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Monday, 12 January 2015

The Stonehenge bluestone lithology list

After all the recent geological work, what we have is a picture of multiple bluestone sources, many of which have been fixed with a degree of confidence, but with others still unknown.  The known lithologies, are itemised in the list below.  As Myris reminds us, there has been much confusion in the past about terminology, with old-fashioned terms being replaced by modern ones, and with some rock groups being amalgamated while others are split up and redefined.

Our attempts at understanding are further confused by our geological friends, who have more than once in the last few years placed rock samples into particular groups and have then changed their minds and re-classified them............

So let's try, not for the first time, to do a service to mankind by providing an up-to-date list. 

** There are 31 dolerite orthostats, of which 14 were sampled in 1991 and 2008.  Some are standing stones and some are stumps.  Some are spotted and some are unspotted.  Bevins, Ixer and Pearce (2014) analysed 22 Stonehenge dolerite samples, and suggested that they were clustered into three groups, with one sample petrologically distinct from all three.  Every one of the samples is unique, and the possibility must be entertained that every one has come from a different geographical location in eastern Preseli.

** There are five crystal vitric ash flow tuffs represented in the orthostat collection (stones 40, 48, 46, 38, 52c).  There seem to be four distinct types.  There is not much debris to match these in the Stonehenge debitage, but similar fragments are found in the great cursus field (Pitts, 1982).  Research is ongoing, but orthostats 38 and 48 and related fragments may come from north Pembrokeshire (Bevins et al, 2012).

** There are four other volcanic ashes -- stumps 32c, 33e, 33f, 41d. (These are not yet analysed.)

** There is one calcareous volcanic ash stump -- number 40c (This is not yet analysed.)

** There are two micaceous sandstone stumps -- numbered 40g and 42c.  There are also lumps of Lower Palaeozoic sandstone scattered about in the debitage -- the largest lump weighs c 8.5 kgs.  There are possible sources in the Lower Palaeozoic rocks of north-west Pembrokeshire.

** The Altar Stone (stone 80) is a greenish calcareous sandstone, probably from the Senni Beds of Carmarthenshire or Powys. (It is not from Milford Haven.)  There is some debitage related to this stone, but it cannot be established that the fragments came from the Altar Stone itself (Ixer and Bevins, 2013).

** In the debitage there are many fragments of rhyolites, some with planar fabrics -- and most matching the rock outcrops at Craig Rhosyfelin.  There appear to be three distinct groups of "rhyolites with fabric", all assumed to have come from the Pont Saeson area.  There may be a match with orthostat 32e or 32d (Ixer and Bevins, 2011).

** There are also some basic tuffs in the collection of fragments from the debitage -- two lithologically different types (Ixer and Bevins, 2013). Also from the Fishguard Volcanic Series?

** Other lithics in the stone collections from the debitage (eg. haematite (from the Reading Beds?), greensand, slate, limestones, Mesozoic sandstones and gabbros) appear genuine, and need further research.

Ixer and Bevins (2014) state that there are “about ten types of bluestone” represented in the orthostat / debitage samples, but they also show that these have nonetheless come from around 20 different locations.  It is estimated on the basis of the above points that there are at least 30 different rock types represented in the full "bluestone assemblage" -- especially when due respect is given in this count to cobbles as well as orthostats and flakes, for reasons frequently recited on this blog.  It should also be noted that the majority of the 43 bluestone monoliths/stumps at Stonehenge have still not been sampled and analysed.  The new work reinforces the idea that the Stonehenge bluestone assemblage is made up entirely of rock types from the west. 

Finally, a plea to the geologists -- if there is anything wrong with this list, please tell us about it, and give us your corrections and citations.  I'll publish them without fear or favour!


Constantinos Ragazas said...


If your impressive list of some 30 various stones found at Stonehenge are from a hypothesized glaciation of Salisbury Plain, shouldn't such also be found more widely spread on the Stonehenge surroundings? Why only confined within that circular area that defines Stonehenge?

Please post this along with your explanation!


Ps I have submitted an earlier comment two days ago under your "Mount Hood" post. Do you know what happened to it? Should I resubmit it?

Myris of Alexandria said...

No I think the pet rock boys would broadly accept that. It is a splitters list and the prb have been moving towards a more clumpers feel.
The ferret club offering for 2015 starring another Giza clumps some of the tuffs.
Like St Thomas I would like to thrust my hand into the void and see the petrography of the tuffs/calcareous tuffs in the buried orthostats. I suspect they would reduce to a smaller number,
About 10 lithologies seems about right.
It makes the transporting agency maddeningly eclectic if not eccentric. Matched only by the foibles of the Researchers and those that watch and comment and of course the New Acers and the whingers.

