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Thursday, 26 September 2013

The Rhosyfelin Fracture Pattern

This is probably all very familiar to the geologists, but let's look at the Rhosyfelin fracture pattern exposed on the rock face.  Here are two versions of the same photo, one in its pristine condition, and the other annotated:



On the annotated photo, the orange lines show the dominant fracture pattern which causes the rock face to break up into elongated slabs or columns.  The blue lines show another fracture trend, running along the face from one end to the other.  One very long fracture can be followed for 50m or more -- others are shorter.  But here we have an explanation for the manner in which the rock face breaks up into "upper slabs" and "lower slabs".  The lower ones, when they break off the face, won't travel very far, but the higher ones, pushed off by physical and biological processes, will fall with considerable momentum, and can travel some way from the rock face.

22 comments:

Phil Morgan said...

Surely the same reasoning can be applied to human quarrying?
Our ancestors would have taken advantage of the fracture lines just as we would today.

BRIAN JOHN said...

Perfectly true, Phil. I wouldn't deny that if there were quarrymen hoofing about at Rhosyfelin they could perfectly well just collect up convenient stones that had already fallen off the rock face -- it's not necessary to assume that they would have themselves levered slabs off the fresh rock face.

Phil M. said...

Brian,
You make me smile, it should have been fairly clear that I meant our ancestors would have taken advantage of the fracture lines in the rock, not of stones randomly scattered about the location; stones that may well have been hidden by soil, as they were until excavated in modern times.

BRIAN JOHN said...

Well, this is a point that needs exploring. Back in the Neolithic, does "quarrying" mean going through the process of physically extracting rocks from their original or "living rock" positions, by leverage, fire/water technigues, wetting wedge techniques etc -- or can it mean simply collecting up slabs or monoliths that have already fallen, and transporting them away? I suspect that different people mean different things when they use this term....... the former obviously assumes a much greater level of technical expertise than the latter.

Phil M. said...

I believe that we have seriously under estimated the abilities of our prehistoric ancestors; not only their in relation to their ability to manipulate awkward and heavy loads with relative ease and maximum safety, but also their grasp of fairly sophisticated mathematics, and their organisation and management skills.

They prevailed in conditions under which the majority of contributors to this blog, myself included, would have succumbed, and it might be true that without the resourcefulness of these people, we wouldn't be here to discuss their abilities, or lack of. So let us not be too critical of them, they were very knowledgeable folk.

My understanding of the term 'Quarrying' is the controlled, and intentional, extraction of the desired quantity, grade, and size of material from a chosen rock face.

The collecting of slabs and monoliths that have already fallen, and transporting them away, can be classed as the scavenging of opportunists.

Which brings us back to the question of whether the individual stones transported to Stonehenge had meaning for the those sending them, and for those receiving them.

If the stones had no intrinsic value, then scavenging would have been sufficient to satisfy the need. However, if there was importance attached to the stones, then the majority of samples would have required quarrying. Nevertheless, I accept that luck would have given rise to some successful scavenging.

So:
1). Did the stones have meaning ---- I believe so;
2)Did humans quarry them ---- again I believe so;
3). Did glaciers transport them ---- possibly, but it required humans to finish the job;
4).Did humans transport them ---definitely, even if only for part of the journey.

Discussion is open to all.

Jon Morris said...

Isn't this dig just a privately funded venture to see what can be found from this place? As far as I'm aware, there is no new evidence that has resulted in one team or another being gifted the right to do this work: It's just one team who have gone to take a look on a hunch. Because of the way it is funded, their work will be subject to vetting by experts: Anyone who goes looking on a hunch will be pre-disposed to finding what they want.

So I'm a little puzzled by all the interest in it: Before the dig, there was no new evidence showing that Rhosyfelin is special. So, prior to the review, anything that comes out of the dig is just rumour. In the absence of any peer review material, why are we interested in analysing rumours about what has been found?

