I have just had a sudden thought in relation to this post:
and the help given by Rob in determining what this rock might be and where it might have come from.
Rob kindly said this about it, having looked at the sample:
The strange rock found on the shore of the estuary, across the river from the Parrog, Newport. Felsite, maybe from North Wales? Not certain. Siliceous igneous rock with feldspar megacrysts and a sub-spheroidal fabric. Crystal tuff??
I have had an inspiration that it looks very similar to the rocks outcropping in the highest part of Tycanol Wood, beneath the tors of Carnedd Meibion Owen. Here is one of my photos from that site:
There do seem to be similarities in colour, texture, and that strange sinuous / knotted / gnarled surface texture as well. There are a lot of strange rocks between Carnedd Meibion Owen and Newport on the coast -- and maybe other outcrops of something similar.
I accept Rob's point that you can't do any sensible petrology from photos, so all we can do is speculate in an amateurish sort of way. Crystal tuff? Felsite? Richard Bevins knows the rocks from this area very well -- Richard, if you see this, any comments please?
PS. Message from Rob: he suggests that Richard is of the view that this might be a microtonalite. Or maybe several microtonalites -- can't say whether they come from the same outcrop, obviously, without very detailed analysis.........
By the way, has anybody yet identified that strange erratic called the Trefael Stone?