Underneath Rainbow Basin is the massive batholith that lies below much of the western Mojave. Made from a type of rock called quartz monzonite, this batholith dates to either the Cretaceous, or possibly the late Jurassic period. 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. Sediments deposited in this basin became the sedimentary rocks that are most visible in Rainbow Basin today. 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. The thick sedimentary layers can be divided into three distinct formations. The lowest is called the Jackhammer Formation, and it is composed of layers of sandstone, siltstone, limestone, and conglomerate, all probably dating to the early Miocene Epoch. Above this is the Pickhandle Formation. The sediments making up this formation are mostly of volcanic origin – tuff, rhyolite and andesite, indicating that they were laid down during a period of active volcanism. That time was probably during the early Miocene. The highest of the three formations is the Barstow Formation, which is made up of layers of conglomerate, limestone, sandstone, and shale. This formation dates to the middle to late Miocene and it contains one of the largest Cenozoicfossil assemblages in North America. Most of the sediment that makes up the layers in this formation was stream-laid, but there is a white layer of rhyolitic tuff near the top. Finally, on top of everything else, is a relatively thin layer of fanglomerate laid down during the late Pleistocene. Differential erosion of rocks of different hardness finished the job of sculpting the formations into the fantastic shapes that can be seen in Rainbow Basin today.
Fossil beds
The majority of the fossil beds in Rainbow Basin are found within the sedimentary rocks of the Barstow Formation. They include many animals not found in California today, including camels, horses, mastodons, and flamingos,. 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. in Rainbow Basin.
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 cannot be collected without a permit. Anyone planning a trip to this natural landmark should keep in mind that it is a desert area, and take appropriate safety precautions.
