This beautiful stretch of coast is composed of the rocks of the Franciscan Complex, a chaotic mix of deep sea sands and muds, fragments of ocean crust and mantle, and even the occasional limestone reef deposit. The rocks formed in the trench of a subduction zone that lay off the coast of California in the time of the dinosaurs. The subduction zone is long gone, but the geologic violence that contributed to their origin continues in the lateral movement of the San Andreas fault which passes a short distance offshore. Large earthquakes like the one that devastated San Francisco in 1906 (close to magnitude 8) can tear the fault over a distance of 200 miles, and shift the crust 10-20 feet over much of that distance. The quakes occur every 100-150 years or so (do the math...).
The San Andreas has been active for more than 20 million years, and in that time has transported a wide swath of California and Baja California northward some 200 miles. Part of the evidence can be seen in the gratuitous sunset picture below, as one can pick out some small islands on the horizon. These are the Farallon Islands, which are composed of granitic rock related to the Sierra Nevada batholith exposed far to the south. The small islands are an important bird rookery, and the only human inhabitants are a few biology researchers.
The storm that threatened to wash out our field trip yesterday (but conveniently waited until the evening to let loose) cleaned up the sky and the gave us a beautiful sunset.