In the valley below the two hillforts in Drumelzier is supposed to be the site of Merlin's grave. RCAHMS puts it at NT13413453 (this is the hillfort side of the river), where Burnfoot Pool is marked. RCAHMS says:
"According to legend which is at least as old as the 15th century, the wizard Merlin was buried 200 yds NNW of Drumelzier Church, on the level haugh close to the right bank of the River Tweed. No structural remains are now to be seen, or have ever been recorded, at the place in question, but it is possible that the tradition may have been originated from the discovery of a Bronze Age cist.
RCAHMS 1967, visited 1956.
There is nothing to be seen at this site which lies in a field. The tradition still survives.
Visited by OS(IA) 11 August 1972."
No doubt while there's Merlin postcards to be sold, the tradition still survives. I also found this slightly confusing piece in 'Notes and Queries' for May 23rd, 1942:
Nearly fourteen hundred years ago Merlin (Myrddin Wyllt). the bard and prophet of the Strathclyde Britons, withdrew himself from an uncongenial world after the collapse of paganism at the battle of Ardderyd. The gateway "through which he departed was a whitethorn in full bloom at Drummelzier on the right bank of the Upper Tweed. We are able to fix the date of his disappearance satisfactorily, since the battle is recorded as having been fought in the year 575. A still-living tradition which I met with last year says that while Merlin lay entranced under the tree the spiders (fairies? or their emissaries?) gathered from all sides and bound him in their threads, so that he vanished from human eyes into the land of Faerie. But his spirit can still" be invoked and consulted at " Merlin's Thorn "—which must be a descendant of the original tree.
Something else on the confluence of rivers and Merlin's Grave at Drumelzier:
The rivulet of Powsail falls into the Tween a little below a small eminence called Merlin's Grave, near Drumelzier. Whether the prophet or wizard Merlin was buried here or not, Dr Penicuik, who notices both the grave and the rhyme, cannot certify. The following popular version of the rhyme [of Thomas the Rhymer?] is better than that which he has printed, and, I fear, improved:-
When Tweed and Powsail meet at Merlin's grave
Scotland and England that day ae king shall have.
Accordingly, it is said that, on the day of King James's coronation as monarch of Great Britain, there was such a flood in both the Tweed and the Powsail, that their waters met at Merlin's Grave. An ingenious friend remarks, though I cannot entirely go along with him, that the lines might be originally intended to attest the improbability of the two hostile kingdoms ever being united under one sovereign and as a means of keeping alive, at least in Scotland, the spirit of disunion. It will appear to modern scepticism that the rhyme was made after the event.
p29 of 'Select Writings of Robert Chambers' 1847. Online at Google Books.