road South out of Swindon - which crosses over the M4, main artery of the "Golden Corridor" - is, unmistakeably, the A361.
However, the signs say it's the A4361. Someone, somewhere has screwed around with Destiny.
At the top of the hill, opposite Wroughton aerodrome, I stopped in a lay-by to take in the view of the Vale of the White Horse, with Swindon sprawling across its floor. I was joined by another sloucher. He turned out to be a retired lorry driver. This road had been one of his regular routes.
He told me that, in those days, this bit of road was known as the B361. Exactly when (or why) it got this promotion to "A" status (with associated number change) - I don't know.
I'm going to ignore it. Until further notice, read all references to A361 as A4361 (or vice versa). After Avebury, when it crosses the A4, our road gets its proper number back.
The land rises in front of us. We're about to climb up on to the Marlborough Downs. Here, a soft scarp of high ground overlooks the Vale of the White Horse. We're heading for a gap in this scarp.
Along the edge of that scarp runs the Ridgeway - an ancient pathway linking Northern Hampshire, through Wiltshire and Berkshire to the Eastern parts of England. "Ridgeway" is a modern name for a string of ancient paths which were used for millennia - moving animals and goods along the drier, firmer ridges, away from the forested, muddy, dangerous valleys.
Today it's a major leisure resource for walkers, but once, it would have been at least as important, economically, as the motorway which parallels it.
It also played an important part in the early history of this country, particularly at this point, where the Ridgeway takes a sharp turn South, to run parallel with our road.
Inside this right-angled turn is Barbury Castle.