Le Pied du Boeuf is an indentation in a rock above Fontenelle Bay, which looks like a hoof.
An old tale is that a saint chased the devil from the island and he then leapt north, leaving a hoof mark behind.
Photographer Nick Despres went to photograph the stone on Tuesday afternoon and was surprised to find someone had poured concrete into the hole and put a granite spike in it.
He had to double check he was in the right place, and did not have the wrong stone, before reporting the damage.
Assistant States archaeologist Tanya Walls visited the site yesterday.
‘Someone had filled it up with concrete and put a stone in it,’ she said.
‘It appears to have been done yesterday [Tuesday] as the person who noticed it said the concrete was still soft.’
Fortunately it appears the cement had been mixed poorly and it was possible to remove the mixture yesterday using hand tools, without damaging the stone.
However it was unclear why someone would have done this.
Vale Commons Council manages a lot of land in the area, but president Peter Blake said it was not aware of anything happening to the stone.