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Saturday, 12 December 2015

Ice moulding in North Pembrokeshire

After all this stuff about imaginary quarries, let's get back to the real world.  I have come across some more photos showing the extent of ice moulding across North Pembrokeshire.  This is of course much more evident on hills and hillocks than it is on the lower land -- but there are many ice-moulded slabs of rock on the clifftops as well, particularly along the north Pembrokeshire coast between St Dogmaels and St David's.

 Heavily ice-moulded and "cleaned" rocky hillock near Dinas


 Beautiful moulded forms on Carnllidi, near St David's
 
 Carnllidi again, showing the down-glacier face of a feature that has some similarities with a roche moutonnee

 Heavy ice moulding features on Garn Fawr, not far from Strumble Head on the Pen Caer Peninsula

 It is generally assumed that these features, and many like them, are a result of glacial activity by the Irish Sea Glacier during the Devensian episode.  In contrast, look at these two photos of Maiden Castle near Trefgarn, in the centre of Pembrokeshire.  These crags are so fragile and indeed ruinous that we are increasingly convinced that the Devensian ice edge must have lain a short distance away, to the north.  That interpretation also seems to match the distribution of glacial sediments in the area.




11 comments:

Hugh Thomas said...

I have often studied the lay of the land in and around Preselau and can see an almost north-south moulding going on even with the larger hills.
On the southern slopes to the main east/west ridge are several spurs which jut out and seem like thier east west flanks have been smoothed down , Craig Talfynnydd being just one.
I have also pondered the grassy plateaux that extend northwards from the craggs of Carn Goeddog to where it finally dips down to Brynberrian moor.
This turtle back shaped plateaux extends northwards from below Carn Bica which crowns the northern extent of Craig Talfynnydd and Carn Goeddog on the same bearing downslope. Please correct me if I am way off with this but is there a chance that this plateaux was once much higher in profile and repeated ice sheets have worn away the old hill to the outcrops at Carn Goeddog we see today ? If this is the case then whole hills could have been erased and dumped east and south over millions of years ? Or am I way off with this ? I have often wondered just how much of the Preselau range is now missing and how far was it taken over millenia . ? ;)

BRIAN JOHN said...

Hugh, nobody knows how much rock has been removed by erosion from over the top of Mynydd Presely. Hundreds, if not thousands of metres, probably, since the youngest rocks now exposed are c 450 million years old. Also, many of the igneous rocks now at the surface were originally emplaced and cooled at great depth beneath the surface. Myris will probably tell us more about that, since that's his subject. We have to be very careful when interpreting the lie of the land, because structural factors come into play -- dykes and sills in particular, in terrain such as this where volcanic processes have been at work. How much erosion has there been off the tops and flanks of the hills during the Ice Age? Again, difficult to say -- but glacial erosion is often highly selective, with one area eroded dramatically and another area close by hardly affected at all. All that I can suggest is that in the Anglian Glaciation entrainment area, on the northern flanks of Preseli, tens of metres of bedrock might well have been eroded where glaciological conditions were suitable. Look up "entrainment" in the search box.......

chris johnson said...

I am curious whether Myris responds to your (Brian's) suggestion that the exact chemistry of the rhosyfelin/stonehenge debitage match might be replicated along the fault and even further down the valley.

I had formed a mental model of volcanic lavas producing a unique cocktail of elements such that, with sufficiently fine measurements, it is indeed possible to fingerprint a few square metres of rock. Were Myris prepared to descend to a low level explanation of the limits of petrological sampling in this context then I suspect we would all become wiser, me at least.

Myris, how about it?

BRIAN JOHN said...

I have offered Myris the opportunity to publish on this blog as many thin sections as he likes, in pursuit of just that objective.

Myris of Alexandria said...

No I said along strike. The fault lies with you, I know of no fault and along the valley what valley? There is a reason for technical terms. The term is strike.
Read what is written and strike out any faults.
Read what is said not what you think I say.
I shall never publish original data on this or any other blog but only in the literature.
You still do not get it Brian it would be pearls before swine. There are enough photomicrographs in the literature to prove the case to those who are qualified to judge. The opinion of the untrained is valueless and not worth pursuing.
M

Myris of Alexandria said...

Incidentally we now know the exact age of the rhyolites from CRyf down to the nearest few hundred thousand years. The data are very new.
M

BRIAN JOHN said...

More tolerance, please, Myris. It's easy for people to use the wrong terms sometimes -- you do it too! OK -- Chris meant to refer to the fracture planes that coincide with the rock face. You know which valley he means.

BRIAN JOHN said...

It ill becomes you, Myris, to refer to our small community of contributors as swine. Wrong word, perhaps?

chris johnson said...

I fail to see how using the term "strike" would have changed a straight question into a straight question.

How precise is the methodology used by Ixer/Bevins?

BRIAN JOHN said...

Myris -- "There are enough photomicrographs in the literature to prove the case to those who are qualified to judge. The opinion of the untrained is valueless and not worth pursuing." You and I will disagree as to whether you have proved a case. One interesting point -- how come you have published all the Rhosyfelin stuff in archaeological journals, rather than geological ones? That might also lead one to ask how good the peer review process might have been....... I do hope the journals involved used other petrologists as reviewers.

Myris said...

Petrography is thought of as an old-fashioned skill and the geological journals do not publish petrographical pictures nor detailed petrographical descriptions anymore anywhere. They have not done so for 30 years.
As the pet rock boys want their work to be fully documented and there for later workers they found a journal that with a little persuasion agreed to be that journal of record. They are extremely grateful that Wiltshire Studies has continued to support them. It does not harm that many of the earlier lithic studies were published in the for-runner of Wiltshire Studies.
Main stream geology has no interest in SH or its provenancing as the work is considered to be trivial.
Why would petrologists be good reviewers none of the pet rock boy's Wiltshire Studies paper have had any petrology in them. It is rare to find a petrologist who is also a good petrographer (too many black boxes and almost impossible to find a 'total petrographer'.

Had the work been petrological I am pretty certain it would have been published in main stream, highly regarded geological journals. But then watch this space!



You are only a swine if you think you like a swine- it was really a Durrington Wall analogy and ill-judged. But the point remains, petrography at this level is not the same as modern art, everyone can have an opinion, they are very welcome, and there are plenty of rhyolite photomicrographs spread amongst 5/6papers for them to do so but only qualified opinions count and it is a complete waste of time to think otherwise or to pander to by-standers. Would you ask for dozens more chest X-rays and the WVS ladies for their opinion when the consultant says....

Use the correct terminology "strike of the foliation" is what was said and meant. That has a specific direction at CRyf, faults and there may be faulting at CRyf may or may not coincide with foliation planes.

M