Here's a view of the coastal path near Aberdaron.
Just down in that cleft, beyond the bit of fence, there are some layers of gritty stuff fully of pebbles. I commented that it looked very soft for the rocks around here and G explained that it was just a glacial deposit. However, I reckon it could pass as a rock down in some southern coastal areas. :) The Lleyn rocks are, however, of much harder stuff, originally formed by volcanoes just like the Japanese ones that are active today.
Just off the coast lies Ynys Enlli or Bardsey Island to give it its English name. Supposedly 20,000 saints are buried there and it does lay claim to possibly being the real Avalon.