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Been looking through my husband's slides. There is a stone museum at Margam (south wales). There's about half dozen flat rectangular domed stones similar to St.Pats, not proper celtic cross, they are dated 10th C, but probably earlier. Simple cross on front face mostly, but one has a wheel depicted with 6 spokes and nothing else. Its top is cowled very similar to the early romano/native altars found in this part of Somerset. I suspect both religons lived side by side, it would have been easy enough to walk out to a stone and say a prayer to whatever god you believed in. Why are there so many single standing stones in the Welsh landscape today? scratching stone, waymarker, or symbolic references to gods that once existed from the b/a onwards. just a thought...
Moss

The simple cross is a late motif when applied to Christianity. Crosses started off as Chi-Rho motifs and simply became simplified into what we know as a cross. The Christian symbol of the cross has nothing to do with the crucifixion, but originates in the Chi-Rho design.

Chi & Rho = the first two letters of Christ's name in Greek - Christos

http://www2.cumberlandcollege.edu/acad/rel/webpage/cross/crxr.htm

After looking like this the loop on the top of the Rho became a hook and then eventually disappeared altogether. Also the vertical shaft lengthened and the two diagonals moved closer together and became the horizontal bar.

http://www.ancientstoneart.co.uk/gallery/scot/s8echoid.htm

If you were to undertake a study of the early Christian cross slabs around Ireland you can very accurately date them by the design of the cross they carry.