Bede's Ecclesiastical History of England, ed. by A.M. Sellar, , at sacred-texts.com
In the year of our Lord 189, Severus, an African, born at Leptis, in the province of Tripolis, became emperor. He was the seventeenth from Augustus; and reigned seventeen years. Being naturally of a harsh disposition, and engaged in many wars, he governed the state vigorously, but with much trouble. Having been victorious in all the grievous civil wars which happened in his time, he was drawn into Britain by the revolt of almost all the confederated tribes; and, after many great and severe battles, he thought fit to divide that part of the island, which he had recovered, from the other unconquered nations, not with a wall, as some imagine, but with a rampart. For a wall is made of stones, but a rampart, with which camps are fortified to repel the assaults of enemies, is made of sods, cut out of the earth, and raised high above the ground, like a wall, having in front of it the trench whence the sods were taken, with strong stakes of wood fixed above it. Thus Severus drew a great trench and strong rampart, fortified with several towers, from sea to sea. And there, at York, he fell sick afterwards and died, leaving two sons, Bassianus and Geta; of whom Geta died, adjudged an enemy of the State; but Bassianus, having taken the surname of Antonius, obtained the empire.