BRIAN JOHN said...

Jon -- do you mean: "Because of the way it is funded, their work will NOT be subject to vetting by experts: Anyone who goes looking on a hunch will be pre-disposed to finding what they want." ?? If that's what you mean, I agree with every word!

BRIAN JOHN said...

Phil -- belief is a fine thing. I prefer to go on evidence -- -- if that stands up, then we have some basis for belief. Don't agree with you that if a stone has some sort of value, that means it must have been physically removed from a rock face. Scavenged rocks or fragments can have value too -- archaeology is full of examples of gold nuggets found and valued, flint nodules found and valued, and erratics found and valued. I suspect that our Neolithic ancestors were not in the least bit worried about geological "provenance" -- that's a modern obsession of great interest to geologists but not necessarily to anybody else....

chris johnson said...

Were the rhosyfelin rocks special?

- the place is spectacular. The white crags are dramatic and compelling. I have not seen anything quite like it. It reminds me of chalk, and anyone from the chalk lands would surely have noticed.
- the polished and cracked stone is bright blue and alive with riverine patterns and swirling shapes. Quite remarkable. An artist from the Boyne Valley would have been absorbed!
- In the right light the fractured stone looks like ancient oak. The neolithic people would have seen a similarity with their wooden monuments.
- under the microscope the texture and colours are inspiring of words like Jovian to modern eyes
- I suspect a dowser would experience strong reactions, if only because of the amount of quartz lying around.

So yes, a special place. Would stone age people have thought so? Almost certainly. And it is where the sewyn, salmon, and trout go to spawn.

Anonymous said...

You might find this funny (on Stonehenge)http://theoccasionalpigeon.blogspot.co.uk/2013/09/stonehenge-in-surprise-turner-prize.html

Jon Morris said...

Because of the way it is funded, their work will NOT be subject to vetting by experts

I know nothing about the project: The Riverside Project did well, so I imagine that this project will be along the same lines. There was that bloke Vince somebody-or-other who works (or worked) for one of the Redbricks and who produced an awful lot of rubbish recently. I don't think this project will go the same way but who knows?

Having said that, I can't say that I was particularly impressed with the Channel 4 programme on Stonehenge: That was a bit of a cause for concern.

Wait and see what happens with it I guess: As far as I can see, it's all rumour and speculation at the moment.

Jon Morris said...

"Were the rhosyfelin rocks special?"

Yes, it's lovely around there. I was in the area a little while back (even passed by Brian's house, but he was off on a jaunt at the time).

It's a special landscape around there, but I am not aware that prior to the dig there was any new evidence showing that it was specially related to Stonehenge: Do you know otherwise Chris?

BRIAN JOHN said...

I don't agree with Chris that there is anything especially wonderful about Rhosyfelin -- there are other wonderful rhyolite crags and tors and crags made of other equally interesting rocks in North Pembs. We are in danger of attributing "specialness" with hindsight here -- or seeking justifications or rationales for whatever human activity we deem to have been going on in a particular spot. Let's remind ourselves that there was nothing special about Rhosyfelin until the geologists (Rob Ixer and Richard Bevins) did a rather interesting piece of provenancing -- my matching up some of the Stonehenge rhyolitic debitage with the rock outcrops art Rhosyfelin. That was an exciting piece of geology -- nothing at all to do with archaeology.

However, in order to maximise the impact of their work, they chose to publish in archaeological rather than geological journals, knowing that a lot of archaeologists would get very excited, on the basis that they were already obsessed with finding bluestone quarries. The rest, as they say, is history.......

Jon is quite right -- prior the the Ixer / Bevins publications there was not the slightest interest in this site. And yes, the archaeologists arrived at the site in 2011 having already decided that they would find a Neolithic Quarry.

BRIAN JOHN said...

Oops -- sorry. That should be "by" and not "my"......

chris johnson said...

