part of the Transverse Ranges of Southern California, a mountain chain formed by tectonic forces between the North American and Pacific Plates along the San Andreas Fault. An early version of the range rose in the Miocene, between eleven and five million years ago, but has largely eroded. The range was shaped into its present form during the Pleistocene epoch beginning approximately two million years ago, with regional uplift continuing to the present. The rocks that make up the mountains are much more ancient than the mountains themselves – ranging from 18 million years to 1.7 billion years old. The San Andreas Fault was also responsible for the formation of both major mountain passes that mark the east and west ends of the range. These mountains are shaped by several primary tectonic or fault blocks – the Big Bear block, which forms the large montane plateau that characterizes the northern portions of the range; and the more complex and fractured San Gorgonio, Wilson Creek and Yucaipa Ridge blocks, which form the rugged and heavily dissected southern parts of the mountains. Because of their large, steep rise above the surrounding terrain, the San Bernardinos have been subject to great amounts of erosion that have carved out numerous river gorges. Rocks and sediment from the mountains are deposited on the surrounding valley floors as massive alluvial fans. Regional alluvial deposits can reach depths of or more, and their permeable soils constitute several major groundwater basins. History Indigenous peoples Archaeological discoveries in the San Bernardino Valley suggest that humans have populated the region for at least 10,000-12,000 years. Several Native American groups held the lands surrounding the San Bernardinos. These included the Tongva, who occupied the Inland Empire area southwest of the mountains; the Cahuilla, who lived in the Coachella Valley and Salton Sea basin; and the Serrano and Chemehuevi peoples, whose territory comprised land north and northeast of the San Bernardinos, adjacent to the Mojave Desert. Most of these tribes did not have permanent settlements in the mountains, with the possible exception of a few groups of Serrano. Indigenous peoples traveled into the mountains in the summer to hunt deer and rabbits, gather acorns, berries and nuts, and seek refuge from the desert heat. They established well-traveled trade routes, some of which were later used by Europeans to explore and settle the region. The precipitous Mojave Road (or Mojave Trail) crested the