Geology
The San Bernardinos are 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 1,000 feet (300 m) or more, and their permeable soils constitute several major groundwater basins.
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