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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.


Hugh Thomas said...

During the "alleged Human transport period " would this piece of rock have been sitting proud of the surface i.e on the rubble , sorry * cough* rails below it ? I seem to remember rather a lot of soil etc gradually being removed from around it , smaller pieces of rock that made it look like it was the largest part of a collapse from the outcrop. It may be sitting proud of the surface now but only a few years ago it was buried.... There is at least one other sizeable piece of rock that looks very much like this type in the area between the enclosure near Carn Goeddog and Carn Goeddog itself..If I knew how to post a photo here I could provide one...

BRIAN JOHN said...

Hugh -- yes, before the dig started in 2011 this big stone was hidden beneath the surface, embedded in spoil and rubble which has been selectively removed since then.

There are lots of bluish rhyolites on Preseli, but the one at Rhosyfelin can only really be identified by very careful geological work. For example, Carn Alw is made of rhyolite.

Was the stone visible in the Neolithic? I suspect that the top of it might have been visible, but we need to see the radiocarbon dates to get any further guidance on where the contemporary ground surface was. I have discussed this a lot in earlier posts.

Evergreen said...

Another probably embarrassingly basic question for Myris or yourself Brian, but do these stones 'split' cleanly? Is it possible the 'proto orthostat' could have been quarried with the intention to split it into smaller sized blocks, more suitable for a SH bluestone? Just a bit of thinking out loud.

I'm struggling a little with the weight miscalculation too. It seems too basic an error. Is something being missed or overlooked here?

chris johnson said...

Excellent and totally credible analysis of how the Rhosyfelin orthostat came to be "quarried" - by the hand of God, no less.

It has always looked to me like a picnic table.

BRIAN JOHN said...

Evergreen -- Myris can give us the geology specialist view, but as far as I can see this rock does not split cleanly like slate. The foliation plane fractures seem to me to be discontinuous, which makes life very complicated. You might get slaps with a "clean face" up to 3 or 4 m long, but not any longer, I reckon. If you were to try and slice up the big "proto-orthostat" slab at Rhosyfelin you would not get two slightly thinner pillars -- you would get a frightful mess. There are too many fractures running through and across the rock.

BRIAN JOHN said...

By the way, I am still very much open to the idea that Neolithic man might have found something useful here in the way of cutting edges and flakes. So if the archaeologists in the famous forthcoming comprehensive paper choose to argue that this was a Neolithic knife factory, and if their evidence is convincing enough, and if that fits in with the Holocene stratigraphy, I would happily go along with that.....

Myris of Alexandria said...

I think we are seeing the explanation for all the post BA Cryf debitage at SH.
The proto-orthostat was only sampled once by geologists(I think) and not from the top face but rather unobtrusively, the obvious damage is from souvenir hunters, New Age nutters getting their rocks off (or angry geomorphologists trying to break up the artefact?) See Mike Pitts showing the same thing at SH.
My guess, New Age Rockers,
Did you see that Judd's pamphlet made over a ton on ebay.

BRIAN JOHN said...

Myris -- re your comment "I think we are seeing the explanation for all the post BA Cryf debitage at SH....."
Don't follow you. Explanation please?

BRIAN JOHN said...

Re the damage on the orthostat, geomorphologists would never do such a thing. Angry archaeologists, more like. And that damage pales into insignificance compared to the mayhem caused by their digging machine in the upper part of the site.....

Myris of Alexandria said...

It was a complex way of saying both the Cryf debitage found throughout the Stonehenge landscape and the bits knocked off the proto-orthostat were formed by souvenir hunters wanting magical gifts.
Give it a few millennia and the area around Cryf will be full of discarded
Proto orthostat debris.
I did not think for a moment that geomorphologists would vandalize such an important artefact.

BRIAN JOHN said...
This comment has been removed by the author.
BRIAN JOHN said...

So the big stone is a "proto-orthostat" and an "artefact"? Codswallop. A big stone is just a big stone, as any fule kno (as Nigel Molesworth would say............)

Myris of Alexandria said...

Brian it is like shooting fish in a barrel.

BRIAN JOHN said...

Am I supposed to feel sorry for the people who put the fish in the barrel, and who drew attention to them with such great enthusiasm over the last 4 years?