Rainbow Basin
| Rainbow Basin Natural Area | |
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The famous Rainbow Basin syncline.
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| Location | San Bernardino County, California |
| Nearest city | Barstow |
| Coordinates | 35°01′46″N 117°02′12″W / 35.02944°N 117.03667°WCoordinates: 35°01′46″N 117°02′12″W / 35.02944°N 117.03667°W |
| Area | 1,961 acres (794 ha) |
| Designated | 1966 |
Rainbow Basin is a geological formation in the Calico Peaks range, located approximately 8 miles (13 km) north of Barstow in the Mojave Desert in San Bernardino County, California.
The Rainbow Basin has been designated a National Natural Landmark and is in the Bureau of Land Management managed Rainbow Basin Natural Area. Rainbow Basin is a mixture of private and public land, but it is managed by the Bureau of Land Management. 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 beds, which have provided scientists with valuable information about life during the middle Miocene epoch, between 12 and 16 million years ago; and to the northeast the Calico Early Man Site.
Underneath Rainbow Basin is the massive batholith 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, or possibly the late Jurassic period (Dibblee 1967:43). Early in the Cenozoic 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 that are most visible in Rainbow Basin today (Dibblee 1968:56-57). Further compression, uplift, and finally extension left these sedimentary 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).
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