The 623-acre Thacher Ranch, a parcel near Marin’s border with Sonoma known to many as Rocky Canyon Ranch, is the latest ranch to be safe-guarded from the pressures of estate development after the Marin Agricultural Land Trust closed on a $1.8 million agricultural conservation easement, the nonprofit’s 73rd easement in its over 30-decade history. Second generation rancher Will Thacher, who has run the ranch since his father died in 2007, leases 500 acres for cattle and uses the rest for his 40 Suffolk-cross sheep, which he raises for meat for Bay Area restaurants. Last year he purchased 40 new sheep—a different breed, East Friesians—for a future artisan cheese-making operation, which will be funded by the easement. “We want to contribute to what is now our little hub, in Tomales and Petaluma, of artisan cheese,” Mr. Thacher explained. (He added that the money will also support his large family. “I have eight children,” he said.) He hopes that his daughter, a recent graduate of New York University who took an interest in urban farming on the East Coast, will work on the ranch. Mr. Thacher’s parents, Jim, an attorney in San Francisco, and Gladys, bought the ranch in 1964; it was Jim’s wish that the ranch be protected. Mr. Thacher said he will continue to work with groups like the National Resource Conservation Service to protect the ranch’s habitat and wildlife; numerous native plants make their home on the land and Chileno Creek, which supports steelhead trout and a few coho salmon, run through it. The ranch is the first to be protected using the county’s 2012 Measure A funds, a quarter-cent sales tax approved by voters that designates 20 percent of its annual revenue toward agricultural easements.