You have two basic options for exploring the rich woodland of Sonoma Mountain. You can join the throngs of tourists who visit Jack London State Park near Glen Ellen. Or, if you time it right, you can sneak up on the mountain from the other direction. From the first week in March through mid-May, and again from mid-September through mid-December, the Nature Conservancy offers docent-led tours of leafy Fairfield Osborn Preserve.

Fairfield Osborn is one of the North Bay’s last undiscovered jewels. Deeded from the Roth family to the Nature Conservancy in 1972 (and named for Joan Roth’s father, a noted ecologist), then handed over to Sonoma State University in 1998, the 210-acre park is remarkably untrammeled. This is partly by design. In transferring management of the site, the Conservancy stipulated that it be blocked to unfettered public use. During the soggy winter and fire-alarm summer, it is closed altogether. Surprisingly, this limited access - in the form of guided hikes each Saturday and Sunday - goes virtually ignored. Often, a single family, or even a single human, shows up for the educational tours.

Because of the sparse attendance, the docents can tailor hikes to your abilities and interests. The preserve’s 6 1/2 miles of trails are inappropriate for wheelchairs, but children and the elderly can easily enjoy them. The highest elevation gain you will experience is about 500 feet. The longest hike would be about four hours.

Fairfield Osborn’s knowledgeable guides might take you on a wildflower tour de force, or on a visual hunt for resident golden eagles. Exploring this realm back in 1972, the Nature Conservancy discovered that Copeland Creek, in its upper reaches, is the only perennially fishless stream in California. But Fairfield Osborn Preserve could just as easily be singled out for its multitude of micro-systems, including oak woodland, coniferous forest, freshwater marsh and chaparral. In short, the park has something for everyone - and practically no one knows it’s there.

Fairfield Osborn Preserve is on Lichau (pronounced LEE-how) Road, a few miles east of Rohnert Park. You need reservations only for the preserve’s weekend field workshops (which range from “The Geology of Sonoma Mountain” to “Edible and Medicinal Plants”). Otherwise, just show up at 9:45 AM. The rambles cost $3 for each teen and adult; they are free to kids 12 and under. No dogs, please. For questions, contact the park manager at (707) 795-5069, or by e-mail at fairfield.osborn@sonoma.edu.