Brian,
I wonder where you mean that is similar? You probably know these hills better than anyone alive. I have walked them maybe 50 or 60 times. Still I do not know where you mean.

Yes, there are many spectacular rock formations, especially along the coast. And lots of dolerite outcrops. But nothing like rhosyfelin that I know of.

Jon Morris said...

But nothing like rhosyfelin that I know of.

I think I drove past it on the way somewhere else: I didn't notice it at all. What is special about it Chris?

BRIAN JOHN said...

Of course Rosyfelin is "unique" Chris, in the way that every locality is unique. What is it that makes Rhosyfelin special? The colour of the rock? The craggy nature of the outcrops? The fact that it conforms to some idea that we might have about "beauty" or "wilderness"? There are other craggy outcrops on valley sides at Felin-y-gigfran, Ty Canol and Nant-y-Bugail near Trecwn -- and locations that are more "spectacular" at Carnedd Meibion Owen, Maiden Castle, Lion Rock near Trefgarn and at maybe a dozen or more sites on Preseli, including Carn Alw, Foel Drigarn, Carn Meini (of course!), Carn Goedog, Carn Bica etc etc. If we want to, we can make a case that every single one of those places is "uniquely impressive."

chris johnson said...

Jon,
Rhosyfelin is not easy to see from the main road. When you take the minor road you are too busy worrying about the next bend, sheer hill, and whether you really should drive your car through a river (literally) to be paying close attention to the scenery. When you did not notice being there then it is safe bet that you were not there.

I mentioned 5 points that I think are especially special. Very noticeable is the white (chalk) colour of the crags when viewed from a distance and the sheer scale of them when viewed from the river valley. This is nothing like the places Brian mentioned that I have seen such as Ty Canol, Carn Meini, Carn Alw, Carn Goedog, or Carn Meibion Owen - impressive though they are.. Definitely not Foel Drigarn which I climbed recently. Maybe Maiden Castle - I shall take a look on the web for photos.

BRIAN JOHN said...

Chris -- I think you are in danger here of "hunting for significance" -- on the basis of characteristics that you deem to be attractive or colourful or special. How do you know that the craggy grandeur of Maiden Castle was not even more significant from the point of view of the Neolithic inhabitants? And the Rhosyfelin that we see today, with all the vegetation cleared, is far more prominent than I ever remember it, having passed it many times on the minor road across the ford without a thought. In Neolithic times it would have been largely hidden by bushes and trees, and would not have been a prominent feature at all. Another example of an archaeological artifice.......

Jon Morris said...

When you take the minor road you are too busy worrying about the next bend, sheer hill, and whether you really should drive your car through a river (literally) to be paying close attention to the scenery.

Yes, it was very scary that route. I didn't drive through a river so must have gone a different way to the one I thought we had taken (was navigating by GPS).

Dave Maynard said...

Chris pointed to the physical attractiveness of the rocks at Craig Rhosyfelin. How visible are the rocks here? I was struck when on the road west of Rhosyfelin to see one of the Carnedd Meibion Owen outcrops neatly framed by the hedges on either side of the road in the warm early autumn sun. That was about 2.5km away, Craig Rhosyfelin was 300m behind me and invisible.
In North Pembrokeshire we are totally dominated (oppressed?) by rocky outcrops. Each has a unique visible impact in the landscape. Carn Meini and its brothers can be seen from Narberth and all over southern Pembrokeshire, Strumble Head is littered with them and Carn Ingli has generated a series of books written by someone, whose name I can’t quite remember. All of these form part of a landscape that is widely acclaimed.

Yet, Craig Rhosyfelin is down in a dip and very hard to see from anywhere, even within 500m. Perhaps Castell Mawr might have a view of the site, but I’m not sure. There must be other inconspicuous rock outcrops in North Pembrokeshire, what is different about this one?
Dave

chris johnson said...

Just sorted some photos I took of Tycanol and what do you know? White crags...

)Don't want to see any diggers there).