Cattle will scratch themselves on whatever they can find - and they are a principal cause of damage to whatever it is they find - just because they're so heavy (and thick-skinned).

But a closer examination of the images of the Popping Stone - I've been up that river but on the Northumberland bank - suggests there are four stones. Two look to have been placed there and two seem to have jammed up against them, presumably by the force of the river in spate.

More pictures please !