The Cheviots are very old. Looking at their rolling outlines today it’s hard to think of them as volcanoes, but some 400 million years ago two continental plates, one carrying what is now Scotland and the other England, collided. In doing so they closed up the body of water that had separated them.
The collision may have lasted for 20 million years and resulted in violent volcanic activity, followed by massive outpourings of a sticky lava consisting mainly of andesite. Then there was the intrusion of a large granitic magma plug which crystallised and now forms the Cheviot itself.
To the north of the Cheviots the Scottish Southern Uplands were formed from oceanic sediments piling up on the edge of the Scottish continental plate as the collision with England progressed. Similar rocks from this time can be seen in the bed of the River Coquet near Makendon, a mile or so downstream from its source.
After the vulcanism, faulting, erosion and deposition caused further changes. The Breamish and Harthope valleys, for example, are the sites of fault lines, while the agricultural land to the north and south of the hills comes from erosional deposition. The fell sandstone of the Simonside Hills in Northumberland is the result of thick fluvial deposits of sand.