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:
http://brian-mountainman.blogspot.co.uk/2012/11/rhosyfelin-is-it-quarry-site.html
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.