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Tuesday, 4 August 2015

Carreg Coetan and the Enigmatic Dolerites


Now for another enigma.  Where did the dolerite boulders of NE Pembs come from?  It's very easy to explain the presence of dolerite boulders almost everywhere else in Pembrokeshire as a consequence of glacial erratic transport, but in NE Pembs -- in the area between Dinas Island and Cemais Head, incorporating the country around Newport, Moylgrove and Cardigan -- there are dolerite boulders dotted around here and there which cannot easily be explained by reference to the glacial maps.  Many years ago the geologist OT Jones pondered on their presence.  The ice during the Devensian glaciation came in from the N and NW, having crossed the area now known as Cardigan Bay.  As far as we know, all of the dolerite intrusions associated with the Fishguard Volcanic Series are to the south of Newport (see the BGS map above), and so they could not have been picked up and moved northwards by ice  moving in the opposite direction!!

There are some possible explanations.

Maybe the dolerite boulders in the area have come from dolerite exposures in the Pen Caer - Strumble Head area by ice moving directly west - east.  There are some dolerite outcrops on the peninsula.  We know from striations on the coast that the ice did move in this direction at some stage.

Another possibility is that the dolerites have come from the Cader Idris area, or even North Wales, and were transported by Welsh ice and Irish Sea ice before being dumped in this area.

A third possibility is that they have come from dolerite outcrops which are now beneath the sea bed.  This seems quite feasible, given that the Penyraber and Cwm-yr-Eglwys mudstone formations are right at the base of the sedimentary sequence overlying the Fishguard Volcanics.  There are complex faults in the area, and I am sure there are some igneous rocks exposed in the cliff faces of Newport Bay, although they are not shown on geological maps.

A fourth possibility is that the boulders are really from the Preseli outcrops, and that they have been transported northwards by ice flowing from a Preseli ice cap either before or after the Devensian LGM of Last Glacial Maximum.  There are a few anolalies in the evidence, and I have suggested several times on this blog that at some stage Welsh ice crossed North Pembrokeshire, flowing from the east towards the west.  But it's doubtful that such ice would have carried dolerite boulders, for the prevailing rock types to the east are Ordovician and Silurian sedimentary rocks -- sandstones, gritstones, shales and mudstones.

Then a fifth possibility.  Could the stones have been transported to the locations of assorted Neolithic burial chambers and Bronze Age standing stones by prehistoric people?  OT Jones was convinced that assorted dolerite pillars used as gateposts and in house building in this area were in fact collected from the Preseli Hills by landowners and farmers during and after the Middle Ages;  I would go along with that, and there is indeed much evidence of "modern stone gathering"and maybe even quarrying of dolerite outcrops within historical time -- simply because the stones were easy to transport and use.  But if we go back to the Neolithic, the evidence of stone transport is not strong.  Those archaeologists who have looked at the cromlechs of the area -- like Pentre Ifan,  Cerrig y Gof, Llech y Dribedd and Trellyffaint -- have always concluded that the boulders used have been glacial erratics used more or less where they were found.

Carreg Coetan Arthur, in Newport, is a good example.  It is in a small enclosure not far from the estuary, and it incorporates a huge capstone resting on just two uprights.  It is very precariously balanced, and it is a wonder that it has not collapsed.  Two other uprights almost touch the capstone, but daylight is visible at the tip of each stone.





All of the stones at this site (capstone, pillars and several other fallen stones embedded in the turf) are made (as far as I can see) of unspotted dolerite which looks like the dolerite of the Carningli-Newport area, which outcrops just a couple of km to the south.  The stones are well weathered, and have rounded or abraded edges typical of glacial erratics.  So they LOOK like glacial erratics, and they are most certainly not quarried from anywhere.  It has to be assumed that they were used more or less where found.  The massive capstone also has a worn and weathered upper surface and a less weathered and somewhat flaky lower side;  I assume that (as in the case of the Pentre Ifan capstone) it once lay embedded in the ground and may even have been overridden by ice before being exposed and subjected to 15,000 years of weathering.  Then somebody came along with the bright idea of levering it upwards, placing props or pillars beneath it, and making it into a burial chamber......

