The hidden faces of the stones

Terence Meaden has found, on several megaliths in the South of England, human and animal faces and bodies; they seem to be mainly natural features, though, according to him, some are likely to have been slightly arranged.
Polimorphic anthropomorfism, according to Pedro Alvim, involving fingers, teeth, phalus, vulvas, and, like a syntesis, the human shape.
One question arises, anyway: how old are the weathered scars on the stones?