Rainbow Basin, National Natural Landmark in the Mojave Desert, San Bernardino County, California, United States

Rainbow Basin is a protected natural area in the Mojave Desert, about 8 miles north of Barstow. Its eroded bluffs show diagonal bands of rust-red, ochre, and grey sedimentary rock, with a dark vertical crevice cutting through the red band. The formations are deeply folded, most prominently into the Barstow Syncline.

The fossil beds lie mainly in the Barstow Formation, whose middle to late Miocene layers hold one of North America's largest Cenozoic fossil assemblages. In 1941 the site was designated the type reference for the Barstovian Land Mammal Age. The fossils include camels, horses, mastodons and flamingos, animals not found in California today.

Scientists and geology students examine the site for educational purposes, studying rock formations and analyzing different visible strata in the landscape.

Access is via Irwin Road from Barstow, which leads to a one-way dirt loop road through the basin. There is no camping inside Rainbow Basin; Owl Canyon Campground is nearby. Fossils cannot be collected without a permit. The area is desert, so take appropriate safety precautions.

Rainbow Basin was designated the type reference for the Barstovian Land Mammal age by the North American Paleontological Society in 1941, so its Barstow Formation beds serve as the standard for comparing middle Miocene faunas elsewhere. The fossil layers have yielded camels, horses, mastodons and flamingos, animals absent from California today.