Some of the ideas discussed in this blog are published in my book called "The Bluestone Enigma" -- available by post and through good bookshops everywhere. Bad bookshops might not have it....
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Tuesday, 26 May 2015

Where did the Rhosyfelin "proto-orthostat" come from?

We have had some discussion about the origin of that big rock slab Rhosyfelin.  There is some confusion out there.  Prof MPP and his colleagues think that it is now about 15m from its source.  That does not mean 15m from nearest part of the rock face, but the confusion is understandable, and the archaeologists should have made things clearer than they have.  Here is a pic of the relevant part of the face:

You can see the slab with the bucket on it.  MPP clearly does not think the stone has come crashing down from the high pinnacle at the top of the photo, but from one of the "fresher looking" exposures at the extreme right of the photo.  They would be about 15 m away.  He thinks it was quarried from there, and then manhandled or manoevred to its present position by being dragged along on these famous stone "rails"  -- on the subject of which we have spent enough time already......

Myris has informed us that only one rock sample has been taken from the slab itself, but maybe 20 or more have been taken from the rock face -- presumably with a view to establishing exactly where the slab came from.  We can also assume that the provenance has been established to the satisfaction of Drs Ixer and Bevins, who will be joint authors when the big paper is published in the autumn.  We'll see whether the petrographic evidence stands up -- there are certain aspects of it thus far that I have questioned on this blog!

Could the stone have got from one of the recesses on the right of the photo to its present position without human involvement?  I wouldn't see a problem with that -- the gradient is quite steep, and many stones in the neighbourfood have moved a long way from their places of origin, with the assistance of glacier ice, meltwater and periglacial processes.  There are some big slabs quite far out onto the valley floor, near the NE limit of the area excavated thus far.

Let's see the colour of the evidence......

Monday, 25 May 2015

Other elongated pillars out on the valley floor at Rhosyfelin

I recall from various talks that the big flat-topped stone at Rhosyfelin is deemed to be unique because it is too far from the rock face to be in a 'natural' position, and because its alignment approx parallel with the rock face cannot be explained except as a result of human interference.  Somehow the word has got around that it is a very long way from the rock face; not at all, since one end is just 4 m from the rock face and the other end about 5.7 m away. 

As for uniqueness, the site is in a proper mess as a result of the excavations higher up the little meltwater channel; but we can still see three other elongated slabs that are approx parallel with the rock face and some distance away from it.  The one closest to the bend in the channel is a very rough and irregular elongated slab about 2.40 m long and about 3 m from the rock face. 

A second slab, of which we can only see about 1.70 m exposed, also seems to be aligned parallel with the rock face, and is 4.3 m away from it.

A third slab, about 5m further down the valley and also on the valley floor, is 2.20 m long at least.  It seems to be very heavily weathered, maybe because it has been exposed at the ground surface for many thousands of years while the 'proto-orthostat' in the main dig site has been buried beneath solifluction materials. Again it is aligned approx parallel with the rock face, and it is 4 m from the face. 

The archaeologists might argue that these three other elongated blocks were all aligned down-valley and parallel with the rock face because they were all carefully chosen as orthostats and were on their way down to a sort of assembly point or dressing area down on the main valley floor close to the tip of the spur.  I don't buy that at all -- these blocks are highly irregular in shape, and to me they show that some rocks -- just some, out of hundreds -- have slid or rolled into positions well out onto the valley floor either with the assistance of snowbanks or else because of the sheer velocity of their falls, from previous positions high up towards the ridge crest.

The crack of doom on the Rhosyfelin pseudo-proto-orthostat

I have had another look at that crack of doom which runs across the top of the famous stone at Rhosyfelin.  It isn't, after all, related to the two dipping fractures which are visible on the south face of the block, but when one looks carefully at the north face one sees that the fracture runs deep into the heart of the block, with a depth of 27 cms.  That's almost half the depth of the block.  It seems to terminate at another foliation plane which we can see as a little projecting ledge.  Yet again this is evidence that the fractures at Rhosyfelin are 'local' and discontinuous.  But this does mean that the strength and stability of this slab is seriously compromised, and I am more convinced than ever that it could never have been moved anywhere without breaking in half.

