Rainbow Basin, about 8 miles north of Barstow on Fossil Bed Road, is a Bureau of Land Management natural area protecting some of the most colourful geology in the Mojave. The sedimentary rocks have been folded, faulted, and eroded into dramatic formations — layer after layer of red, orange, cream, and green bands that glow in the afternoon light. A 4-mile loop road winds through the basin, passable in a standard vehicle if you take it slowly.
The fossil record here is exceptional. Miocene-era bones of extinct horses, mastodons, camels, and other fauna have been excavated from these beds since the 1940s. You won't find exposed fossils lying around — they've mostly been collected — but the interpretive signs explain what was found and what it means. Go in early morning when the light rakes across the formations and the temperature is manageable. The access road is unpaved but not difficult.