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Wednesday, 4 October 2017

The spotted dolerite enigma


This is an image I took yesterday -- Carn Goedog, on the northern flank of the Preseli ridge, looking pretty well SSE.  I was standing at SN120351.  Carn Goedog is c 2km away.  (We are not very far from Pensarn, where the diggers have been at work this year.)

I have been troubled for a long time by the frequent occurrence of spotted dolerite boulders in places where they should not be...........  I am not talking here about gateposts or pillars, which could have been collected from Carn Goedog by the local farmers, but big rounded boulders which are clearly heavily abraded glacial erratics.  The nearest known source for spotted dolerites would be Carn Goedog, but to get them dumped in a big morainic spread at this location would involve a substantial flow of glacier ice from the south towards the north -- and that can only have involved an active Preseli ice cap.

The other option is that there are other sources for spotted dolerite to the north or north-west of this location, which we do not know about.  The dolerites on Carnedd Meibion Owen are not spotted, and there are some very peculiar rocks that look like ashflow tuffs or ignimbrites in Tycanol Wood.  There are rhyolites at Sychpant, not far from Nevern, and there are dolerite dykes on my own land in Cilgwyn.  The geological map is clearly inadequate in this area.......

That being the case, I wonder how reliable the provenancing of the spotted dolerites at Stonehenge actually is?   Rob Ixer and Richard Bevins tell us that the "best match" is at Carn Goedog and Cerrig Marchogion, but the matches are not perfect, and could it be that some at least of the Stonehenge spotted dolerites could have come from sources as yet unidentified, to the north of the location I examined yesterday?

That would put the cat among the pigeons -- but the geologists really need to check this out.........

Here are some of the pics taken of the surfaces of some spotted dolerite boulders.  The whitish mineral clusters are quite spectacular in some cases, and they actually stand proud of the surface, having resisted weathering processes better than the "matrix" in which they are set.



The erratic assemblage here includes unspotted dolerites, foliated rhyolites, ignimbrites and quartz boulders.  Some of the boulders are heavily weathered and have clearly been exposed for a very long time -- others are quite fresh in appearance, having been dragged out of the ground by a JCB during a building project next to the track. 


35 comments:

chris johnson said...

Perhaps in the Spring we should organise a walking holiday, starting with a little workshop with your good self to learn what we might be looking for. I'm in!

BRIAN JOHN said...

Good idea, Chris! I would be up for that -- I always enjoy walking in the wilderness with others -- everybody sees different things.... and we all learn from each other.

TonyH said...

That would be good! My knowledge of Preseli on the ground is very superficial. I believe there is a Youth Hostel in Newport.

Tony

TonyH said...

Maybe Mel Brooks (b 1926, "still active" says Wiki) would be up for performing an outdoor, walking tribute to Preseli, based upon his well - known "Springtime for Hitler and Germany" ditty?

He could re - phrase it "Springtime for Preseli and Pembrokeshire"? I could co - write the new lyrics, with mention of one or two familiar characters.....

The Afanc said...

If there are young maidens involved then please let me know when and where I can meet you.

The Afanc.

BRIAN JOHN said...

In the moonlight, on the river bank. Can't promise any beautiful young maidens, and beware of carefully laid traps......

The Afanc said...

Would the event be classed as a Preseli Tor,or a Preseli Tour?

BRIAN JOHN said...

How about an Elvis Preseli Tour? (blue suede shoes compulsory)

I went on such a tour once, up around Carn Alw, and it was quite wonderful......

TonyH said...

Long before I knew much about bluestones, Myris of Alexandria, and other erratics/eccentrics, I saved up my pocket money and bought "Blue Christmas" by Elvis. But I never bought "Blue Bayou" by Roy Orbinson, though I now have it on CD, thankfully. No bayous in Preseli, sadly, but then they aren't glacial formations, more to do with flood plain rivers.

Mrs Afanc said...

I have wondered whether there are dolerite and spotted dolerite outcrops offshore, which could have been the source of some of these erratics on the north side of the hills?

The Afanc said...

As you know we Afancs lead a quiet life with very little social interaction, but I'm told by relatives that some of the dolorite stuff came from farther east, nevertheless, it may also have come from beneath the old waves.

BRIAN JOHN said...

Yes, I am sure that there are further twists and turns to come on this dolerite business. The Fishguard Volcanic Series is not as well mapped as we might think, and I am sure there are volcanic rocks interbedded with shales and mudstones along the coast between Fishguard and Newport, in places where they are not supposed to be. There are probably dolerite dykes too, in places where they are not supposed to be -- or where they are not obvious at the surface because they are covered with Devensian till. These spotted dolerite erratics on the northern edge of the moor are really quite puzzling. And yes, there could be dolerite dykes on the floor of Cardigan Bay as well. Other dolerite erratics could have been brought into this area from Carningli, Carnedd Meibion Owen or even Pen Caer -- but I don't know of any outcrops with this degree of spottiness..... Watch this space!

The Afanc said...

My relatives have been in touch and provided a slight correction, they say "dolorite from far to the east".