A place of beauty.....

Every now and then we need to remind ourselves that Rhosyfelin is a place of extraordinary beauty, tucked away in its wooded gorge, and with birdsong echoing through the woods.  This is the south/east side of the ridge this afternoon.  A nice contrast with the terrible archaeological mess  -- in more senses than one -- left by the archaeologists on the other side of the ridge.....

Rock mechanics at Rhosyfelin

Here again is Adam Stanford's fantastic Gigapan image of the Rhosyfelin rock face, scree bank and "proto-orthostat."  It helps to give the context for the previous post.

If we look at the big stone, the right (fresh) end was originally at the bottom of the standing pinnacle, resting on a jagged fracture or set of fractures,  and the  left (weathered) end was the one at the top, exposed to the atmosphere.  The bottom (buried) face of the stone would originally have been positioned approximately along the rock face as we see it today, facing NW.

By going to the source of the image you can zoom in very closely and examine things in detail.  Thoroughly recommended!

ADDED 25 May;  Here is that rather interesting diagram, courtesy Phil Morgan, showing the two mechanisms of rockfalling that we should be thinking about.  Although the rockfall debris has accumulated in an almighty jumle of blocks, slabs and fragments apparently at random, there are some signs of order.  One of the most interesting is that the weathered -- top -- ends of the pillars and pinnacles that have fallen tend to be furthest away from the rock face.  That means that the lateral fractures that run along the face, and which are at right angles to the foliation planes and the foliation surface fractures, have acted as ledges or hinges when the slabs have toppled over and outwards.

The top part of the rock face appears to have evolved as in Fig 4. and the lower part of the face might have evolved by sliding, as in Fig 5.  In reality the whole scenario has been far more messy, since accumulated banks of rockfall debris have provided a rough surface which, when snow-covered, could have acted to assist in the movement of blocks well away from the rock face.  Rolling and sideways sliding are also feasible.

Sunday, 24 May 2015

The famous Rhosyfelin "proto-orthostat" -- the most useless bluestone ever?

The pseudo-proto-orthostat at Rhosyfelin, carefully protected from visiting tourists and marauding geomorphologists......

Largely as a result of media hype generated by Prof MPP and his colleagues, the large stone at Rhosyfelin which was first unearthed in 2011 has gained considerable notoriety.  It is known to thousands of faithful followers as "the bluestone that never made it to Stonehenge" or as "the bluestone that confirmed the presence of a Neolithic quarry at Rhosyfelin."

On p 286 of his 2012 book, Prof MPP refers to "an ancient ground surface", "hammer stones" and "a monolith left behind when quarrying ended" as showing that this is "the Pompeii of prehistoric stone quarries."  All a bit over the top, you might think.  But warming to his task, the good professor then says of the discovery of the stone:  "This was the smoking gun; the game was up for anyone still trying to argue that the bluestones were not quarried in Preseli during the Neolithic, and then taken to Wiltshire....."

In the midst of all this excitement, let's take a deep breath and take a look at the famous stone, leaving aside all the  speculation about fulcrums and rails, pillars and wedges and so forth.

The facts about the stone

As I have already indicated, it is made of local blue foliated rhyolite.  It is over 3m long and is probably twice as heavy as Prof MPP suggests -- at around 8 tonnes.  So it is much larger than any of the Stonehenge bluestones, and on that basis alone one might wonder how on earth our heroic Neolithic ancestors planned to get it out of the Brynberian Valley and all the way to Stonehenge, all in one piece. That would have been a Herculenean task, even if the rock was solid and robust, which it is not......

So let's take a look at it in detail.  We can of course only see five of the six sides, since its bottom face is hard to examine without risking life and limb.  The elongated, roughly rectilinear stone lies approximately E-W, so we can refer to the faces as "E,W, N, S and top" without any great risk of confusion.  The E and W faces are the "ends" of the rock and the N and S faces are the elongated "sides".

