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Thursday, 1 August 2013

A veritable cornucopia of spotted dolerites

I drove past Bethel Chapel in Mynachlogddu today, and was forcefully reminded of the way in which spotted dolerites from the nearby outcrops of Carn Meini etc have been used in local architecture.  The whole of the chapel facade (finished in 1875) is built of shaped blocks of spotted dolerite -- many different types.  Big spots, little spots, scattered spots, dense spots, white ones and cream ones, and so on and so on.

This stone must have been incredibly difficult to work, but somehow or other the faithful builders of the chapel have managed to make most of the blocks rectangular.  The only stones that are NOT spotted dolerite are the window cills and lintels, and those curved window surrounds -- those are all, I think, made from soft grey local slate which is very easy to work and shape.

I dare say that Rob or Richard could give a comprehensive geology lesson on spotted dolerites without having to move away from the churchyard.  I don't know enough about these different types to know which outcrops they were taken from -- but there are historical records of decisions being made by the chapel deacons to go up onto Preseli to collect stone, and of horses and carts coming down with loads specifically for the chapel restoration.

Here are some close-ups.  In some cases the weathered surface has been left, giving a nice rusty or buff colouring, and in other cases the blue-grey of the fresh worked surface is quite striking:

You can click to enlarge any of these photos.  What interests me here is the question of WHY, in or around 1870 -75, the congregation here chose local spotted dolerite for their chapel facade.  Did it have any particular religious or spiritual significance for them?  I doubt that very much -- chapels were generally built with stone that was cheap and durable, and attractive -- they obviously wanted their chapel to be striking, as an offering to the glory of God.  But if sandstone or limestone had been the local rock, they would have used that instead.  And it WAS cheap -- they needed to pay nothing for it, for there it was, within a mile or two, up on the common, ready to be carted away.

Interestingly enough, in the years 1946-48, during the famous local episode called "The Battle of Preseli", local ministers and political leaders created a great campaign to resist the efforts by the MOD to turn the whole of Preseli into a military training range, by building on HH Thomas's thesis of "the sacred stones."  It suited their campaign strategy very well to claim that the spotted dolerites were sacred, and always had been, and that it would be an outrage and an insult to the sensitivities of the Welsh if the MOD had gone ahead with its plans.  Shock!  Horror!  Military firing range planned for sacred Welsh mountains......!!

Anyway, it worked.  Churchill and the rest of the Government of the day were swayed by this emotional and pseudo-spiritual argument, and chose Castlemartin and the Brecon Beacons instead.  That doesn't alter the fact that the campaign to save Preseli was based on spiritual mumbo-jumbo, conjured out of thin air by the bards and religious leaders of the day.......  all credit to them, for in reality there never was any great feeling in history that either the spotted dolerite or Preseli had anything sacred about them.  They had MYTHOLOGICAL connections, of course, since the uplands figure prominently in the Mabinogion -- but so do many other places in Pembrokeshire, and you could argue that Cwm Cych, Narberth and the Pembrokeshire Islands were much more "special" than Preseli in the minds of the medieval storytellers.


Anonymous said...

What are the “spots” in the dolerite made of?


chris johnson said...

One surprising fact is that the stones of Gors Fawr are close by and seem not to have been used.

BRIAN JOHN said...

I doubt that even in 1875 the good deacons of Bethel would have sanctioned the use of stones from an existing prehistoric stone setting. Anyway, the stones at Gors Fawr are not particularly interesting, and are no good for shaping and stone facing.

Myris of a said...

The spots
One of the main aspects of the present study has been
to look very closely at the iconic spots. These distinctive,
pale spots originally described as feldspars (and often
still called that in much of the archaeological literature)
have been more recently interpreted as metamorphic

porphyroblasts comprising plagioclase and alteration
products and belonging to the low-grade metamorphism
of the dolerites (Bevins et al 1989).
Petrographically the spots are seen to comprise
?relict chrome spinel together with relict feldspar; plus
?secondary feldspar; secondary, fine-grained clinozoisite;
the two chlorite minerals chamosite and clinochlore; finegrained
muscovite and a coarse-grained epidote group
Whole rock X-ray diffraction of the pale spots has
confirmed the presence of albite, (a secondary feldspar)
epidote and chlorite (Richard Bevins pers comm 2007) and
qualitative chemical analyses by SEM EDAX of a white
spot about a chrome spinel has suggested the presence
of clinozoisite, muscovite and clay minerals (Chris Blake
pers comm 2007). The spots are or were undoubtedly
feldspathic but now carry more alteration material than
feldspar. It is significant that iron-rich chrome spinel (not
a possible secondary mineral) only occurs within these
spots (it is not found in the usual groundmass of the
dolerites) suggesting that the original plagioclase ‘knot’ is
not metamorphic but igneous.
The pale spots with iron-rich chrome spinel should
be regarded as the key identification signature for Carn
Menyn dolerites and the presence of this mineral within
macroscopically ‘unspotted’ dolerites suggests that they
are in truth crypto-spotted dolerites. This lessening of
the importance of spotted and unspotted dolerites at
Stonehenge may be of significance.

