The Fourstones Fairy Stone/Fairy Trough is apparently lost. This is, however, the background to this curious object, presumably part of a medieval cross?

there is a current story in the neighbourhood, that one of [the four stones of Fourstones] was called the Fairy-stone, because in the rebellion in 1715 the focus of this altar was formed into a square recess with a cover to receive the correspondence of the rebel chiefs, and that a little boy clad in green came in the twilight of every evening to rescue the letters left in it for Lord Derwentwater, and deposit his answers which were spirited away in the same manner by the agents of his friends. This fairy stone, however, certainly had existence, for a person 80 years of age, remembered its situation to the south of the village, near the old road, and that it was squared, and had a square cistern hewn out of its top, which was called the Fairy-trough, and traditionally said to have had a pillar fixed in it. This is an exact description of the base of an old church-yard, village, boundary, or memorial cross; but no trace of either the stone or the road remains (Richardson 80)