cathedrals and large abbeys follow the two basic forms found in France, that with paired towers as at Southwell Minster and that with framing turrets as at Rochester Cathedral. Portals are usually arched and decorated with chevrons and other geometric ornament, barbaric faces and spirals. There are a few carved Romanesque tympanums, with a Christ in Majesty at Rochester Cathedral. The ornamentation of portals in Ireland have distinctive elements of Celtic design as at the gabled portal of Clonfert Cathedral. Side porches are common and are often the usual mode of entrance, the western portal only being opened for major festivals. Blind arcading is used as a major decorative feature, often around internal walls. Windows are comparatively large and may be arranged in tiers as in the transepts of Peterborough Cathedral. Paired windows occur in towers. Naves of cathedrals and abbey churches are of great length, and transepts are of strong projection. Chancels of cathedrals and abbey churches are also very long. The chancels of cathedrals and abbeys were round and with an ambulatory in the French manner, as indicated at Peterborough and Norwich Cathedrals but none have survived unchanged. Large central towers are characteristic, as at Tewkesbury Abbey and Norwich Cathedral. Many round towers occur in Ireland. They are also found in Saxon (Pre-Romanesque) architecture in England as stair towers attached to larger towers of square plan. The nave rises in three stages, arcade, gallery and clerestory. The arcade has two forms: arches resting on large cylindrical masonry columns as at Gloucester and Hereford Cathedrals, and arches springing from composite piers as at Peterborough and Ely Cathedrals. Durham Cathedral has alternating piers and columns. Crypts are groin vaulted, as at Canterbury Cathedral. Nearly every large Norman church has a later, Gothic high vault, except at Peterborough and Ely Cathedrals which have retained trussed wooden ceilings. The vaults at Durham are of unique importance, that of the south aisle being the oldest ribbed vault in the world, and that of the nave being the earliest pointed ribbed vault in the world. Ribbed vaults of the Norman period exist over the aisles at Peterborough Cathedral and other large churches. Barrel vaults are rare, examples being St John's Chapel, Tower of London and several 12th century monastic churches in Ireland including Cormac's Chapel and St Flannan's oratory. Notable examples Durham Cathedral, England Peterborough Cathedral, England Ely Cathedral, England Southwell Cathedral, England Rochester