Читать книгу 101 Hikes in Southern California. Exploring Mountains, Seashore, and Desert онлайн

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Geologists figure that the San Gabriels are being squeezed horizontally about a tenth of an inch each year, and being thrust upward much more rapidly than that. Caught in this tectonic frenzy, the San Gabriel Mountains are surging upward as fast as any mountain range on the planet. They are also disintegrating at a spectacular rate. Although the San Gabriels consist mainly of durable granitic rocks, much like those in the sturdy Sierra Nevada, the San Gabriel rocks have been through a tectonic meat grinder. The tops of the San Gabriels are fairly rounded, but their slopes are often appallingly steep and unstable. An average of 7 tons of material disappears from each acre of the front face each year, most of it coming to rest behind debris barriers and dams in the L.A. Basin below.

The San Gabriel Mountains themselves are relatively young as upthrust units—only a few million years old. This is not true of the ages of most of the rocks that compose them. Some rocks exposed here are representative of the oldest found on the Pacific Coast—more than 600 million years old.