They are part of a geologic province known as the Salinian Block which broke off the North American plate as the San Andreas Fault and Gulf of California came into being.

The higher portions of the Peninsular Ranges, especially the west-facing slopes, are home to coniferous and mixed evergreen forests.

Cleveland National Forest covers much of the higher Southern California Peninsular Ranges.

These isolated forests are predominantly composed of tamarack pine (Pinus contorta subsp.

It was sheared off the mainland, becoming at one time an island, and evolved in relative isolation from the northern part of the peninsula and ranges.