ARCHAEOLOGISTS who discovered the remains of what was once Wales' largest city have now unearthed a new mystery.

Stuart Wilson, of Monmouth Archaeological Society, who is leading the dig for the lost city of Trellech, discovered a block of disjointed stones, too thick to be a wall, with a purpose, as yet, unclear.

One of the stones was found with small and unusual markings, which may be significant.

"The stones we've uncovered so far were low-status buildings, and the owners would not have had much in the way of wealth," said 25-year-old Mr Wilson, of Agincourt Street, Monmouth.

"But despite the ground being very acidic, our latest discovery is a stone with three very unusual carved markings on it.

"At the present we don't know what this is or the relevance of the whole section, but it will be an exciting challenge to find out.

"Of course, the markings may well be caused by worms or insects over the years, but it is also possible that the stone may have been some sort of moulding."

Since the Argus reported the discovery last week, media interest in the 13th- century city, which lies on the Catbrook Road, just outside the modern village of Trellech, has been intense.

The lost city is believed to have been destroyed by fire more than 700 years ago, but before that catastrophe, with more than 400 buildings, it represented the largest populated area in Wales.

Mr Wilson said the four-week dig in a field above The Hostry cottage was uncovering building foundations that seemed to go deeper than first anticipated.

"The deeper a dig site is, the more likely you are to find preserved items, so we are optimistic," said Mr Wilson, who has a job toll-collecting at the Severn Bridge when he is not pursuing his hobby.

"There is no doubt this is the original Trellech and I have even bought the field opposite from the council so we can expand our dig to there."

The current project is due to finish on Sunday, when the findings will be covered and reburied until the society decides to return.