There's a stone built cottage near to my house that has similar grooves on the walls outside.
The cottage was once a school house and the grooves are a consequence of the pupils sharpening whatever the used to use to write on slates with or so legend has it.
I'm not sure thse stones are a myth when dealing with a volcanic tuff , or whinstone you shirley work from coarse to fine.
It could always be a shepherd sharpening his docking shears before the days of laggy bands.