From ‘Rude Stone Monuments in all Countries – their age and uses’ by James Fergusson, 1872.
I think all antiquarians will agree with Sir Gardner Wilkinson in assuming that this is the stone of Cetti mentioned in the Welsh Triads... the 88th* Triad speaks of the three mightly achievements of the Isle of Britain; the raising of the stone of Cetti, the building of the work of Emmrys, and the heaping of the pile of Cyvragnon.
He suggests the work of Emmrys is Stonehenge and the pile of Cyvragnon, Silbury Hill.
*Not sure if this is true yet – need to find the original? The triads are rather dodgy as if they are the ones collected by Iolo Williams, he did make some of them up.