Was this also the history of the other burial chambers to the north of the Nevern River?  Watch this space....



17 comments:

TonyH said...

With regard to your 5th possibility, that prehistoric people, in your view, probably did not move cumbersome MEGAlithic dolerite boulders any great distance, I have come across an interesting Research Study of the so - called Exmoor MINIliths by Mark Gillings,Josh Pollard and Jeremy Taylor.

This may be found on the Leicester University Archaeology Dept website, at:-

www2.le.ac.uk/departments/archaeology/research/projects/miniliths-of-exmoor

For those interested, one of the areas studied in detail is Badgworthy Water, associated with the dreaded Doone family in "Lorna Doone"

Geo Cur said...



“With regard to your 5th possibility, that prehistoric people, in your view, probably did not move cumbersome MEGAlithic dolerite boulders any great distance, “

Depending on what is considered cumbersome and any great distance we do know that prehistoric people certainly did move heavier boulders than the capstone of Carreg Coetan distances of over 7-8 km and further .
Possibly not dolerite but I don’t really believe that would make much difference .
The certainty is due to the fact that they were moved in areas that had no glaciation and there is no other reasonable “explanation” .

The miniliths of Exmoor tend to be sandstone or slate .

BRIAN JOHN said...

Nice little study, Tony -- thanks for drawing attention to it. I am impressed that the authors have issued annual reports during the progress of the dig over a number of years. That is, I fear, not something that can be said of all extended digs........

Geo, we have covered all of this many times before. You think your evidence for the transport of capstones over distances of 8 km or more is convincing. I don't find it so, as we have discovered often on the pages of this blog.

Geo Cur said...


"You think your evidence for the transport of capstones over distances of 8 km or more is convincing "

There was no mention of moving capstones 7-8 km . the comment was "we do know that prehistoric people certainly did move heavier boulders than the capstone of Carreg Coetan distances of over 7-8 km and further ."

How else do you explain the movement of much bigger stones than the Carreg Coetan capstone over greater distances than 7-8 km in areas where glaciation can't be used as an explanation .

TonyH said...

Who rattled your cage, Geo Cur? Your lip seemed to be visibly curling (in my imagination, at least) at my comment!

I am quaking in my boots down 'ere in Wiltshire, in an area not notorious for earthquakes, but quite renowned for megaliths, miniliths, and even microliths.

My comment was just meant to highlight a research study's fascinating findings in an area and subject (viz Exmoor miniliths) that contrasts with the geographic area and subject (N.E. Preseli dolerite boulders) of this Post. I thought some folk might find it worth having a look at for its own sake! No glaciers needing to be summoned up, nor Merlin - moved "bluestones". I await your quotation - marks around some part of this comment in your reactionary, rumbling reply.....


BRIAN JOHN said...

No no -- I think Geo's cage was rattled by my comment, not yours... such is life. Capstones, supports, lintels, doorsteps, gateposts, all equally interesting.

Geo Cur said...


Your comment about prehsitoric people not moving cumbersome boulders great distances was wrong Tony . That is why I quoted it .
No lip curled ,just a sigh .

I always thought that mention of these transports ratteld others cages .

Your comment bore little relation to the minilths on Exmoor which are neither cumbersome or transported great distances .

Geo Cur said...



what was reactioary or rumbling about noting the fact that prehistoric people did indeed transport big rocks great distances ?

TonyH said...

Maybe I was distracted by the amazing news of bowler Chris Broad's achievements in the Ashes cricket v the Australians at Trent Bridge today, Geo, and the adrenalin - thus released - somehow got displaced onto this Blog!! I probably got carried away, no offence intended, Geo.

