OK - this may be a clue or it may not but we may as well look at all the evidence that we can muster.
I looked at the Old and New Statistical Accounts for Lochmaben but there was very little on the quarries - a ref to Corncockle and one other. The minister seemed to be more concerned with the agriculture and the Castle

However, I then went back to the baptisms of Alexander Jardine's and Jean Saunders' children and looked at the earlier one (Margaret) in 1793. This one gives their residence as
Bridgesmuir. This is still on the modern maps - to the North East of Lochmaben on the Nethercleugh burn.
Do any local people know if there are/were any quarries in that area?
Gadget
Added - just seen a Broomhill slack and a Broomhill quarry just NE of Lochmaben as well - slack -> slag, etc. Again, just throwing out some thoughts