Here is a photographic record: 

The relatively fresh top and weathered N face of the slab.  The top surface is conformable with the foliations in the rock, and coincides with a foliation plane fracture.  The pitted surface, with foliation "ridges", is typical of the exposed surfaces on the crest of the rhyolite ridge today. One might speculate that the top was on the "inside" of the crag, protected from intensive weathering, when the slab was in its original position. If one looks carefully, one can see a fracture running across the top face of the rock, approx halfway along its length.

Close-up of the top face and the S face -- both relatively fresh and thus protected from weathering prior to the rockfall that brought the slab crashing down.  The tip of the pen shows the fracture that threatens to split the rock into two; it may be related to the dipping parallel fractures that we see on the S face.

The corner between the E face (end) at the right edge of the photo, and the elongated S face.  We see intense weathering features in the foreground, and in addition a weathered section of a substantial foliation plane fracture running along the S face.  Since the E face was the top of the rock when it was in its original position, the weathering (involving the rotting of at least some of the minerals) of this outcropping fracture plane is a consequence of concentrated rainwater running directly down the side of the slab, presumably in a fissure.

A close-up of the weathered E face of the rock, showing outcropping foliations, picked out by weathering over a very long period of time. This must originally have been the top of the pinnacle when the rock was in situ on the ridge.  

A close-up of part of the relatively fresh W (end) face of the rock, showing abundant parallel fractures unrelated to foliation planes.  Some of these fractures can also be traced on the top of the slab. The chunks knocked off have probably provided samples for the geologists to look at, and are nothing to do with Neolithic quarrymen.

[Note added on 25 May:  I suspect that the face that is now the "bottom face" of the recumbent stone was originally the one aligned approximately with the rock face as we see it today, ie coinciding with the sub-planar fracture surface which is so much discussed.  When it was in its original position, the faces were originally aligned as follows:
 E end was at the top of the pinnacle
W end was at the bottom
Top of the recumbent stone was facing SE (embedded in the crag)
Bottom of the recumbent stone was facing NW (probably exposed)
S face was facing NE (probably protected)
N face was facing SW (probably exposed)
The stone does not appear to have rolled into position.  Rather, it has fallen and slid, twisting in the process so that the bottom of the broken pinnacle now rests further from the rock face than the top end.  This is a three-dimensional jigsaw puzzle!  The archaeologists will of course say that the stone was manipulated into its current position by the famous Neolithic quarrymen, since it is too far from the face to be in a natural position.  I think it much more likely that snow, glacier ice or meltwater have played a role, given the context within which the stone is found.]

[Further Note added:  we had an interesting discussion in 2012 -- thanks to Phil Morgan -- on the rock mechanics of the site, reported here:
The diagrams are particularly helpful.  There were also lots of useful comments.]

In short, this old pinnacle is in a very fragile state, and it is a miracle that it has survived without breaking in two.  There are multiple fractures exposed on the flanks of the slab, unrelated to the foliation planes.  These render the slab completely unsuitable for long-distance transport or even for use in the immediate neighbourhood, as the slightest percussive impact might result in either breakage into two more or less equal portions, resulting in the loss of its "pillar" shape, or in the spalling off of large irregular chunks of rock such as those we see today beneath and on the flanks of the rock after the clearance work of the archaeologists.

In other words, this is a completely useless orthostat, since it has no integrity or inherent internal strength.  So it is a fantasy to pretend that it was in some way "valuable" or worth any expenditure of time and effort by our heroic ancestors.

Craig Rhosyfelin -- a seriously cracked-up crag

 Part of the rock face exposed during the archaeological dig.  If you get close enough to it and look along it, it certainly looks very flat, and one might think of it as "unnatural" in some way, and therefore a quarry face.  However...... things are not so simple.

I was intrigued the other day with Myris's comment that the "quarry face" at Rhosyfelin is  "a large planar surface that appears not to be structural in nature (absence of slickensides) and is difficult to explain if totally natural.  Not a fault plane nor master joint plane.........."