BRIAN JOHN said...

Dolerite, maybe? And how about the Mendips? I have seen a reference to at least one dolerite dyke, but have no idea what it looks like......

AG said...

Hi Brian
There are volcanics (Basalt) and (Tuff)exposed in the headlands to the north and south of Weston Super Mare.

A Lab report describes a rock sample of bedrock found at the far reaches of Wookey Hole Cave,as "XRD shows it to be composed of Quartz" "A felsic volcanic or shallow intrusive igneous rock, fine crystals are visible in the matrix with phenocrysts of quartz visible on the fresh broken surface as well"
Technical name a "Porphoritic Felsite"

Clasts of Picrite have been found in fields nearby, but these "were" thought to be railway ballast?

I think I sent you the report?

Cheers
Alex

TonyH said...

The Afanc and Mrs Afanc

I think you're both fabulous (both in the Sixties meaning of the word;and the Dictionary Definition), thank you for coming on!

The Afanc said...

Thanks to Tony for the kind words, and an observation on Brian's post of the 9th October @ 15:29 re: "Dolerite, maybe?", ----- replace 'maybe' with 'definitely', (samples collected from an old, but very real, quarry).

Far to the East, but no where near the Mendips.


A thought for today

From the window of our shitehouse,
I cold see South Bishop lighthouse,
If it wasn't for the mountains in between.

BRIAN JOHN said...

Now I am getting confused, Afanc. Don't like being confused. What on earth are you talking about? What dolerite? Where from? Where found? Your riddles are reminiscent of Mystic Myris........

Sun dried Myris said...

Oh Boys. Cannot you wait for a single month but must have me abandon my Nilotic pleasures for your increasingly insane/inane nonsense.
Anyone who used ignimbrite in a SH bluestone context should be publically whipped and have their spouse and bairns East bourne to Van Dieman’s Land in old rusty chains. Or at a minimum learn to read the literature.
Look up ignimbrite it is NOT the same as flow-banded or tuff (it was the continued use of this stupidity that caused us to hot foot up the Nile in disbelief and gross irritation).
Fiamme have NOT been recognised by any competent petrographer (In a STONEHENGE context) ignore the Thorpe et al paper, apart from the appendices the petrography in that paper is distinctly fifth-rate, geochem, however, very very good for its time.
Most vitric tuffs/rhyolitic tuffs are not ignimbrites. IGNIMBRITES BOLLOX. Are we now clear? BOLLOX IGNIMBRITES.

Spotted dolerites ah wood for the trees. All of this s stupidity and what has it to do with Stonehenge.
Again the pet rock boy and friends have spelled it out clear and often that macroscopical and petrographical identification/classification of the dolerites is inadequate. As Thorpe et al showed, geochem is the only useful determinative method. They did a good job in eliminating all the possible rubbish contenders. Including volcanics on the south of the Preseli Hill, the volcanics in the Mendips (apart from the Silurian to the East) there is not enough volcanic material in the Mendips to make a window sill let alone a 2 tonne orthostat. Also the material is so altered you can shape it by blowing it. AGAIN ALL THIS IS IN THORPE ET AL.
Bloody read the literature. There is nothing more irritating (Pace ignimbrite misuse) than an amateur with joy in their eyes telling you afresh, AN INSIGHT, something that has been investigated and dismissed for decades.
As for uncovered close to the surface Preselite in the Irish Sea look at the off-shore geology maps.
Spotted dolerite erratics in weird places is of potential geological interest it has nothing to do with the geographical origin of the quarried bluestone orthostats.
Almost the only thing you can guarantee in a XRD diffractogram (Cu source) is the peak at 29.6 –the main quartz peak.
I remember a foolish woman at a conference showing a diffractogram of an igneous rock axe-head showing lots of feldspar peaks and saying she knew exactly where the source rock was because it contained feldspar. (She made a career of such poor crass scholarship/research). Feldspars are the most abundant mineral group in the Earth’s crust so much so that the primary classification of all igneous rocks is based on the chemical composition of feldspar.
The current spotted dolerite talk is not as bad as that, but did immediately remind me of that horror show and so drew me reluctantly away from Africa’s warming sun, our Divine Apollo.
Ich bin der Babysitter von den ganzen Welt. sigh.
M



AG said...

Pardon me for breathing! I'was just reporting what professionals have writ and what Brian asked for?

BRIAN JOHN said...

Ah, Myris, how the rest of us aspire to the sunlit highlands of enlightenment, which you have for so long occupied all on your own! What's all this fuss about ignimbrites? Why do you hate them so much? Read more carefully. I'm not talking here about ignimbrites at Stonehenge -- I am talking about possible ignimbrite boulders on the edge of Brynberian Moor. I am not going to re-label them as flow-banded tuffs just because you think that is what they might be. If you want to disagree with me, go and look at the boulders! You have the grid ref. "Ignimbrite" is a nice old-fashioned word for the smashed-up rocks associated with explosive or pyroclastic volcanic activity. Volcanic breccias, agglomerates, autobrecciated gunk,disaggregated and welded tuffs etc are all in there -- and Richard and his colleagues have shown how complex the Fishguard Volcanics are in the Pen Caer / Strumble Head area. But the word that encompasses all these complex variations for the results of explosive eruptions, with highly technical names, is "ignimbrite" -- I like it, and shall continue to use it, since I am not expert enough to know my welded ashflow tuffs from my autobrecciated rhyolites from my volcaniclastic disaggregated reconstituted dacites. All I know is that some of the boulders on the edge of Brynberian Moor remind me of some of the seriously smashed-up volcanics exposes in the cliffs of Pen Caer. Some pretty good geologists taught me that they can be referred to as ignimbrites.

