Wrong about the stone, and the right answer is better than mine
What I said
That the stone came from the quarry six miles north, on the basis of colour and the fact that everybody says so.
Why that is wrong
The fossil content is wrong for that bed by about forty million years. That is not a marginal error and colour is a useless indicator.
What is actually true
It is almost certainly reused Roman material, which would explain three other things I had noticed and not connected.
