A variation on the theme of drystone walls, since we got home I’ve learnt this is a slate pillar fence, sort of a picket fence made of stone. They are found in only a small area of north Wales, apart from the odd escapee, which you can read about in the link. This was a particularly tidy example, others were a bit more ‘rough and ready’. They date from the mid 19thC and were made from poorer quality slate than is used from roofing slate.
Slabs of slate, driven into the ground, linked together with wire
Of course Wales is famous for its slate.
This type of walling is not as common as drystone walls, but there was still a fair number to be seen.