The pre-Norman churches have some peculiarities. They are remarkable for the height of the walls, as compared with the width of the building. Thus at Monkwearmouth and Jarrow (erected by Benedict Biscop in the seventh century), the width of the nave is 18 feet, while the height of the walls is 30 feet. The carved lacertine figures of the porch at Monkwearmouth have likewise a Celtic character. Square towers at the west end of the nave form common features of these churches, and the jambs of the doors and windows are often inclining, like those of Ireland. Some of these features may be observed in one or two of our Scottish churches, such as that of St. Regulus at St. Andrews and Restenot Priory. As the Roman influence prevailed, that of the Columbans waned, till, finally, that of Rome was, after the Synod of Whitby in 664, definitely adopted, and the Columbans were driven off. After the expulsion of the Columbans from Northumbria, the Roman forms and observances were gradually extended over the southern parts of Scotland, then included in the dominions of Oswy, King of Northumbria. Various circumstances tended to aid this process. When the victory of the Picts at Dunichen, in 685, terminated the rule of the Angles in Scotland, Nectan, king of the Celtic kingdom, was brought into contact with the Roman missionaries, whom he found in his extended southern provinces, and became, in 710, a convert to their ideas. He seems to have warmly espoused their cause, and desired that their rules and forms should be universally adopted throughout his kingdom. But the Columbans still clinging to their own observances, King Nectan at length, in 717, issued a decree, expelling from his dominions all ecclesiastics who refused to conform to the Roman practices. Up to this period there had been an increasing tendency to asceticism in the Columban Church, leading the monks to forsake the cœnobitical or monastic life in common, and to adopt that of the hermit or Anchorite. This had the effect of breaking up the monastic system which had hitherto succeeded so well amongst the Celtic tribes of Ireland and Scotland, and also tended to encourage the introduction of the secular hierarchy of the Roman system.[30] The hermits were known on the Continent as Deicolæ, or Worshippers of God, and in this country by the title of Keledei or Culdees. The similar order which arose in the Celtic Church afterwards played an important part in Scottish ecclesiastical matters. They first appear in Scotland after the expulsion of the Columbans--the establishment of St. Serf on an island in Lochleven being of this school.