The eastern shore of Tomales Bay rests on the American tectonic plate, and the western shore, the Point Reyes Peninsula rests on the Pacific tectonic plate. The San Andreas Fault Line runs down the middle of Tomales Bay. Scientists consider Tomales Bay one of the most pristine bays in America.
The constant movement of land masses the occurs at the San Andreas Fault has created a rift that separates the peninsula comprising the Point Reyes National Seashore and the mainland. The long, narrow body of water that fills the rift is Tomales Bay. The town of Inverness lies near the head of the bay.