My main intention was to draw attention to that Exmoor minilith research survey purely for its own sake, but whilst also recalling that some other contributors to this Blog (e.g. ?Hugh?) have remarked on finding partially concealed stones on the Preseli landscape, and trying to make sense out of their distribution.

I have spent plenty of time both on Exmoor and the Peak District moorland, having lived close to both these uplands, trying to locate small prehistoric sites or just examining, out of interest, fairly haphazardly parts of the landscape near prehistoric sites e.g Gardom's Edge in the Peak and finding cupmarks, etc. Brian's Post mentions the possibility that farmers from medieval times onwards manoeuvred some doleritic stones on Preseli. This Exmoor study talks about early, probably prehistoric farmers, doing similar things, although not necessarily for purely farming reasons e.g. in the creation of stone rows or other shapes.

chris johnson said...

For some reason buried in the psyche of homo sapiens sapiens we like to move big stones around. We see evidence of this from around the globe, from Korea to Madagascar, from Malta to Marlborough, India, Europe, Africa, Asia. And it is not done for utilitarian reasons unless you include religion as a utiliteranian satisfying of a basic necessity.

Today we see this instinct at play in front gardens everywhere. Some people like to look at big rocks that they had put in a particular place.

There is plenty of evidence from Pembrokeshire in the Neolithic. The question for which evidence is lacking is how far these stones were brought.

Geo Cur said...



Read the original minilith paper when it first appeared Tony ,they are odd in that they are so small and insignificant .
I was more concerned with clarifying that prehistoric peoples did indeed move large stones great distances in areas where there was no possible explanation for the transport being attributed to glaciation .
As for mistaking the perfectly natural for something supposedly significant prehistorically ,or misattributing ,see
http://www.themodernantiquarian.com/site/14271/non_rock_art.html for a recent example of what had been described as a spectacular example of prehistoric cup marks /rock art ,when it clearly isn't .

TonyH said...

Cricket correction: it was, of course, STUART Broad, not Chris. The latter is Stuart's father, also a very important England player, batsman in his case.

TonyH said...

Couldn't make your link to the Modern Antiquarian item on non - rock art work, Geo.

Perhaps you could also give us a few keywords, or a full title, which may help to summon the item up? Thanks, Tony.

Plenty of professionally - recognised cup marks on the afore - mentioned Gardom's Edge in the Peak. See, for example, "Prehistory In The Peak", Mark Edmonds & Tim Seaborne, 2001, page 109.

I also dug fairly nearby at Barbrook in the '60's.

Geo Cur said...



This one worked when I tried it .

http://www.themodernantiquarian.com/site/14271/non_rock_art.html
It’s the first fieldnotes and some pics related to Fereneze . It is rock art but just not prehistoric ,but I hadn’t created a page for that category .

The original mention was .
http://www.wosas.net/news/fereneze.html

The above were also "professionaly recognised" .
Wehn it comes to recognition there are no professional rock art experts in the UK ,a handful of amateur experts though .

TonyH said...

Are you a contributor to The Modern Antiquarian, Geo, on a regular basis, or are you indeed its editor? (You say "but I hadn't created a page for that category".)

I have noticed Professor Mark Edmonds is working recently on the Ness of Brodgar Neolithic discoveries.

Geo Cur said...


Tony , I contribute to TMA on an irregular basis .When a monument/category that wasn't already in the database of the site is added , the contributor has to create a new page for the site . You could do so .Does that makes all the contributors editors ?
In this case nobody had previously created a category of "rock art ,non prehistoric" and as I had created a page some time ago for "non rock art " i.e. what gets mistaken for rock art I included the Fereneze example . It is of course out of place because it is not natural and is indeed rock art ,but I'm sure anyone who looked at it appreciated that .

Yes ,noticed the prof is at the Ness .

TonyH said...

Thanks Geo, your remarks about TMA informative, will bear that in mind.