The vallum is the strangest part of Hadrian's Wall, and a good place to see it is on the flat ground near Bowness-on-Solway. It is a broad, deep ditch with a flat bottom, flanked by two mounds of earth set back from the edge, and it ran along the south side of the wall for its whole length. Nobody is certain what it was for, but the best theory is that it marked the rear boundary of a military zone, a line that civilians could cross only at controlled points, which fits with the idea of the wall as a border of customs and security as well as defence.
Here at the western end the wall reached the Solway Firth at Bowness, where the Romans had a fort called Maia, and then the frontier did not stop. A chain of small forts and towers continued south along the Cumbrian coast, watching the sea for raiders from Scotland and Ireland, a reminder that the Romans were as worried about the sea as the land. The estuary itself, with its wide mud flats, tidal bore and wading birds, forms part of a coastal conservation area, and Bowness Common behind it is a rare raised bog.
The landscape is flat and open, a green plain edged by the grey firth and the blue hills of Scotland beyond, and the weather can be dramatic. The Solway tides are among the fastest in Britain, and the old sailors' saying is that they can come in faster than a galloping horse. The air is salty, birds fill the sky, and in winter huge flocks of barnacle geese arrive from Svalbard to feed on the marshes, a spectacle that draws birdwatchers from all over.
Riding on the lanes of the Solway plain is easy, with long straights, gentle bends and light traffic, though the wind can be fierce and cold. Surfaces are generally good, but watch for mud and slurry on farm roads and for sheep and cattle. The roads are mostly flat, so it is a relaxed place to ride, and the sunsets across the firth are glorious.
Villages on the coast have a pub and a shop at most, so fill up before leaving the main road. Carlisle and Annan are the larger towns, and Annan lies across the firth, so for fuel go to Carlisle or Silloth. Fish and chips, followed by a walk along the shore, makes a good evening.