I commented on it at the time,  and have been back to take another look.  I am now more than ever impressed that the whole crag is so seriously riddled with multiple fractures running in all directions that it is a miracle that any of has survived the ravages of time and the Ice Age.  There are many posts touching on this subject, for example:

I published this rough illustration of the fractures in two dimensions.  The third dimension is the fracture plane(s) running along the face itself.

Things are actually a great deal more complicated still.  Let's concentrate for a moment on the "large planar surface" which might also be called a "sub-planar foliation surface" since it coincides with the foliations in the rock which have been given so much attention by the geologists.  It is not actually a single sub-planar surface at all, but a rock face made up of fragments of multiple surfaces.  Some parts of the rock face project at least 1.5 m out beyond other parts -- so we have recesses and projections when we examine the site carefully.  When we look at the tip of the spur, we see some of these surfaces end-on:

 Close-up of the tip of the spur.  Here we can see the steep dip of the "sub-planar foliation fracture surface" on the right, and then behind it, across the photo from right to left, a further 6 (at least) other fractures, more or less parallel.

 A close-up of another part of the spur tip.  There are multiple fracture surfaces coinciding with the foliation planes, some of them 10 cms apart and others less than 1 cm apart.

Looking in the other direction, down the slope from near the right edge of the photo with the blue and orange lines on, we get another cross-section of what is going on in the interior of the crag.  Again, we see multiple fractures related to the foliation planes in the rock.  There are at least ten pronounced fractures, and many more hairline cracks.

Now here's a question for the geologists, which was raised in a letter to me from Barry.  Why do these foliation plane fractures occur, in some places very closely spaced, and in others widely spaced?  Is it something to do with crystal alignment or arrangement, or something to do with cooling surfaces?  Rob and Richard, advice please?

So much for the fractures related to the foliation planes.  Then we get to all the other fractures.  I started to record them by compass directions, but gave up because there are so many of them, some traceable over 50m or more, undulating up and down along the rock face, and others restricted to just one semi-detached slab.  The message is that hundreds of these fractures are discontinuous and localised, developed in response to quite localised stresses.  So I would agree with Myris that there is no master fault plane or joint plane here, and certainly no bedding plane since we do not have any identifiable "beds" of rock as we do on sedimentary rock exposures. These are, after all, metamorphosed volcanic rocks, and the main processes of deformation shortly after their emplacement would have been related to cooling and contraction and maybe loading beneath later igneous and sedimentary accumulations.

So here are two more pics just to confuse things even further.

 Another exposure on the rock face, showing one long and undulating fracture running from left to right, and a series of steeply plunging fractures which are not parallel.  Just to the left of centre, there is a V-shaped arrangement of fractures.

This is the most complex arrangement of fractures on the whole rock face.  In a word, wholesale chaos in three dimensions......

Back to the original point.  This is certainly a highly complex rock face -- worth a structural geology field trip on its own account -- but I do not agree that it is difficult to explain if totally natural.  I see nothing whatsoever on the rock face to suggest human intervention, apart from the cosmetic work recently undertaken by the archaeologists. 

What intrigues me rather more is the role of geomorphology in all of this.  I mentioned the possible role of cooling, contraction and loading beneath later Ordovician and later rocks, but the more I think about it the more convinced I become that the rock wall is actually a meltwater channel wall, swept clean by turbulent meltwater flowing over the col at the base of the spur and then downslope in a subglacial channel.  I'm also quite attracted by the idea that there may have been actual fracturing of the crag as a result of ice loading during the Anglian Glaciation and -- to a lesser extent -- during the Devensian or Last Glaciation.  This process of compression / tension / pressure release might well have been a powerful factor in explaining the entrainment of blocks into the base of the over-riding glacier.  Another possibility is that we are looking at the results of unloading or pressure release linked to ice wastage.  We see evidence of this happening in many glacial environments today -- I have discussed this before in the blog.

Quarry face?  I suggest that we just forget the idea.