Caledonia was originally the region of the Great Glen in Highland Scotland inhabited by the Caledonii.
As such, in Classical usage Caledonia came to mean Scotland north of the Forth-Clyde isthmus. But in Welsh tradition - as is evidenced by the presence of Merlin at 1) Arthuret just north of Carlisle, 2) Drumelzier on the Tweed 3) the region near Glasgow, and 4) a mountain in the central Lowlands [see my The Mysteries of Avalon for a discussion of this last) - the Coed Celidon would appear to be at the heart of the Scottish Lowlands. It is generally accepted by scholars that this is indeed the location of the great wood in the Welsh sources.
We may be able to pinpoint the location of the Coed Celidon battle more precisely.
It is possible that a river-name in the area, believed to be a truly ancient hydronym, may have contributed to the idea in early Welsh tradition that Celidon lay in this part of the Scottish Lowlands.
Caddon Water, a tributary of the Tweed, has a Roman road. The etymology of Caddon (Keledenlee, 1175, Kaledene, 1296) is interesting.
From Alan James of BLITON:
Nicolaisen included Caddon Water among the *cal-eto- river-names. The final syllable is probably OE -denu added by Northumbrian English, though a secondary suffix isn't impossible. It is a very common hydronymic formation; *cal-eddoes indeed occur in ethnic names too ("hard men"), including that of the Calidonii.”
When I asked Mr. James whether this name could have contributed to the region thereabouts becoming known as the Celidon Wood, he responded:
“Well, yes, a name like *caleden could readily have attracted folk- or learned etymologising and dinnseannachas. I think it would have contracted to something like the modern form Caddon by the 15th ct, so I doubt whether such a thought would have arisen in the early modern period, when renewed interest in Tacitus etc., and even 'Nennius', gave rise to a good deal of fanciful etymologising.
But it's in an area with a good many P-Celtic pns, many of which I consider to be 'late' Cumbric, i.e. 10th-11th ct, when I think there was something of a revival/ reintroduction of the language in the upper Tweed/ Moorfoots/ Lauderdale area, and my hunch is that was the period when Arthurian and other (semi-) legendary associations were being attached to locations in that area, as in the Solway basin.
But I don't think the water-name would have been been given at that time, it's an 'ancient' hydronym that might have come to be associated with Caledonia because of the (accidental) similarity.”
There are remnants of a fort at Caddonlee by Caddonfoot
/details/caddonlee/). The famous Eildon Hills fort at the Roman period Trimontium on Dere Street is only a dozen or so kilometers to the east of the mouth of the Caddon. Several other hillforts are in the area and a Roman road went from Trimontium west along the Tweed to the Easter Happrew fort beyond Peebles.