And in that posting I forgot the point of the whole comment - that the A5 divided the English-ruled land from the Danelaw, with its ample place name evidence for Scandinavian language input, so it should be expected to be an isogloss - that this legal entity of a boundary matches a soil boundary is curious, and in my view requires explanation - it doesn't show up on the usual, solid, geology maps, so I had missed it at first. Drift maps show the glacial stuff, which, certainly in Essex, just north of the Thames, shows up as a pronounced positive feature in the landscape. Solid maps show chalk in East Anglia - it's not always near enough the surface for roots to find it.

Penny