"Caldwell is an ancient Scottish name that was first used by the Strathclyde-Briton people of the Scottish/English Borderlands. It is a name for someone who lived in Renfrewshire."

http://www.houseofnames.com/xq/asp.c/qx/caldwell-coat-arms.htm

So says a heraldry site!

This is the first support I have seen from an independent source to the effect that Caldwell is a locality name in a general sense rather than a locality name pointing to a specific location.

I don't know where the reasning has come from:

My interpretation is "Gald-wallys" or Lowland Welsh which would allow thta there could be Caldwell's in any other part of Strathclyde when surnames were first adopted.

By this time Cumbria was lost, Galloway included Carrick, but Carrick was just being detached incorporated into Ayrshire as its Southernmost District and might accordingly have some new Strathclyde settlers bringing their new surnames with them. Strathclyde didn't reach to Argyll but would have had some influence inot the Lothians. I cannot answer for the lack of Caldwell's in Annandale under this theory, but perhaps it was under the influence of Galloway at the time or settled by English.

We might be just a tribe of Lowland Britons.

Tom