The evidence comes from royal writs addressed to Siward as earl in these shires. Siward's predecessors as earl in these areas were other Scandinavians, Thuri and Bjorn son of Earl Ulf; the former was styled "earl of the Midlanders" ("comes mediterraneorum"), showing that this earldom represented the earlier polity of the Middle Angles of Mercia. It was this area, rather than Northumbria, to which Siward's descendants were most attached. Likewise, it has been argued that Siward brought Cumberland, thought by some historians to have been lost to Strathclyde, back under Northumbrian lordship. The evidence comes from a document known to historians as "Gospatric's Writ". This is a written instruction, issued either by the future Gospatric, Earl of Northumbria, or Gospatric, son of Earl Uhtred, that was addressed to all Gospatric's kindred and to the notables dwelling in the "all the lands that were Cumbrian" ("on eallun þam landann þeo Cōmbres"); it ordered that one Thorfinn mac Thore be free in all things ("þ Thorfynn mac Thore beo swa freo in eallan ðynges") in Allerdale, and that no man is to break the peace which was given by Gospatric and Earl Siward. Historians such as Charles Phythian-Adams believed that such phraseology indicated that Siward conquered the region from its previous rulers, although others, like William Kapelle, believed that the region had come, were it ever lost, back into English power before Siward's time. A little can be said about Siward's relations with the Northumbrian church, in particular with regard to his relations with Durham. As a result of Siward's marriage to Ælfflæd, Siward gained possession of a group of estates in Teesside claimed by the bishops of Durham. Acquisition of these estates might have brought opposition from the Bishop of Durham, but Æthelric the incumbent had been expelled by the clergy of Durham in either 1045 or 1046 and, according to the "Libellus de Exordio", only returned by bribing Siward. According to the "Libellus", the clergy were "terrified and overwhelmed by the fearful power of the earl" and "were compelled willy nilly to be reconciled to the bishop, and to admit him into his episcopal see".