If Serth means steep or obscene then it could refer to its proximity to the steep sided cliffs, or as I beleive obscene refering to that old obscene religion and just to confound them they carved there own religous symbol on it. So Maen Serth could be "Big stone of the old obscene religion"
The stone is soooo obviously a way marker, People making there way over the hills wouldn't immediately head down through the pass, which in ancient days was probably covered in trees, they would follow the stones up and along the ridge. Any way the stone looks prehistoric,
and even Coflein states it as Bronze age as if there was never any doubt, not even mentioning it's so called disputed antiquity.
If you ever go to this stone for gods sake go the long way round dont go up the cliffs, its exciting but a bit dangerous.
Visited 6th April 2003: Maen Serth was our goal on the walk along the ridge. Since our visit I've learned that it's antiquity is in doubt, which I suspected at the time given that it stands right on top of the hill. As we approached from the west the stone looked at first like an elongated trig point. The track runs to the south of the stone, so a minor deviation takes you right up to it.
Maen Serth is a beautiful slender thing. Unfortunately it's set in concrete, and surrounded by lines of erosion caused by my good friends the motorcyclists. The setting more than makes up for this, with beautiful views in all directions. Fortunately we weren't met by any other walkers or of-road types while we were at the stone. In fact all was tranquil, and (despite the fact we live on a secluded rural hillside) I enjoyed sitting on a secluded rural hillside next to a standing stone listening to the Skylark. It's a shame if this baby is Medieval rather than Bronze Age. I'll post more up about the stone when I've done some more research.
In his book The Spirit Paths of Wales (ISBN: 1-85284-289-X) Laurence Main describes Maen Serth:
Maen Serth (Steep Stone) is a prominent standing stone which stands 7ft 2ins high, 6ins thick and 1ft 8ins broad, at an altitude of 15000ft. It was erected in the Bronze Age and had a cross carved on it around AD 800. The Welsh chieftain of Elvel, Einion Clud, was murdered by the English here in a tournament in the late 12th century.
Main writes confidently about this site and others, but doesn't include any of his sources.
On the BBC web site (see the link below) a short article by Roger Hulm gives a more detailed account of the legend, this time identifying the murderer as Roger Mortimer (a Norman):
The story goes that in the late 12th century, there was a dispute over the land surrounding Rhayader between the Norman 'Marcher' Lord, Roger Mortimer, and the Welsh chieftains - Rhys ap Gruffydd and the two brothers, Cadwallon and Einon Clud. During a temporary 'lull' in hostilities, at Christmas 1176, these men had gathered at Rhys' castle to feast and joust.
At the jousting event, the Welshman Einon Clud defeated Roger Mortimer but the Norman was a bad sportsman. He ambushed Einon on the hill above Rhayader and killed him as he was returning home. A short time later, the other brother was also ambushed by Mortimer on the same hill and killed.
It was said that Maen Serth was the place of the double killing which has given another local name to the stone - the 'Princes' Stone'.
I've now found out some more about Maen Serth. The Cadw records, indicate that it's not prehistoric, but it remains listed on the National Monument Record as a Bronze Age standing stone.
Apparently the stone has a cross carved on the east south east side of it, but this is now practically indiscernible. There's a large lump of rock missing from the top of the stone, and it's probable that most of the carving was lost when this came away. The top of the stone was missing when photographed in 1911, but what remains of the cross has deteriorated since then.
In 1933 the stone was leaning at 45 degrees, but was subsequently set straight in concrete (there's no indication of who did this). The stone is 8 feet tall (that's nearly two and a half metres).