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Roman Britain was treated as a single province till Severus (who died in A.D. 211) divided it into two, called Lower and Upper Britain, Britannia Inferior and Britannia Superior,[1] so that henceforward the term Britannia came to be used not only for the island or even for Roman Britain, but also for portions of Roman Britain which was now known as Britanniae or the Britains. Dion Cassius[2] gives us to understand that the legions at Caerlleon on the Usk and Chester on the Dee, were in Upper Britain, while that located at York was in Lower Britain. As the Romans, like other people, allowed the ready test of running water to decide what was upper and what was lower, it is natural to suppose that Upper Britain was mainly that part of Roman Britain which the legions had to approach by marching in the direction of the sources of the Thames and of the streams which meet to form the Humber. When, however, Upper and Lower Britain came to be distinguished as provinces, the question of what was expedient would also play its part in the new arrangements. And as the territory north of Chester would go more conveniently both for geographical and military reasons with that north of the Humber, the whole of this district falling under the surveillance of the official who resided at York, which we know to have been in Lower Britain, it is in no way improbable that Upper Britain as a province
