The Franciscan Complex is a world-renowned example of the rocks that form in a subduction zone as an oceanic plate dives (subducts) beneath another plate. The subducting plate has the sediments and volcanoes and other features on its top scraped off onto the front and underside of the upper plate. Some of this material is carried down to great depths, where it turns into unusual rock types, and returns to the surface by methods that are still uncertain. The result is melange
, a chaotic mixture of many different rock types, of different ages, that have been mixed and churned in a tectonic mill.
Shell Beach, between Bodega Bay and the Russian River, is an outstanding display of the subduction-zone setting and its colorful variety of rocks. As it is part of Sonoma Coast State Park, all collecting, excavating and hammering is prohibited. The weather is changeable and often foggy. For more geologic detail, don't miss Terry Wright's guide, prepared for his students at Sonoma State University.