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Friday, 6 May 2016

Cambrian basal conglomerate boulder at Tafarn y Bwlch



There are a great many rock types that I do not recognize in the field, but one that I'm pretty secure about is the Cambrian basal conglomerate (Caerfai series) that outcrops in the area around Caerfai and St Non's in the St David's Peninsula.  I learned to use it as an erratic marker when I was doing my doctorate field research in 1962-65.  It's pretty spectacular, reddish or purple or pink in colour, with a vast range of coloured pebbles contained within it.  It's right at the base of the Cambrian series of rocks, overlying Pre-Cambrian volcanics.  There's nothing like it anywhere else in west Wales, and I think it's pretty unique in the British Isles too; there are other "basal conglomerates" in the Harlech area, Anglesey and other parts of North Wales, and in the Midlands, but I gather that they do not look the same........

Anyway, imagine my surprise when I came across a boulder of red Cambrian basal conglomerate today, at the base of an amazing new hedge bank being built by a very clever young Polish fellow at Tafarn y Bwlch, where a new Visitor Centre will be built over the coming months.  He's not sure where the boulder came from, but probably it came from the Pantgwyn Quarry near Moylgrove -- the source of the great bulk of the fluvioglacial cobbles and boulders brought into this site:


I think we can discount the idea that this boulder found its way from St David's to the northern flank of Mynydd Preseli either as ship's ballast or through some other form of human interference.  So it must have been transported by some natural agency or other -- and glacier ice has to be the most likely candidate.  But that means ice flowing from the SW towards the NE.  Hmmm -- quite a puzzle.  Must give the matter some more thought........

Could there be other outcrops of this rock far out in Cardigan Bay, or in St George's Channel?

In the meantime, if there is any geologist out there who knows one Cambrian basal conglomerate from another, please get in touch or post a comment!

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Postscript

I have found boulders of this conglomerate before, in the base of the old chapel at St Non's:

http://brian-mountainman.blogspot.co.uk/2014/04/st-nons-and-stonehenge.html

1 comment:

BRIAN JOHN said...

I have now looked at this boulder again, and I am not so sure it is a Cambrian basal conglomerate. I now think it might be from one of the ORS outcrops -- maybe from Ireland or Scotland.........