The summit of Sonoma Mountain, the county's 2,463-foot-high landmark vista, will be purchased by the county's Open Space District under a $10 million deal approved Tuesday by Sonoma County supervisors.

"We are fortunate to have the opportunity to purchase this significant property and create an unbroken band of 5,500 acres of publicly protected land on Sonoma Mountain," Valerie Brown, the county supervisor representing Sonoma Valley, said in a statement.

Wendy Eliot, conservation director of the Sonoma Land Trust, said the 283-acre acquisition adds Sonoma Mountain to the list of publicly owned Bay Area peaks that includes Mount Tamalpais, Mount St. Helena, Mount Diablo and San Bruno Mountain.

"Now, they'll be able to hike to the top of Sonoma Mountain, too," she said.

Officials said it was too early to predict when the public will have access to the mountaintop, which rises above Petaluma to the west and Glen Ellen to the east. Acquisition also fills in a missing piece of the patchwork of properties on the ridge that runs 25 miles from San Pablo Bay to Bennett Valley.

For about 150 years, the site known as Sonoma Mountain Ranch has been owned by the Stevenson family, which reportedly obtained it in a presidential land grant. Since the death of John L. Stevenson a decade ago, the property has been in probate court with Oakland attorney Michael Walsh acting as executor.

Eliot said the sudden availability for purchase in September prompted the Land Trust to put down $125,000 to secure a contract to buy the ranch. A number of properties on the slopes of Sonoma Mountain have deed restrictions that prevent public access, but the ranch was not one of them.

Officials with the Land Trust, a private nonprofit agency dedicated to bringing private lands into the public realm, and the county Agricultural Preservation and Open Space District, funded by a quarter-cent sales tax, said the effort to secure Sonoma Mountain constituted an acquisition partnership that had worked effectively earlier this year in putting the Jenner Headlands in the public domain.

Over the past 15 years, the Open Space District has amassed through purchase or conservation easement thousands of acres on the Sonoma Mountain ridge. Those lands include McCrea, Lafferty, Morelli, Wilroth, Skiles, Cooper's Grove and Quailbrook Cook properties.

Escrow is expected to close by the end of the year on the deal that commits $9.95 million in sales tax revenues. In addition, the staff of the state Coastal Conservancy has recommended that its governing board approve $1.5 million toward the project.

The ranch is a mix of oak woodlands, grasslands and wetlands and contains headwaters of streams in which steelhead spawn.

Dee Swanhuyser, North Bay representative of the Bay Area Ridge Trail Council, said the addition of Sonoma Mountain "puts 550 miles of trails on the ridge tops" that circle the San Francisco Bay.

"The voters of Sonoma County are the ones who put the money on the table," Swanhuyser said.

Eventually, the property would be turned over to the state parks system or to the regional parks department, depending upon whether either agency can accommodate it in its budget.

You can reach Staff Writer Bleys W. Rose at 521-5431 or