And please stop referring to "quarried bluestone orthostats" -- you know I get very upset when geologists stop being scientfic.

Evergreen said...

idiot/stupid/inane question here - When I look at a geology map of the British Isles and I see Intrusive and Volcanic Igneous rock regions in Ireland, Northern and South-West England, am I to assume that these regions either do not contain the dolerite, rhyolite, or whatever 'tuffs' or 'ash' are of interest to us here, or is it that these types do appear in these areas but have been examined and discounted as possible matches for SH stones?

Again sorry for the idiot/stupid/inane question.


BRIAN JOHN said...

Carry on breathing, AG! Interesting info -- tell us more, whether or not this has anything to do with the bluestone orthostats at Stonehenge.

BRIAN JOHN said...

Yes, Evergreen -- there are a lot of igneous rocks all over the place in Western Britain. A lot of those were ruled out as "Stonehenge bluestone sources" by the old geologists who were pretty good at looking at hand specimens and describing textures and mineral assemblages. Thin section analyses moved the science along rapidly, and gradually a database built up of what the petrography of these various rocks looks like. Then along came geochemistry and all sorts of other techniques. More progress, and more sources ruled out..... what I like about the work of Richard Bevins and Rob Ixer and other modern geologists is the recognition of metamorphic effects over the lifetime of various rocks -- whether sedimentary or igneous. That allows provenancing to be even more accurate. No doubt Myris will correct me if I am wrong on that.....

Evergreen said...

Thank you Brian!

TonyH said...

Afanc/ I think/:-..... it can be safely stated that ignimbrites or no ignimbrites, and whether or not The AFANC is lurking underneath a Preseli bridge, MYRIS is definitely back and in the building! (even though he claims to be Carrying On Up The Nile)

We all love your mega - metamorphic extrusions, my dear old thing.

The Afanc said...

It should be noted that Afancs are not confined to the West Wales area, and over the years they've spread far and wide, so, in an attempt to ease Brian's confusion regarding a nuisance, in the form of a stray dolerite outcrop, I submit the following:

The Dolerite Outcrop

Like a bone with a dog,
I detect on this blog,
An occasional dose of the panics.
But what makes me smile,
Is, by many a mile,
It’s not part of the Fishguard Volcanics.

What’s pleasing to me, and perhaps MPP.
Is, (contrary to young Brian’s axiom),
The bloody extrusion, is not an illusion,
And crops East of the Glacial Maximum.

Bard Afanc.






BRIAN JOHN said...

I am intensely relaxed about stray rocks, dolerite or otherwise, popping up all over the place. So stop messing about, Afanc, and tell us what you are on about. Nice poem, but bad geology. If it's an extrusion, its not dolerite. And if it's dolerite, it's not an extrusion. And yes, there are dolerites and other intrusives in the Midlands. So what? And which glacial maximum are you referring to?

TonyH said...

What we must remember is, like The Iceberg, two thirds of The Afanc lies below the surface (deep within Its own sub - conscious).

And don't get too intense about your relaxed state, Brian! You may find yourself distinctly underwhelmed.

The Afanc said...

I'm not talking about stray rocks, where did that idea come from?

I'm highlighting a small but visible dolerite outcrop, where rock has clearly been worked by an abandoned/old quarry, situated on an overland route from Preseli to Stonehenge, but far from Preseli, and, to my knowledge, beyond all the glacial maximums. Now, should samples from this quarry match anything at Stonehenge, what would be the effect on the glacial transport theory?

As the Alexandrian often proclaims ------ "read the primary literature", which in this instance should be ------- "study the geological maps".

Regarding the use of the term 'extrusion' rather than 'intrusion' then "Ooops" was said , but it still rhymed.

How much geological expertise do you really expect a simple Afanc to possess?
Ask me one on lurking!!

BRIAN JOHN said...

Mr Afanc -- red herrings and white elephants again. Before we all get weary, just tell us what the grid reference is, and we'll tell you whether we are overwhelmed or underwhelmed.

TonyH said...

Red herrings,white elephants and oxymorons abound - this is a marvellously metaphorical blogsite! Me, I love the wild geese too (and its followers, too numerous to mention).

TonyH said...

.......then there was the PARTRIDGE Family's big hit "Afanc/ I think I love you"..........

Gordon said...

Could Afanc be referring to Gullet quarry in Malvern?

BRIAN JOHN said...

No idea, Gordon. He will tell us if he wants to.........