The central valleys of northern Sonoma and Mendocino Counties are transtensional pull-apart basins formed by the tectonics of the Maacama Fault, and are framed by the foothills of the Coastal Belt Thrust. The Maacama Fault is the major right lateral transform/strike-slip fault of the San Andreas system in Northern California, and forms the primary plate boundary between the Pacific and North American Plates in Northern California.
