Rainbow Basin is a geological formation in the

Calico Peaks The Calico Peaks are geologically and historically colorful mountains in the Calico Mountains Range in the Mojave Desert of San Bernardino County and Inyo County, California. They are located just north of Barstow, Yermo, and Interstate 15. P ...

range, located approximately north of Barstow in the

Mojave Desert The Mojave Desert ( ; mov, Hayikwiir Mat'aar; es, Desierto de Mojave) is a desert in the rain shadow of the Sierra Nevada mountains in the Southwestern United States. It is named for the indigenous Mojave people. It is located primarily ...

San Bernardino County San Bernardino County (), officially the County of San Bernardino, 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 the 2020 U.S. Census, the population was 2,181, ...

California California is a state in the Western United States, located along the Pacific Coast. With nearly 39.2million residents across a total area of approximately , it is the most populous U.S. state and the 3rd largest by area. It is also the m ...

. 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 ...

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. Headquartered in Washington DC, and with oversight over , it governs one eighth of the country's ...

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: 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 preserved ...

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

Miocene The Miocene ( ) is the first 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 means "less recen ...

epoch, between 12 and 16 million years ago; and to the northeast the

Calico Early Man Site The Calico Early Man Site is an archaeological site in an ancient Pleistocene lake located near Barstow in San Bernardino County in the central Mojave Desert of Southern California. This site is on and in late middle-Pleistocene fanglomerates ...

Geology

Underneath Rainbow Basin is the massive

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

that lies below much of the western Mojave (Dibblee 1968:15). Made from a type of rock called

quartz monzonite Quartz monzonite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagio ...

, this batholith dates to either the

Cretaceous The Cretaceous ( ) is a geological period that lasted from about 145 to 66 million years ago (Mya). It is the third and final period of the Mesozoic Era, as well as the longest. At around 79 million years, it is the longest geological period of ...

, or possibly the late

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

period (Dibblee 1967:43). Early in the

Cenozoic The Cenozoic ( ; ) is Earth's current geological era, representing the last 66million years of Earth's history. It is characterised by the dominance of mammals, birds and flowering plants, a cooling and drying climate, and the current configu ...

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 that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particle ...

that are most visible in Rainbow Basin today (Dibblee 1968:56-57). Further compression,

uplift Uplift may refer to: Science * Geologic uplift, a geological process ** Tectonic uplift, a geological process * Stellar uplift, the theoretical prospect of moving a stellar mass * Uplift mountains * Llano Uplift * Nemaha Uplift Business * Upli ...

, and finally extension left these

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

formations deeply folded, the most prominent fold being the Barstow

Syncline In structural geology, a syncline is a fold with younger layers closer to the center of the structure, whereas an anticline is the inverse of a syncline. A synclinorium (plural synclinoriums or synclinoria) is a large syncline with superimpose ...

. 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 A jackhammer (pneumatic drill or demolition hammer in British English) is a pneumatic or electro-mechanical tool that combines a hammer directly with a chisel. It was invented by William Mcreavy, who then sold the patent to Charles Brady King ...

, and it is composed of layers of

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 silicat ...

siltstone Siltstone, also known as aleurolite, is a clastic sedimentary rock that is composed mostly of silt. It is a form of mudrock with a low clay mineral content, which can be distinguished from shale by its lack of fissility.Blatt ''et al.'' 1980, ...

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms w ...

, and

conglomerate Conglomerate or conglomeration may refer to: * Conglomerate (company) * Conglomerate (geology) * Conglomerate (mathematics) In popular culture: * The Conglomerate (American group), a production crew and musical group founded by Busta Rhymes ** ...

, all probably dating to the early

Epoch (Dibblee 1968:18). Above this is the

Pickhandle Formation A pickaxe, pick-axe, or pick is a generally T-shaped hand tool used for prying. Its head is typically metal, attached perpendicularly to a longer handle, traditionally made of wood, occasionally metal, and increasingly fiberglass. A stand ...

. The sediments making up this formation are mostly of

volcanic A volcano is a rupture 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 found where tectonic plat ...

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 ...

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

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 predo ...

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

volcanism Volcanism, vulcanism or volcanicity is the phenomenon of eruption of molten rock (magma) onto the surface of the Earth or a solid-surface planet or moon, where lava, pyroclastics, and volcanic gases erupt through a break in the surface called a ...

. That time was probably during the early Miocene (Dibblee 1968:23). The highest of the three formations is the

Barstow Formation The Barstow Formation is a series of limestones, conglomerates, sandstones, siltstones and shales exposed in the Mojave Desert near Barstow in San Bernardino County, California.Dibblee, T.W., Jr. (1967). Areal Geology of the Western Mojave Des ...

, which is made up of layers of

, and

shale Shale is a fine-grained, clastic sedimentary rock formed from mud that is a mix of flakes of clay minerals (hydrous aluminium phyllosilicates, e.g. kaolin, Al2 Si2 O5( OH)4) and tiny fragments (silt-sized particles) of other minerals, especiall ...

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

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

fanglomerate Conglomerate () is a clastic rock, clastic sedimentary rock that is composed of a substantial fraction of rounded to subangular gravel-size clasts. A conglomerate typically contains a matrix (geology), matrix of finer-grained sediments, such 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 ...

deposits) laid down during the late

Pleistocene The Pleistocene ( , often referred to as the ''Ice age'') is the geological Epoch (geology), epoch that lasted from about 2,580,000 to 11,700 years ago, spanning the Earth's most recent period of repeated glaciations. Before a change was fina ...

(Bureau of Land Management 1992:31). Differential

erosion Erosion is the action of surface processes (such as water flow or wind) that removes soil, rock, or dissolved material from one location on the Earth's crust, and then transports it to another location where it is deposited. Erosion is d ...

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: la, camelus and grc-gre, κάμηλος (''kamēlos'') from Hebrew or Phoenician: גָמָל ''gāmāl''.) is an even-toed ungulate in the genus ''Camelus'' that bears distinctive fatty deposits known as "humps" on its back. ...

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 million yea ...

mastodon A mastodon ( 'breast' + 'tooth') is any proboscidean belonging to the extinct genus ''Mammut'' (family Mammutidae). Mastodons inhabited North and Central America during the late Miocene or late Pliocene up to their extinction at the end of the ...

s, and

flamingo Flamingos or flamingoes are a type of wading bird in the family Phoenicopteridae, which is the only extant family in the order Phoenicopteriformes. There are four flamingo species distributed throughout the Americas (including the Caribbea ...

s (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 barren area of landscape where little precipitation occurs and, consequently, living conditions are hostile for plant and animal life. The lack of vegetation exposes the unprotected surface of the ground to denudation. About on ...

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
* ttp://www.nature.nps.gov/nnl/site.cfm?Site=RABA-CA Rainbow Basin

National Park Service The National Park Service (NPS) is an agency of the United States federal government within the U.S. Department of the Interior that manages all national parks, most national monuments, and other natural, historical, and recreational properti ...

{{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