Half a million years ago or so, these were part of the Santa Rosa Mountains. Over millennia, layers of sediment eroding from the rising Santa Rosa Mountains accumulated here, forming the sand and mudstone of these hills. The climate has changed significantly since these depositions occurred, becoming much warmer and dryer. There are fossils embedded within these rocks of numerous large herbivores, including camels, horses, mammoths, and ground sloths, as well as meat-eating predators such as the saber-toothed cat, the bone-eating dog, and the wolf coyote, showing that a cooler, wetter, more verdant landscape existed here over much of the past 2 million years. If you stop on the way to look at the layered sandstone for fossils, remember that if a fossil is found, it is protected: no excavating or taking of samples is allowed.