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…