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Saturday, 4 October 2014

Another mystery erratic from Abermawr




Another erratic which has dropped out of the Irish Sea till at Abermawr.  It's obviously very ancient and has a heavily weathered surface, but the striking thing about it is the presence of these large pinkish crystal agglomerations which are slightly reminiscent of the spotted dolerotes at Carn Meini and Carn Goedog.  The surface of the stone is pinkish brown, but when it is broken open the interior is seen to be very hard crystalline blue-grey.

Anybody prepared to hazard a guess as to where it might have come from?

POSTSCRIPT

 Looking through some old photos, I also found this -- an erratic found on Newport beach, in an exposure beneath the sand dunes.  Could it be from the same place?


Post-Postscript

Now we have another suggestion from one of our blogging community that maybe this is a porphyry.  Here is the idiot's guide to volcanic / igneous rocks:

http://www.nwnature.net/cam/science/rocks/web_rocks/

and this is what porphyry looks like. 


Well, not identical, but maybe in the right general area.  And there happens to be a source of porphyry on Lambay Island off the Irish coast near Dublin. I haven't been able to find a good photo of what this particular porphyry looks like, but it would of course make perfect sense for erratics from Lambay to be incorporated into the Irish Sea Glacier, transported southwards, and then dumped in glacial deposits on the North Pembrokeshire coast.







8 comments:

Myris of Alexandria said...

Again identifying rocks from photographs is for fools.
However it is not spotted dolerite.
The alteration spots are round, these are angular.
M

BRIAN JOHN said...

First photos, then hand specimens under a hand lens, and then thin sections. It all has to start somewhere, preferably in the hands of fools rather than charlatans.

Myris of Alexandria said...

It starts and ends with thin sections
The trouble with photos is they are deceptive.
Macroscopical identification is also fraught with difficulties, even experts are only correct 70 80 per cent.
To rephrase an opening line. No identification (provenance) is better than wrong identification (provenance).
Charlatans come in all shapes, sizes and persuasions. They can be recognised by always being correct
despite being corrected.
M

Myris of Alexandria said...

I was not suggesting asking for a photo identification is foolish but that anyone responding with a definitive answer is daft.
M

BRIAN JOHN said...

Thin sections are all very well if you are a petrologist intent on publishing a peer-reviewed paper on something or other. But generations of geologists have had to learn to identify various rock types from hand samples with a lens giving 8x magnification or whatever. The hammer and the acid bottle also come in handy. For example, in Pembs I'm reasonably confident that I can identify most lumps of rock and make a reasonably intelligent guess as to which part of the county they might have come from -- and in cases such as that, 100% accuracy does not actually matter. Sid, for example, knows his stuff from long experience! Horses for courses.

Myris of Alexandria said...

Were you a mycologist would you say the same thing.
I am trying to avoid getting colic.
We are not dealing with simple rocks
but the unusual and non distinctive.
As my old friend would say there is only one truth.
M

Myris of Alexandria said...

Mmmmmm, the first erratic could be a porphyritic.
Lambay Island has an axe quarry I think the only convincing one in the Irish Free State.
The Northern Six have the famous Rathlin Island and Tievebulliagh (spelling?)IPG group IX quarries.
In the British Isles Group IX is the largest group.
M
Maybe it is not an erratic but a lost traded roughout.I know that despite the main belief here, some axes were anthropogenically transported from their origin to there find spot.
M

Myris of Alexandria said...

Whoops their not there, written bct,
Before croissant time.
M