Charterhouse-on-Mendip has had the most impressive of advocates, from the time of Smith & Rivet’s The Place-Names of Roman Britain (1979). But it has only ever been a tentative identification and doesn’t seem to fit the meagre scraps of information we have too well. Yes, ‘Charterhouse Roman Town’ was important as a lead-mining centre but:
1. The Roman name has been reconstructed as ‘Vebriacum’, because the letters VEB are found on a number of lead ingots found nearby and farther afield. I’m not sure why Vebriacum is the supposed form: in France a, probably pre-Roman, toponym was Latinised (and is attested) as Vebritum giving rise to Vebret (Cantal) and other toponyms suggest a Celtic Veb- origin. If Celtic and Roman forms were Veb-, why did Ptolemy call it Iscalis?
2. Isca- strongly suggests a name based on ‘isca’ – British ‘water’, perhaps on a river which the Celts called Isca; but Charterhouse isn’t on any significant bit of water at all. The nearest river flowed through Cheddar, the old River Axe (now the Cheddar Yeo), a couple of miles away. Cheddar had a small Roman port and the Axe would have been close enough to be used for transporting lead, but surely not close enough to Charterhouse for it to be characterised as ‘the place by the water’. That would be more likely to be Cheddar itself.
3. Charterhouse was an important centre for the Romans, but if Iscalis was a polis of the Belgae, as Ptolemy said, we are really looking for a place with significant Iron Age archaeological remains. Although there are signs that lead was mined at Charterhouse during the Iron Age, it was the Romans who developed it into a sizable industry. There was an Iron Age hillfort to the east of the village, but I can’t find anything to suggest it had special importance.
4. (And least likely to be of any importance at all) My reconstruction of the various Ptolemaic coordinates gave this:
It’s a bit rough and has obvious drawbacks (Iscalis wouldn’t have been in the middle of the Severn estuary for one thing); but allowing for triple inaccuracies, Ptolemy’s, the transcriber’s and mine – an inexactitude of inaccuracies – it does suggest something. Of the 10 crossing points, 9 are closer to the coastal possibilities near the mouth of the Axe or the mouth of the Parrett, real ‘watery places’. Against that, the one inland point is where the alignments London-Bath-Iscalis and Canterbury-Bath-Iscalis meet. I might suppose that London, Canterbury and Bath would be more likely to be accurately located than other towns around the country.
But, on the whole, I’m going to rule out Charterhouse as being not important enough to the Belgae and too far from any water. Also that the evidence seems to be that it was called something else.