Читать книгу Afoot & Afield: Orange County. A Comprehensive Hiking Guide онлайн

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Hikers following this guidebook will discover many interesting and sometimes colorful exposures of marine sedimentary rocks in places like the Chino Hills, San Joaquin Hills, and foothills of the Santa Ana Mountains. These sediments, uplifted by a variety of geologic processes, are continuations of the formations that lie deep underground in the center of the Los Angeles Basin.

The marine sediments you will see—sandstone, siltstone, shale, and conglomerate—tend to be rather soft and easily eroded. Along the south coast, where the San Joaquin Hills meet the sea, several wave-cut “marine terraces” are identifiable on the coastal headlands. They exhibit a record of changing sea levels and gradual uplift over the past 1–2 million years. In many places, the terraces themselves are deeply cut by drainage channels—the coastal canyons—which themselves are quite recent features.

The Santa Ana Mountains, along with the San Jacinto Mountains, lie at the northwest tip of the extensive Peninsular Ranges province, stretching south toward the tip of Baja California (the province, in fact, derives its name from Baja’s peninsular shape). Each range in this province possesses a core of granitic (granitelike) rock, overlain in many places by a veneer of older metamorphic rocks. Many of the Peninsular Ranges, including the Santa Anas, are raised and tilted fault blocks, typically with steep east escarpments and more gradually inclined western slopes.