I have been occasionally reading about, and intermittently visiting, Hadrian's Wall for over fifty years - but I have never (until today) seen an even remotely plausible explanation for one of its most obvious features: the Vallum.
The Vallum is (or was) a wide, flat-bottomed ditch situated in straight lines at variable distances on the south side of the Wall - flanked by smallish mounds. There is another, v-shaped, classic defensive ditch on the north side of Hadrian's Wall: i.e. facing the potential enemy outside the Roman Empire.
So the Vallum is on the non-dangerous side of the wall, it is not the right shape for a defensive wall, and is all-but useless - nonetheless it is an enormous earthwork, about 70 miles long; which took a vast amount of labour, far more effort than would make sense if the Vallum was intended as merely some kind of boundary line.
So why on earth did the Romans build it - what did they hope to achieve?
Until just now, I have never seen an explanation of the Vallum which was even plausible - let alone potentially true.
But I have found one at last! - on the website of a chap called Geoff Carter, who seems to reside somewhere not far from me, and specializes in reverse-engineering archaeological phenomena.
I suggest that anyone interested should Read The Whole Thing in order not only to evaluate the evidence-cited; but to experience what strikes me as an exemplary way of reasoning about this kind of archaeological problem.
I found Carter's way of approaching the problem of the Vallum so impressive that - even before I had reached his conclusions - I felt confident that his answer would be coherent and plausible.
So what is Geoff Carter's explanation for the Vallum? It is that the Vallum, as we observe it, represents a partially-completed road: more exactly the foundation ditch for a road planned to extend along the whole length of Hadrian's Wall. The parallel mounds are simply spoil heaps from this extraction.
Carter says that the vastly ambitious plan for a road (which, if finished, like the Wall would have endured hundreds of years: the Romans built to last) was abandoned very soon after the Wall was finished, within about a decade.
After which brief timespan, the Vallum was often filled-in in places (presumably to allow easier crossing) and cut-across by later structures.
I find this a completely satisfying explanation for the Vallum - and am looking forward to returning again to the Wall this spring (when the ground is less sodden than at present) to re-explore some of the favourite places, with this new understanding in the back of my mind.