A strange and honorable thing happened Monday at an old cemetery near Graton.

Three reverent, community-minded sorts plucked up a large, upper portion of a broken tombstone that wasn’t supposed to be there in the Green Valley Cemetery. Perhaps only Heaven knows who put it there, and why.

The base of the grave marker of Petra Johnston, a young mother who died in 1861, is where it belongs: in a cemetery in Half Moon Bay.

Santa Rosa’s Jeremy Nichols, a cemetarian and current president of the Sonoma County Historical Society, helped Half Moon Bay residents Ulli and Caesar Bisono wiggle the partial stone out of the ground at the Graton graveyard and place it in their pickup.

The Bisoros then carted it back to Half Moon Bay, where, in time, it will be reattached to its base on Petra Johnston’s grave at the Pilarcitos Cemetery. Once the headstone is repaired, some sort of re-consecration ceremony will be planned by volunteers with the foundation that keeps up the splendid, prominent, old Half Moon Bay house in which Petra lived with her husband, Gold Rush entrepreneur James Johnston.

It may forever remain a mystery why and when and how Petra Johnson’s headstone was broken in two and the top piece was transported about 90 miles north to the cemetery on Green Valley Road.

Dave Cresson of the Half Moon Bay History Association said the local lore is that the upper portion of the tombstone was broken off and carried away in the early 1970s.

It was only about 20 years ago that west Sonoma County history lover Tony Pires found the large piece buried in the Graton cemetery, unearthed it, attached a good-enough base and set it upright.

Just months ago, an inquiring journalist named J’aime Rubio spotted the base of Petra Johnston’s headstone in Half Moon Bay and launched the inquiry that culminated in the civic-minded Bisonos driving to Sonoma County on Monday to truck the upper part of the stone back to its rightful place.