Tomales Bay is a long, narrow inlet of the Pacific Ocean in Marin County in northern California in the United States. It is approximately 15 miles (25 km) long and averages nearly 1.0 miles (1.6 km) wide, effectively separating the Point Reyes Peninsula from the mainland of Marin County. It is located approximately 30 miles (48 km) northwest of San Francisco. The bay forms the eastern boundary of Point Reyes National Seashore. Tomales Bay is recognized for protection by the California Bays and Estuaries Policy. On its northern end it opens out onto Bodega Bay, which shelters it from the direct current of the Pacific. The bay is formed along a submerged portion of the San Andreas Fault.