About 480 to 360 million years ago, the ancient continents of Avalonia and Laurentia drove together in the Caledonian orogeny - the same mountain-building event that raised the highlands of Scotland, Scandinavia, and the Appalachians on the far side of the then-closing Iapetus Ocean. The Cheviots are what remains here. At the centre of the range an outcrop of early Devonian granite, the Cheviot Pluton, was forced up through the crust. Around it a halo of andesitic lava flows, tuffs, and agglomerates of the Cheviot Volcanic Formation records ancient eruptions. Igneous dykes radiate outward from the pluton like spokes. The Cheviot itself - 815 metres, the highest point in Northumberland - is a great rounded dome of weathered granite, and the rocks underneath ordinary moorland turf are the cooled remains of a volcano older than fish.

The northern Cheviots are pierced by five main valleys, each with its own character: College Valley, Harthope, Breamish, Bowmont, and Heatherhope. College Valley is the most famous, owned by an estate trust and accessible by car only on permit. Harthope rises toward The Cheviot itself. The Breamish carves a long green strath where Iron Age hillforts crown nearly every promontory. Bowmont feeds north into Scotland near Kirk Yetholm. Heatherhope sits high and remote. The southern Cheviots, by contrast, slope down to the River Coquet, and the northern Cheviots end above the Tweed. Between them, on the watershed, runs that Border Ridge fence. Walking from Byrness to Kirk Yetholm along it takes most hikers two days, and most hikers do not forget the second day.

These hills supplied the English language with one of its oldest ballads. The Battle of Otterburn was fought south of the Cheviots in 1388, when the Percy Earl of Northumberland clashed with the Scottish Earl of Douglas; a separate, almost certainly legendary battle - in which only 110 people were said to have survived - became the basis for The Ballad of Chevy Chase. The dispute may have started, the ballad suggests, with a hunting party that strayed across the border and was interpreted as an invasion. Two further battles followed within Cheviot reach: Homildon Hill near Wooler in 1402, where Percy defeated Douglas, and Hedgeley Moor near Powburn in 1464, fought in the Wars of the Roses. The hills look quiet now. They are layered with the bones of fights nobody can find the exact ground of any more.

Wooler, at the eastern foot, calls itself the Gateway to the Cheviots, and the title is accurate - the A697 runs through it, the visitors arrive there first. The hills are home to the Cheviot sheep breed, white-faced, hardy, the foundation stock of countless British and overseas flocks. They also support the Cheviot primitive feral goat, classified as a Native Breed at Risk, descended from herds that escaped from medieval farms and went wild on the highest ground. Most of the English side is mapped as open country under the Countryside and Rights of Way Act 2000; the Scottish side has equivalent access under the Land Reform Act 2003. The Border Ridge fence is just a fence. Everything underneath belongs to whoever walks it.