"Wonder four is the stone that walks at night-time above the valley of Citheinn, also formerly it is thrown down the watery hollow Cereuus, which is in the middle of the sea which is called Mene, and on the morrow above the bank on top of said valley is is discovered without doubt."1,2
This wonder has been identified by Walter Gill* as the
Maen Morddwyd ["The Thigh Stone"], apparently
built into the wall of the ruined church of
to Llanidan House in south-western Anglesey.
The connection relies on a description by Gerald of Wales in his
"Journey through Wales" of a thigh-bone shaped stone that always returned to its home, even if, as happened in c.1098 CE, it was thrown into the sea. He also reported
it had a number of somewhat more dissolute properties (see
Thomas Pennant, who reported on a visit to the House in the 1770s,
stated that Gerald's stone was built into the church (*p.9).
In 1833 Lewis* reported that the stone was said to have been built into the
wall but was "
not sufficiently conspicuous to be easily distinguished". By 1849 the church was at least partly destroyed
and it isn't entirely clear what happened to the stone4. The remaining parts of the church are now a private burial chapel**
Cereuus is identified in the "Sawley" variants of the Historia as Pwll Ceris2, also known as the "Swellies"5: an area between the two bridges over the Menai Straits (hense Mene), which is known for its chaotic tidal whirlpools. This site was also identified, probably independently, by Llanidan's vicar, Henry Rowlands (*p.4). The valley of Citheinn is unknown, but is probably the major valley of the Cefni which runs just north of Llanidan6.
You can read more on the Myth of the stone.