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Wednesday, 21 January 2015

Erratic pebbles - Abermawr and Flat Holm


Since we are on about erratic behaviour just now...........here we have a nice little collection of small pebbles (up to 3 cms in length) from Abermawr and other North Pembs beaches.  These are on the Cardigan Bay coast.  We can assume that there is nothing here from Ramsey Island and the outer end of St Davids Peninsula -- although one or two do look similar to some of the pebbles I saw yesterday on the beach near Sleek Stone.  Note that these pebbles are wet......



 This is a photo of the beach on the east side of Flat Holm -- mostly limestones and other Carboniferous rocks, but if you click to enlarge, you'll see some igneous material in there too.....


 And this is the collection on the windowsill of the farmhouse on Flat Holm -- collected by Linda and others.  Varied and colourful, as as we might expect, from a wide variety of sources.  Sid Howells staggered off the island back in October with a much more comprehensive collection in his rucksack -- and we look forward in due course to seeing what treasures might be included.

We might expect a collection of pebbles from the Croyde - Saunton area to be similar -- although of course we would expect to find some pebbles exclusive to North Devon.














15 comments:

chris johnson said...

The coast in Pembrokeshire is truly fascinating. I love to visit Freshwater West where there is a spectacular variety of coloured stones - if I was younger I might be inspired to become a geologist, or even a geomorphologist.

As a believer myself that stone agers valued stones for their aesthetic qualities, it is almost proof positive for glaciation theories that no Freshwater West stones are found at Stonehenge/Avebury to my knowledge.

TonyH said...

THAT REMINDS ME:
I must go down to the seas again
To the lonely seas and sky....

Myris of Alexandria said...

There is no doubt but that the Neolithic loved coloured stone,green and blue especially.
There is even much mileage in believing that the coloured ores of copper used as beads were the precursor to the earliest copper smelting.
"Vanity not neccesity,is the mother of invention" to quote an up and coming book review.
M

BRIAN JOHN said...

Ah -- new departure for you, Myris? before we know it, you'll be covering the London Fashion Week. We look forward to receiving your reports.......

Myris of Alexandria said...

It is often held in the Natural History museum.
No just a run of the mill book review. The review not the book although I have reviewed too many of those.
Libel forbids me to name names.
M

Myris of Alexandria said...

Fresh erratic news.
A suggested porphyry cobble from the Irish sea till found at Abermawr last year is now known to be an arkosic litharenite,
coarse sandstone with lots of feldspar and rock fragments.
It did look rather porphyritic in hand specimen but what is the old mantra.The rock fragments look FVS. But the big feldpars are more difficult to explain??
The possible tuff erratic found at CRyf is a bit of local tuff.CRyf is surrounded by microtonalite and by tuffaceous rocks.
Macroscopical identification of rocks is a mug's game.
Channelled from DrIxer.
M

BRIAN JOHN said...

Thanks for that news, Myris. So that fragment from Abermawr is not igneous, but sedimentary....... so it's a sort of coarse conglomerate? we had the same dilemma when trying to work out whether that strange boulser found on Flat Holm was igneous or sedimentary -- I recall Chris being of the opinion that it might have come from one of the sedimentary sequences of the South Wales Coalfield, or maybe the Brecon Beacons.

And the tuff from Rhosyfelin is likely to be fairly local. There are certainly lots of dolerite boulders there too -- I wonder where they came from?

Myris of Alexandria said...

Yes a conglomerate BUT the matrix is low grade metamorphic, so not ORS I think and certainly not Carb. In age.
I don't know the Welsh sediments microscopically and in the field it has always rained and we concentrated
on grauwacke sequences.
My GUESS is something lower Palaeozoic, nothing like the Stonehenge LP sst.
We had something very similar in the earlier batch,or from the cup marked stone.
Oh shall check!!!!
M

TonyH said...

Myris/Brian

So do we have any broad idea where that Abermawr sedimentary cobble erratic's provenance was?

BRIAN JOHN said...

Actually nothing would surprise meat Abermawr. One of the commonest types of erratic found on the beach there is Ailsa Craig riebeckite -- from the famous old crag in the Firth of Clyde -- where they get all the curling stones from.....

Myris of Alexandria said...

No it has abundant non-detrital magnetite, this is very unusual.
Trace amounts of gold are associated with magnetite in meta-sediments in the Wicklow Mountains.
I have no idea if there are similar rocks in Wales.
M

BRIAN JOHN said...

Thanks Myris! Very interesting. Gold?!! Tell me more.......!!

Actually it would not be surprising to find material from the Wicklows in North Pembs -- or even Somerset, for that matter. Remember that they found some Ulster "white limestone" in one of the till exposures near Bristol.

BRIAN JOHN said...

Can't remember offhand which erratic the sample came from -- was it this one?
http://brian-mountainman.blogspot.co.uk/2014/10/another-mystery-erratic-from-abermawr.html

Myris of Alexandria said...

Yes it is.
It not a porphyry but a conglomerate.
M

BRIAN JOHN said...

Thanks Myris -- thought as much.... and always things to learn!