I have a small collection of photographs, including ones found on the net and my own, of Norman. The square stone in your photo used to be on the "path" that you walk on here, but has also caused the damage to the feet. It was part of a wall that was made, clearly with good intentions but it's proximity to the feet meant it got kicked into the bones. In November 2014 a friend and I moved the stone away to a position similar to your photo to try to avoid further damage occurring in this way.
I can't help but think that the plastic wrap present at some stage in the past and mouldy food offerings (also removed), didn't help with the decay of the dark organic deposits visible clearly in the older photos but now much reduced.