If the Wall was the barrier you could see, the vallum was the barrier you were not supposed to cross. This is a great flat-bottomed ditch, about six metres wide, with mounds of earth on either side, and it ran along the south of the Wall for most of its length, forming a kind of military zone behind the frontier. Nobody is quite sure of its purpose, though the usual theory is that it marked the rear boundary of the area and controlled the movement of people and goods. At Benwell, within Newcastle, the vallum has one of its best-known features, a stone causeway that allowed the road to cross the ditch near the fort gate, a neat piece of Roman engineering that came to light in the twentieth century.
The setting is suburban, with Victorian terraces and bus routes, and the Roman line is hidden beneath streets and gardens. Riding is a city affair, with traffic lights and one-way streets, so allow extra time, and avoid the rush hours. The route west along the river gives a gentler start, and within a few kilometres the city loosens into the valley of the Tyne, with fields and woodland. Rain falls often here, and the road surface in the city is patched and sometimes greasy.
Benwell is only a short ride from the centre of Newcastle, and Blaydon-on-Tyne, three kilometres west, has nearly fifteen thousand people. Fuel stations are plentiful. For a bite, the neighbouring streets hold plenty of cafes and takeaways, and the city's food scene is lively, from pie and peas to Chinese and Indian restaurants. A short ride away, the Quayside has bars and a view of the great bridges.
Walk the short stretch where the vallum survives, and you can see it as a hollow in the ground, edged by grassy banks. The effect is quiet and slightly eerie, like a scar on the landscape. It is not a thing that makes for a postcard, but it has a kind of stubborn dignity, and it reminds you that the Roman frontier was much more than a wall.