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Saturday, 9 January 2016

Ramsey Island Geology

When we think about erratic transport routes etc, Ramsey Island often comes into the conversation.  With good reason, since there are some quite interesting rock outcrops there.......

Previous post:

If you want to check on other discussions, you can find them on this blog just by entering "Sleek Stone" or "Flat Holm" or "Ramsey Island" into the search box.

The above map and key come from the excellent -- but rather specialised -- British Regional Geology volume for Wales.  The igneous rocks here are approx the same age (around 463 million years old) as those belonging to the Fishguard Volcanic series (Llanvirn: Ordovician) on the northern flank of Mynydd Preseli.  Of particular interest are the igneous tocks from the southern part of the island -- including porphyritic rhyolites, turbiditic tuffs, ash fall tuffs, ash flow tuffs, and brecciated lavas.  Then there is a big microtonalite mass on the NW flank of the island.

For many years it has been known that these rock outcrops have provided entrained material for the overriding Irish Sea Glacier -- on at least two occasions -- and that erratics from Ramsey are widespread.  The super-erratics near the Sleek Stone, Broad Haven, have almost certainly come from Ramsey, and from our 2014 trip to Flat Holm we have strong suspicions that there is a lot of Ramsey material there too.  Sid Howells is hoping to take a careful look at the collected samples this spring -- so watch this space!

 A fabulous "Visit Wales" image of Ramsey Island -- view from the SE.  The igneous rocks are concentrated in the southern (nearest) part of the island, and on the west (left) side of the island, where the hills of Carnllundain and Carnysgubor are located.  Click to enlarge.


Myris said...

MMMMMMMMMMMM porphyritic rhyolite that could describe the lithology of SH48 an orthostat, proved not to come from the Cryf quarry site, I wonder if there be a quarry site on Ramsey Island. Mystical place, easy access on the sea route to Salisbury and less smelly than Milford Haven.
Or am I being naughty.
Incidentally can be have no more talk of ignmbrites. There are no ignimbrites associated with anything seen at Stonehenge. I think this is the fifth/sixth time I have had to correct this highly dateable error. Copying lists of rock names (flagstone!!!!!! is another, surprised you have not been en-trap-ped also) and adding them up to make 30/40 is as useful as having a full-greased fry-up to cure a hangover. The answer of course is stay sober (and enjoy the fry-up).

BRIAN JOHN said...

Ramsey was probably populated by a strange and fearsome race of quarrymen who dug up large interesting stones to sell to passing Neolithic sailors who had lost their monolith cargoes while going round Strumble Head. They probably also charged tolls for travelling monolith traders who passed through Ramsey Sound, and brought curses down on the heads of those who refused to pay tributes. They died out eventually because all their trade disappeared when the A40 road was built, allowing Stonehenge bluestones to be transported at much lower cost via Carmarthen, Brecon and Abergavenny.

What's all this about ignimbrites? Haven't mentioned them in many months, as far as I can remember......

Myris of Alexandria said...

Ah Alzheimer's kicking in. Your post of 5th January 2016!!!!!!!!!!! mentions ignimbrites and flagstones and other incorrect, subsumed, old-fashioned terms (we were lucky not to be en-trap-ped by yet more)that make up your pet. lithics-list.
No one, who knows, has mentioned ignimbrite in a South Wales context for 25/30 years and the last users the OU, despite having two very good petrographers on tap, did not look at the rocks but blindly used the whole rock geochemistry.
So unless you want to advocate a Snowdonian source for SH rocks (and I put little past you)dump ignimbrite and use the correct term rhyolite A-C or just Rhyolite C or just Cryf quarry rhyolite. The 'fiamme' of the 'ignimbrites are the tectonised qtz-rich lenses within the CRyf rhyolites/rhyolitic tuffs.

It is rumoured that a SH volcanics review paper will address this amongst other issues.

For the best modern listings of all the relevant SH lithologies read the pet rock boys two articles in Brit Arch and Current Arch. Between the two you will have a simple, easy to access, comprehensive and comprehensible summary.
Better of course read the original literature, most of which is FREE on line,yes not even the cost of a bottle of White Lightening.

Myris I think said...

Ah perhaps I should read my earlier posts to avoid repetition.
Perhaps we should both go to AA meetings (Alzheimer's Anonymous). "Hello my name is mmmmm...... what is my name, begins with a G?)

BRIAN JOHN said...

Myris, I shall stay clear of changing the terms as used by the old boys, largely because it would be presumptuous of me to say things like "Judd referred to some rocks as X, but he actually meant they were Y, or possibly Z." I'll leave that exercise to you. In any case, do you actually know that the things the old boys referred to as "ignimbrites" were from rhyolite groups A-C? They may have been looking at samples you have never seen.........

Myris said...

Many of the old boys sections are in museum/BGS collections and seen by the pet rock boys or new sections have been made from the original samples.
All your caveats have been made by the pet rock boys -they after all are working for future scholars so insist on getting it correct.
It is all in their papers. All samples , who has seen them before etc etc.
I am not certain when ignibrites first was used, doubt the old boys, but it was its use by the OU that is to be condemned.