Folklore

Eglwyseg
Standing Stone / Menhir

Fair enough, this might not refer to this particular rock. In fact it’s true to say that it probably refers to some random rock on the steep slope of the Creigiau Eglwyseg (Eglwyseg Rocks) below. But it shows you what the rocks are like round here.

Thomas Morris happened to be returning home from Llangollen very late on one Saturday night in the middle of the summer, and by the time he reached near home the day had dawned, when he saw a number of the Tylwyth Teg with a dog walking about hither and thither on the declivity of the Eglwyseg Rocks, which hung threateningly overhead. When he had looked at them for some minutes, he directed his steps towards them; but as they saw him approaching they hid themselves, as he thought, behind a large stone. On reaching the spot, he found under the stone a hole by which they had made their way into their subterranean home.

Maybe midsummer had something to do with the sighting.
From ‘Celtic Folklore, Welsh and Manx’ by John Rhys (1901).