The 630-acre Mitsui Ranch property is located in the Sonoma Mountains of the Coast, approximately five air miles northeast of Petaluma and four air miles east of Rohnert Park. Elevation ranges from 1,979 feet where Copeland Creek exits the property on the western boundary to 2,397 feet near the northwest corner of the ranch.
The Sonoma Mountains are a small, isolated range between the Sonoma Creek and Petaluma River drainages. Sonoma Mountain (called Oonapais by the Coast Miwok, which probably means “Buckeye Mountain”) is the dominant feature of the range, rising to 2,463 feet (751 meters), with views of the Pacific Ocean directly to the west. The Mitsui property comprises the ring of minor peaks on Sonoma Mountain and a large basin between them. The western and northwestern property boundary lines remain unchanged from the original boundary lines of the Rancho Petaluma lands granted to General Mariano G. Vallejo in 1844 (C.C. Tracy, Deputy Surveyor, U.S. Surveyor General, March 1862).