The vallum is the strangest part of Hadrian's Wall, and the one that nobody fully understands. It is a great flat-bottomed ditch, with a mound on each side, running along the south of the Wall for its whole length, and it was dug soon after the Wall was begun. The usual explanation is that it marked the back edge of the military zone, a boundary that civilians were not meant to cross except at controlled crossings, but nobody knows quite what it was for. Between Cockmount Hill and the village of Walltown, the vallum is clear and well preserved, a green trench in the field that you can follow with the eye for a long way.
Walltown Crags, a short walk north, are the sort of place that makes the Wall famous. The Whin Sill rises here as a line of crags, with the Wall running along the top, and the view from the highest point takes in the Tyne Gap, the Pennines and the Cheviots. A former quarry, now a country park, holds a wildflower meadow. Nearby Thirlwall Castle, a ruin from the fourteenth century built with stone from the Wall, recalls the Scottish raids that struck this country for centuries.
The land has a bare, big-sky look, with fields and moor stretching to the horizon. Lapwings, curlews and skylarks nest here in spring, and the wild flowers in the quarry attract butterflies in summer. The air is clean and cold, and clouds throw moving shadows across the hills.
Riding here is excellent. The roads climb from Greenhead in a series of steep bends to the ridge, and the lanes along the line of the Wall have long views and good surfaces. Traffic is moderate in summer, with walkers crossing at parking places, and low in winter. Haltwhistle has fuel, a supermarket and cafés, and Haydon Bridge and Brampton are farther. In winter, snow and ice can close the roads, so check the forecast.