BRIAN JOHN said...

Kostas -- the stones and fragments are NOT restricted to the Stonehenge monument itself. They are quite widely scattered, as some of the old (and new) research shows. If we like, we can even include the Boles Barrow bluestone boulder......

And yes, I did receive your earlier post. I told you ages ago that I will not accept any further posts which talk about frozen lakes in crazy places and entirely unknown processes, based upon a misunderstanding of the local topography. That edict still stands!

Myris of Alexandria said...

Typo alert.
I wish to publically state that I am not anti-Acer, indeed some of my best plants are acers and I have five, one very large and beautiful, in my rear garden.
It is New Agers, mind you most are now drawing their wrinkly pensions by now so about as new as last week's chip paper.

BRIAN JOHN said...

Myris -- thanks for the confirmation that the list is broadly right, and that there is a great need for ALL of the orthostats and stumps to be resampled systematically. Keep on banging away at EH......

I too think that differences in listing lithologies comes down to whether one is a lumper or a splitter. Another matter is whether there is such dramatic variation in petrography within a very small area that four or five "different" lumps of rock might have come from quite a small outcrop or group of outcrops. So could four different rhyolite rock types all have come from Rhosyfelin, or could four different dolerites all have come from Carn Goedog? It seems to me that that is what you are suggesting......

We will agree to differ on whether the other smaller "inconvenient" stones (classed as packing stones, hammer stones and mauls or even as "adventitious" stones) are included in the list. Leaving those aside, are we dealing with about 10 lithologies and therefore -- if you are so inclined -- 10 quarries? As I have said before, I have a few problems with your interpretations -- for example where several types of debitage might be assumed to have come from the same destroyed orthostat.....

It seems to me that there are far more than 10 basic rock types, if one includes your various groups of dolerite and rhyolite (at least 3 of each and some anomalies) and then the different tuffs, other volcanic rocks, old sandstones and newer sandstones. So I'm going to stick to about 20 rock types / provenances for these various rocks that you have looked at, for reasons I have enumerated in my detailed considerations of your papers. Maybe the real answer is "around 15" provenances represented in the orthostat and debitage groups ??!!

Battle on -- all very intriguing.

AG said...

Alex Gee

I have difficulty accepting the quarry theory although because of the way the joints and bedding are, I can quite understand the ease with which a 2-4ton bluestone could have been quarried.

I do however wonder how the quarrying of a 50-60 ton block of sandstone for the altar stone was achieved?

BRIAN JOHN said...

Alex -- where did you get that figure of 50-60 tonnes for the Altar Stone? The conventional wisdom is that it weighs between 6 and 7 tonnes. Does anybody have a better estimate?

chris johnson said...

On the splitter or clumper distinction it is worth pointing out that the sites under discussion are a couple of miles apart at most.

BRIAN JOHN said...

Very true, Chris. The Carngoedog spotted dolerites could have come from a dolerite sill outcropping across a couple of miles of countryside; and the foliated rhyolites could have come from a patch of territory including Rhosyfelin and Pont Saeson. So you could argue that if there were determined quarrymen around, they might have quarried in several neighbouring places while they were about it. On the other hand, if you are a glaciologist you might argue that the glacier bed conditions that allowed entrainment from either Carngoedog or Rhosyfelin would also have applied to wider patches of the landscape in the vicinity of each..........

TonyH said...

As most of the folk on this Blog are well aware, the terrain of Salisbury Plain is predominantly occupied by The British Army. What we could do with is a project to involve some of H.M. Forces and the Salisbury Training Areas' Archaeologist, Graham Brown I think that is.

This might involve the National Grid Referencing of sundry exotic rocks noticed on the Plain. This recording by NGR's would be the first step towards analysing their geology. Hopefully, rocks with a similar provenance in the North Pembrokeshire area would thus be identified.

Since the Stonehenge element of the Avebury & Stonehenge World Heritage Site attracts over a million Visitors each year, surely such a trawl of the Plain would be justifiable. I have no doubt "Stonehenge" receives scores of millions of "hits" every year from all over the Globe. Marketing of matters Stonehenge is self - propelling. The Audience is increasingly knowledgeable and educated, and would welcome such a systematic investigation.

AG said...

oops my mistake


BRIAN JOHN said...

Alex -- how close are we to seeing the results of your work on those anomalously large conduits beneath the Mendips? Is there anything in print thus far?