A distinctive Insular tradition of erecting monumental stone high crosses began by the 8th century, and possibly earlier. They probably followed earlier versions in wood, perhaps faced in metalwork. Some of these 'Celtic' crosses bear inscriptions in ogham. Standing crosses in Ireland and areas under Irish influence tend to be shorter and more massive than their Anglo-Saxon equivalents, which have mostly lost their headpieces, and therefore perhaps required the extra strength provided by the ring. Irish examples with a head in Celtic cross form include the Cross of Kells, Ardboe High Cross, the crosses at Monasterboice, and the Cross of the Scriptures, Clonmacnoise, as well as those in Scotland at Iona and the Kildalton Cross, which may be the earliest to survive in good condition. There are surviving free-standing crosses in Cornwall, including St Piran's cross at Perranporth, and Wales. Other stone crosses are found in the former Northumbria and Scotland, and further south in England, where they merge with the similar Anglo-Saxon cross making tradition, in the Ruthwell Cross for example. By about 1200 the initial wave of cross building came in to an end in Ireland.