The traditions of the country point to these lands as the site of an ancient battle-field. One of the workment present told us that a very old master, with whom he worked on that land when a young man, used to say that there had been foul work there a long time ago. The old farmer would threaten to call up the "old one" who was buried there.
From 'Tumulus at Berriew, Montgomeryshire', in Archaeologia Cambrensis, July 1857.
December the first, 1856, and a concerned Antiquary writes to the editor of Achaeologia Cambrensis:
During the autumn of this year the farmer in occupation of some land [...] in this neighbourhood has ploughed into a tumulus standing on his field; and, finding some upright slabs of stone standing within it, has decided on removing the tumulus altogether. He has, however, had the kindness to accede to his landlord's request that the tumulus may remain intact for a short time longer, until it can be examined by some member of our Association.
[...] The threatened destruction of this tumulus loosens one more link in the chain of historic associations that attach us all to the beautiful and romantic vale of the Severn. Why may not the old unsightly mound still remain? Why destroy, for the advantage of the passing moment, this monument that connects us of the present days with our fathers of bygone ages? The farmer still may have to plough round it instead of over it; the landlord may have to lower his rent one shilling per annum in consequence; but the historic dignity of the country will not be lowered [...]*
Fortunately, D Phillips Lewis stepped in, and in the following April he and his antiquarian chums dug into the remains of the mound. They found three large stones, two over 5 feet. He thought they were igneous 'trap' from Montgomery. They also found heaps of 'charred substances' which they believed were cremations. He concluded: "Mr Evans [the landlord] proposes covering the large stones up again, once he has lowered them into a position that will not interfere with the plough."**
Baza's interesting link tells the story of the site's reexcavation in 2006: many other prehistoric features were also confirmed in the vicinity. The site was built as a henge - the stones perhaps came a little later, and then finally were buried when the circle was turned into a burial mound.
* From a letter to Archaeologia Cambrensis, Jan. 1857, and
** 'Tumulus near Berriew, Montgomeryshire,' Arch. Camb. July 1857.