of carbonate rock, carbonate sedimentary rock which is the main source of the material Lime (material), lime. It is composed mostly of the minerals calcite and aragonite, which are different Polymorphism (materials science) ...

, and conglomerate, all probably dating to the early

Epoch (Dibblee 1968:18). Above this is the Pickhandle Formation. The sediments making up this formation are mostly of

volcanic A volcano is commonly defined as a vent or fissure in the crust of a planetary-mass object, such as Earth, that allows hot lava, volcanic ash, and gases to escape from a magma chamber below the surface. On Earth, volcanoes are most often fo ...

origin –

tuff Tuff is a type of rock made of volcanic ash ejected from a vent during a volcanic eruption. Following ejection and deposition, the ash is lithified into a solid rock. Rock that contains greater than 75% ash is considered tuff, while rock co ...

rhyolite Rhyolite ( ) is the most silica-rich of volcanic rocks. It is generally glassy or fine-grained (aphanitic) in texture (geology), texture, but may be porphyritic, containing larger mineral crystals (phenocrysts) in an otherwise fine-grained matri ...

and

andesite Andesite () is a volcanic rock of intermediate composition. In a general sense, it is the intermediate type between silica-poor basalt and silica-rich rhyolite. It is fine-grained (aphanitic) to porphyritic in texture, and is composed predomina ...

(Dibblee 1967:84), indicating that they were laid down during a period of active

volcanism Volcanism, vulcanism, volcanicity, or volcanic activity is the phenomenon where solids, liquids, gases, and their mixtures erupt to the surface of a solid-surface astronomical body such as a planet or a moon. It is caused by the presence of a he ...

. That time was probably during the early Miocene (Dibblee 1968:23). The highest of the three formations is the Barstow Formation, which is made up of layers of conglomerate,

, and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of Clay mineral, clay minerals (hydrous aluminium phyllosilicates, e.g., Kaolinite, kaolin, aluminium, Al2Silicon, Si2Oxygen, O5(hydroxide, OH)4) and tiny f ...

(Dibblee 1967:86). This formation dates to the middle to late

(Lindsay 1972:15) and it contains one of the largest

assemblages in North America (Dibblee 1968:32). Most of the sediment that makes up the layers in this formation was stream-laid (Dibblee 1968:28), but there is a white layer of rhyolitic

(sometimes called ''marker tuff'') near the top (Dibblee 1967:86). Finally, on top of everything else, is a relatively thin layer of fanglomerate (

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 ...

deposits) laid down during the late

Pleistocene The Pleistocene ( ; referred to colloquially as the ''ice age, Ice Age'') is the geological epoch (geology), epoch that lasted from to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fin ...

(Bureau of Land Management 1992:31). Differential

erosion Erosion is the action of surface processes (such as Surface runoff, water flow or wind) that removes soil, Rock (geology), rock, or dissolved material from one location on the Earth's crust#Crust, Earth's crust and then sediment transport, tran ...

of rocks of different hardness finished the job of sculpting the formations into the fantastic shapes that can be seen in Rainbow Basin today (Dibblee 1968:52).

Fossil beds

The majority of the

beds in Rainbow Basin are found within the

of the Barstow Formation. They include many animals not found in

today, including

camel A camel (from and () from Ancient Semitic: ''gāmāl'') is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. Camels have long been domesticated and, as livestock, they provid ...

horse The horse (''Equus ferus caballus'') is a domesticated, one-toed, hoofed mammal. It belongs to the taxonomic family Equidae and is one of two extant subspecies of ''Equus ferus''. The horse has evolved over the past 45 to 55 mi ...

mastodon A mastodon, from Ancient Greek μαστός (''mastós''), meaning "breast", and ὀδούς (''odoús'') "tooth", is a member of the genus ''Mammut'' (German for 'mammoth'), which was endemic to North America and lived from the late Miocene to ...

s, and flamingos (Lindsay 1972:15), (Bureau of Land Management 1992:33). This unique collection of animals is representative of the ''Barstovian Land Mammal age''. In 1941 Rainbow Basin was designated the type reference for the ''Barstovian Land Mammal age'' by the North American Paleontological Society (Bureau of Land Management 1992:33).

Access

Rainbow Basin is open to the public by means of a one-way dirt road that loops through the basin. There is no camping within Rainbow Basin itself, but the Owl Canyon Campground is nearby.

Fossils A fossil (from Classical Latin , ) is any preserved remains, impression, or trace of any once-living thing from a past geological age. Examples include bones, shells, exoskeletons, stone imprints of animals or microbes, objects preserved ...

cannot be collected without a permit. Anyone planning a trip to this natural landmark should keep in mind that it is a

desert A desert is a landscape where little precipitation occurs and, consequently, living conditions create unique biomes and ecosystems. The lack of vegetation exposes the unprotected surface of the ground to denudation. About one-third of the la ...

area, and take appropriate safety precautions.

References

*Bureau of Land Management **1991 ''Management Plan for the Rainbow Basin Natural Area.'' Barstow Resource Area, California Desert District, Bureau of Land Management, U.S. Department of the Interior, Barstow. * Dibblee Jr., Thomas W. **1967 ''Areal Geology of the Western Mojave Desert, California.'' Geological Survey Professional Paper no. 522. United States Government Printing Office, Washington D. C. **1968 ''Geology of the Fremont Peak and Opal Mountain Quadrangles, California.'' California Division of Mines and Geology, San Francisco. *Lindsay, Everett H. **1972 ''Small Mammal Fossils from the Barstow Formation, California.'' University of California Publications in Geological Sciences Vol. 93. University of California Press, Berkeley.

External links

*Officia
Rainbow Basin Natural Area website
* ttps://www.nps.gov/subjects/nnlandmarks/site.htm?Site=RABA-CA Rainbow Basin

National Park Service The National Park Service (NPS) is an List of federal agencies in the United States, agency of the Federal government of the United States, United States federal government, within the US Department of the Interior. The service manages all List ...

{{authority control Protected areas of the Mojave Desert Protected areas of San Bernardino County, California National Natural Landmarks in California Rock formations of California Geography of San Bernardino County, California Bureau of Land Management areas in California Landforms of San Bernardino County, California