Timothy Darvill1 Geoffrey Wainwright2 Kayt Armstrong3 and Rob Ixer4

TonyH said...

Whoops, forgot to say...! You see, I was strangely drawn to Rasputin by your mention of fine-grained MUSCOVITE in the composition of 'dem spots.......

Not sure whether I am a genuine clairvoyant.


BRIAN JOHN said...

Thanks for the info, Myris -- you are indeed a man of many faces who changes his spots whenever appropriate.......

Constantinos Ragazas said...

… that was a mindful! Thanks. The more we know the less we understand.

Myris of Alexandria said...

Tony why Rasputin and Muscovy -he was a Siberian I think.He visited Mount Athos so is one of the good guys.
He would make a splendid Orthodox Saint alongside The saintly Imperial family. ((I never called St Petersburg anything but that even in the old days (that is real clairvoyance))
In later life of course he became perhaps the best Dr Who and what JOY with the new one potentially a real Gravitas-filled Doctor after that feather-weight prancy pony of late.

BRIAN JOHN said...

Myris -- you are obviously a time lord -- how come you did not get that Dr Who job when it was up for grabs? Now it's gone to that gawky lad from LOCAL HERO, and we just have to hope that he makes a good fist of it.....

Myris of Alexandria said...

Ahhhhhhhhhhh don't tell the other two. They think they are the only ones left but Dr Riversong and I know differently.
I am but a faux-archy, she of course is the real thing.
The new avatar, we shall see, I am hoping for good a time. Lord, but I am.

Constantinos Ragazas said...

Myris of A

Seeing more “spots” … more questions for you!

1)Are these dolerite “spots” 3-D (i.e. have thickness like embedded pebbles) or flat surface? If flat, do they also appear inside the dolerite were we to break open a dolerite stone? If so, how is that possible? What geological process is planar rather than spacial within a rock?

2)If I understood you correctly, the non-spotted dolerite has the same “spots” but these are not formed. If so, what geological process has formed the “spots”. Is it intense heat? Or severe cold? And is that from deep earth or atmospheric?

3)When have these “spots” formed? Any other place in the UK (or anywhere else) that has these “spots” in dolerite rocks?

Curious Kostas

BRIAN JOHN said...

Kostas -- forget about the idea that these specks are planar. They are three-dimensional, and you see them on all surfaces of the spotted dolerite boulders abd stones, no matter how the surface is cut. Look at the last photo -- you can see how the specks stand proud of the weathered surface.

Constantinos Ragazas said...

Brian you write,

“forget about the idea that these specks are planar. They are three-dimensional”

That would make sense to me too. From what I understand from Myris about these “spots”, they also exist (but not visible) in the other “unspotted” dolerite rocks. If so, what geological processes made them appear in the “spotted” dolerites? Was it “heat” or “cold”? If I had to guess it would be severe cold.


Myris of Alexandria said...

Ah Myris was quoting Dr Ixer from 2009? or whenever the paper was written. I believe he thought then that distinction between spotted and unspotted dolerite was largely false, an opinion he no longer holds I believe.
The spots are formed by heat-not great heat geologically speaking something less than 320C, water and much time.
The unspotted dolerites have undergone the same geological history as the spotted dolerites but may have a slightly different geochemistry or petrography so the spots are less well developped.
One problem is that much unspotted dolerite especially in the debitage may be spotted dolerite that is not showing any spots due to too small an area.
More work is needed on the dolerites and this is being done I am sure.
Severe cold does not do much to alter the mineralogy/petrography of rocks -that I know of.

Constantinos Ragazas said...


Thank you for making a simple question that much more complicated! As is the way of sublime Apollo, I am sure.

Do you know of any markings made on any stones or minerals when subjected to prolonged cold/pressure? Or possibly subjected to a “hot/cold” treatment; the effects of which we readily see in both Man and Nature

Balance in my Greek mind leads me to believe in such symmetry in Nature.


Myris of Alexandria said...

Kostas Of course moderation in all things is the golden rule. We both are fine exemplars of that.
Mineralogical changes are kinetically driven often aided by a fluid. Intense cold would be an anathema to that. Effects due to very low temps is more in Brian's field.
Meteorites (we are at the Perseiid climax) ave spent millions or billions of years in the cold of space but show no mineralogical effects. Of course entry into the Earth's atmosphere has ruined their surfaces. Oh better the Moon rocks,
they are basalts show no cryogenic effects.
So sorry no.