Porth Meudwy, Llyn Peninsula -- an interesting erratic

 

Porth Meudwy -- the departure point for the boats to Bardsey Island

The interesting erratic, next to what looks like a chunk of concrete -- probably related to the slipway.

Thanks to Andrew for drawing attention to this erratic on the beach at Porth Meudwy--- and thanks for the photos. Andrew thought it may be a sandstone erratic, but on seeing his close-up shots I thought it was probably igneous. This is confirmed by two senior geologists whose opinions I value. One thought it was probably igneous because "it contains what appear to be euhedral phenocrysts that are probably feldspars."  The other thought it was probably a "porphyritic basalt, heavily altered. Rotted, prob originally zoned feldspars (plagioclase) set in heavily altered but dark, mafic-looking groundmass. Phenocrysts quite obvious (cream/brown, more or less euhedral) with cores most altered (rotted out)."

I thank them for their opinions -- qualified of course by the fact that they have not seen actual samples of the rock or thin sections.........

Anyway, it looks as if the rock has nothing to do with the Altar Stone!  Where has it come from?  Difficult to say --I don't know the geology of North Wales, but Anglesey?  Maybe further afield, in Galloway, Scotland?  Or even the inner Hebrides? 

Three of Andrew's close-up shots of the surface of the erratic boulder