one inevitable division between north and south. Another is provided by underlying geological features which have placed a large tract of high, barren moor land and hilly country - commonly though incorrectly called 'the Cheviots' - between the Solway Firth and a point only fifteen miles up the River Tweed from the North Sea4. Even although those historians who have seen this as some kind of political 'no man's land' are in error - for the line of the Border was always pretty clearly understood in medieval times, and even demarcated in places - nevertheless the tract of inhospitable wilderness formed a geographical 'no man's land' which must often have discouraged one side or the other from sustained aggression. The existence of the Cheviot barrier, averaging around 400 m. above sea level, meant that English aggressors would find it necessary to push as far north as the Firth of Forth to fix a secure and worthwhile frontier, while Scottish aggressors, correspondingly, would wish to incorporate the northern English river systems of Eden and Tyne. For much of the medieval period it was beyond the resources of either the English or Scottish kings to hold such extended frontiers for more than a few years together. There were thus powerful considerations working in favour of the Solway-Esk-Cheviot-Tweed alignment in the formative period when the medieval kingdoms of England and Scotland were taking shape. It was of decisive significance for the location and structure of the Borders that these historic kingdoms grew out of, respectively, a southern (i.e. West Saxon) and a northern (i.e. Scoto-Pictish) kingdom. Only thus can we understand how a region which otherwise possessed a high degree of unity - Northumbria on the eastern side and Cumbria on the western side - came to be divided across the middle5. The unity of at least the northern half of Northumbria (what had been the ancient kingdom of Bemicia) was still recognized well into the thirteenth century before the king of Scots, by the Treaty of York of 1237, was persuaded to abandon finally his ancestral claims to the four northernmost counties of England6. Of these, the two lying east of the Pennine watershed, Northumberland and Durham, formed essential components of Bemicia. The western pair, Cumberland and Westmorland, were equally integral parts of the ancient kingdom of Cumbria or Strathclyde. Even after Scottish royal claims upon them were abandoned in 1237 the bishops of Glasgow could still declare publicly and formally that the southern limit of their diocese was marked by the Rey or Rere Cross on Stainmore Common, the boundary dividing Westmorland from Yorkshire, thus effectively 4 Geological Map of the United Kingdom, North (Institute of Geological Sciences, 3rd edition solid, 1979). 5 Blair, P. Hunter, "The Bemicians and their northern frontier", in: Studies in Early British History, ed. Chadwick, N. (Cambridge, 1954); Kirby, D.P., "Strathclyde and Cumbria: a survey of historical development to 1092", in: Transactions of the Cumberland and Westmorland Antiquarian and Archaeological Society, New Series, 62 (1962). For the general background, see Barrow, G.W.S., The Kingdom of the Scots (1973), 139-61 and Barrow, G.W.S., "Frontier and Settlement: which influenced which? England and Scotland^ 100-1300", in: Medieval Frontier Societies, ed. Bartlett, R. and Mackay, A. (Oxford, 1989), 2-21. 6 Stones, E.L.G., Anglo-Scottish Relations, 1174-1328. Some Selected Documents (Oxford, 1970), 38- 53. 199