Two Caer Brittons? Dumbarton and Bristow | Toponymy | Forum

I have some questions about your back yard.

Running "Caer Britton" in common search engines brings up references to two Caer Britton toponyms. Apparently, one morphed to Dumbarton via p-Celtic>q-Celtic translation; the other morphed via Brittones>Brito>Saxon approximation Bristow>Bristol. But were there ever two Caer Brittons, or do we only derive the "Caer" because the early accounts were written by p-Celtics?

The locations were once both strategically important Brythonic fortresses. Proto-Bristol was the critical link between Brythonic land/tribes from Cornwall to those of Wales, eventually severed by the Saxons at the Battle of Deorham. Proto-Dumbarton anchored the northward Brythonic extension of Strathclyde and Cumbric Brythons to its northern extremity, presumably the cairn marked border near the top of Loch Lomond (with Dal Riata land?).

Both were named by Romans as militarily important strong points of the "Brittones". The northern one, however, had its (I assume Goidelic) ancient descriptive name al Cluith, "The Rock", which persisted into Dark Age or later times. "Celtic culture: a historical encyclopedia", Volume 1, edit. John Koch, gives Caer Glud, Al Clut, Alo Cluathe as the seat of Ystrad Clud/Strathclyde, but also Dun Breatann as a later Irish naming. Clach nam Breatann, at the cairn marked border, is consonant, but of what antiquity? Dun Barton seems a possible conversion of Caer Britton via Dun Britton/Dun Brittones, but not of the older and persisting name, and less credible than from Dun Breatann. Linguistically though, is there a credible trajectory for Britton/Brittone to become Barton by p>q Celtic conversion?

Speculatively, one might also muse on the possibility that both were trading points for the Veniti, given Brythonic possession and similarly strategic locations from the sea trading perspective.

Finally, it's bugged me for years, whence, when and why the "m" in Dumbarton?