The fort here with its double rampart is high above where three streams meet. Perhaps its occupants thought that was something important, more than just for its defensive potential. Later an Abbey was built in the valley, so it obviously suited Christian sensibilities. You can't help wondering how long before that the following spring was revered:
At some distance from the church, in a woody nook, issues a spring named St Bathan's well, which, according to the superstition of ancient times, had the power of healing diseases; and which still, as is the belief in the neighbourhood, neither fogs nor freeze, and even prevents a mill-lead into which it flows from being locked up with ice in the winter.
From the Abbey St Bathans section of the 1830s Statistical Account.
On more than twenty different spots of this moor were large cairns, in many of which fine yellow dust, and in one of which an old spear head, was found. Two unhewn massive stones still stand, about 100 yards distant from each other, which doubtless are monuments of the dead. The real tradition simply bears that here a deadly feud was settled by dint of arms: the upright stones mark the place where the two lords or leaders fell, and the bodies of their followers were thrown into a marshy pool called the Dead Lake, in the adjoining haugh. It is probable that this is the locality of "the Dowie Dens of Yarrow."
About 300 yards westward, when the cultivation of this moor began, the plough struck upon a large flat stone of unhewn greywacke bearing a Latin inscription. Bones and ashes lay beneath it, and on every side the surface presented verdant patches of grass. It was examined by Sir Walter Scott, Dr John Leyden, Mungo Park, and others of antiquarian lore. From the rudeness and indistinctness of the carving upon the hard rock, only the following characters can be deciphered--
"Hic memoria et... hic jacent in tumulo duo filii liberali."
It's slightly curious that the RCAHMS records don't give the latter Yarrow Stone the benefit of the doubt when it comes to a possible prehistoric origin. From 'Reminiscences of Yarrow' by James Russell (2nd edition, 1894).
Another giant-related location in the area was the Giant's Grave. The Canmore record says it was a cairn removed at the start of the 19th century, and there's nothing more to be seen. They say it was supposed to be at NT 0925 2410.
Over against the foot of Hawkshaw-Burn in a Kairn beside the High road is the Giants Grave, so called from a huge and mighty Fellow, that robbed all on the way, but was at length from a Mount in the over side of the River supprised and shor to Death as Tradition goes.
There is a curious mound by the side of the Blackadder, on the north of the stream, called "the King's Grave," which may be a natural eminence, or may have been formed by the debris of a rush of water through a ravine nearly opposite to it, but which certainly has the appearance of having been stirred - dug into - on a part of its extent, the tradition connected with which, seems to carry the mind back to the same stern times [of the sixth century].
The residence, according to this old tale, of a British Chief was surprised by Saxon assailants in his absence, and all who belonged to him were murdered or carried away, with the exception of one infant child who was a twin, who happened to have been carried out at the time in the arms of his nurse, and was by her concealed and preserved.
Many years afterwards this British Chief met a Saxon army, and the place of meeting must have been some where near to these lines. It was proposed by the Saxon leader and agreed to, that the matter in dispute between them should be decided by combat, one champion being chosen from each army. The Saxon champion was the Briton's stolen son, whose life had been spared by his enemies when they put to death the other members of his family who were in their power. It was his twin brother who represented the British host - and the two kinsmen both fell - mutually slain, and lie buried, as the tradition which I seek to give says, under the large and contiguous cairns on the "Twinlaw," a prominent eminence of the Lammermuir range, a few miles to the west. The armies having afterwards engaged in battle on the southern descent of the Lammermoors, near to Wedderlie, the British Chief was himself either mortally wounded or slain in the action, and, on the route of his dispirited army, was interred in that lonely mound by the Blackadder.
It's not very convincing is it? If I'd met my long lost twin I don't think I'd set to murdering him. But that seems to be the story. There is a ballad about it too, which you may read some of in the rest of the article by the Rev. John Walker, here: http://www.biodiversitylibrary.org/item/110629#page/124
in volume 2 of the History of the Berwickshire Naturalists' Club (1864). There's also a version in vol 30 http://www.biodiversitylibrary.org/item/110767#page/326/mode/1up
Local legend connects the stone with the burial place of Merlin the Wizard. Merlin was said to be buried 183m NNW of Drumelzier Church, close to the right bank of the River Tweed. There are no structural remains and none have ever been recorded at the place in question, but it is possible that the tradition may have been originated from the unrecorded descovery of a Bronze Age cist in the area, which links to the Drumelzier Stone.