ORIGINAL STORY AT WIRED.CO.UK
A team of geologists from Britain have pinpointed the exact quarry that Stonehenge’s innermost circle of rocks came from. It’s the first time that a precise source has been found for any of the stones at the prehistoric monument.
Robert Ixer of the University of Leicester and Richard Bevins of the National Museum of Wales painstakingly identified samples from various rock outcrops in Pembrokeshire, Wales.
For nine months the pair used petrography — the study of mineral content and textural relationships within rocks — to find the origins of Stonehenge’s rhyolite debitage stones. These spotted dolerites or bluestones form the inner circle and inner horseshoe of the site.
They found the culprit on a 65-metre-long outcropping called Craig Rhos-y-Felin, near Pont Saeson in north Pembrokeshire. It lies approximately 160 miles from the Stonehenge site.
READ COMPLETE ARTICLE HERE
In our film, Rupert very boldly asserts that the Stonehenge ‘bluestones’ were brought all the way from the Plesilis in Wales – that’s 135 miles as the crow flies. I think we based our certainty (in the face of what was, and still is, a hot topic of controversy) on the then recent discoveries made by the archaeologist Tim Darvill and the compelling argument he made for the stones having come from a particular ‘quarry’ in the hills. He had also put forward some quite convincing reasons as to why the builders this phase of Stonehenge would have gone to such lengths to transport the stones to Salisbury plain – to create it as a place of healing.
Be that as it may, we are very glad that further – and more concrete evidence – for the human transportation (as opposed to the glacial) of the bluestones has been provided by new research. Read on: (ORIGINAL ARTICLE AT ARCHNEWS)
It has been around for the best part of 5,000 years and still holds many mysteries but new research into Stonehenge has overturned established ideas about where some of the rocks came from.
Dr Rob Ixer from the University of Leicester Department of Geology has been studying the famous monument in collaboration with Dr Nick Pearce from Aberystwyth University and Dr Richard Bevins from the National Museum of Wales. Their particular interest was in the ‘bluestones’ which are not the iconic massive uprights and cross-pieces but smaller stones, weighing a mere(!) four tonnes or so each.
Stonehenge is not just a ‘stone circle’ but is structurally quite complex. There is an outer circle of massive ‘sarsen’ stones: uprights and cross-pieces, weighing anything up to 50 tonnes, collected from the Marlborough Downs about 25 miles away. Within this is a ring of bluestones – which predates the outer ring – then a horseshoe of sarsens, then a horseshoe of bluestones, then the central stone commonly referred to as the ‘altar stone’.
Even that’s not all because there are circular earthworks around the stone circle and all manner of stone detritus scattered within and around. Furthermore any investigation into Stonehenge is, of course, complicated by the number of stones which have fallen over or been moved – and complicated even more by the number which have been stood up again! As recently as the 1960s work was carried out to lift some of the fallen stones and set them in concrete bases which was not, strictly speaking, how they were held up five millennia ago…
Most of the bluestones are a type of rock called spotted dolerite, an igneous rock similar to basalt but coarser grained. It was in 1923 that the source of this rock was comprehensively identified as the Mynydd Preseli district, a range of hills to the east of Fishguard, meaning that each of these stones was transported about 240 miles.
However, while the spotted dolerite is distinctive, the origin of the non-dolerite bluestones, which include sandstone, silica-rich rhyolites and volcanic ejecta called basaltic tuffs, are harder to pin down. They have generally been assumed to come from the same location as the dolerites because, well, there are rocks like these in the Preseli Hills.
Rob Ixer and his colleagues analysed samples from the Stonehenge bluestones and found that they matched rocks in the Pont Saeson area just outside Newport. Having established a likely origin, they looked in detail at zircons within the stones. These are tiny crystals of zirconium silicate (about 150?m across) which have distinctive signatures of trace elements within them, such as hafnium, yttrium and scandium.
Long story short, the Stonehenge bluestones matched the Pont Saeson samples extremely closely whilst being markedly different from control samples of similar rocks collected elsewhere.
Rock and rollers
As so often in research, solving one outstanding mystery just raises more questions, in this case regarding transport.
The accepted view of how the bluestones got to Salisbury Plain is that they were transported overland due south to Milford Haven (probably using logs as rollers underneath the stones), then by raft up the Bristol Channel, then more log rolling to take them across to Stonehenge. Which is fine if all the stones started life at the top of the Preseli Hills because it’s all downhill from there.
But if some of the stones came from Pont Saeson, that’s low ground to the North of Mynydd Preseli. In other words, to get those stones to Milford Haven, our Neolithic building gang would have had to transport them over the Preseli Hills. Which seems, frankly, unlikely.
This research builds on work published by the team in 2006 which showed that the ‘altar stone’, previously believed to have originated at Milford Haven, came from somewhere else much, much further away.
This new, detailed chemical analysis of the stones actually has enormous (pre-)historical implications, overturning established theories about how this extraordinary creation was constructed. The mystery of Stonehenge continues…
READ ORIGINAL ARTICLE AT ARCHNEWS
How didn’t the Preseli bluestones get to Stonehenge? Ask your maths teacher.
ARTICLE REPRODUCED BY KIND PERMISSION OF EDWARD PEGLAR. YOU CAN READ THE ARTCLE AT HIS OWN BLOG HERE.
The late Neolithic temple (or whatever) of Stonehenge, on Salisbury Plain, England, is constructed from two types of stones. These are the large ’sarsens’ and the smaller ‘bluestones’.
The sarsens are large slabs of a kind of rock called silcrete. Silcrete forms at the bottom of a sandy soil profile under semi-arid conditions in landscapes where there is little erosion. Silcrete once covered much of southern England. Now it is limited to small pockets in North Wiltshire, such as Fyfield Down and Lockeridge Dene. Whilst a bit of a schlepp, Neolithic oxen and people probably could have dragged silcrete blocks the twenty miles from these locations to Stonehenge. Whether they did… ?
Bluestones are a series of varied rocks which have their origins, for the most part, in the Pembrokeshire peninsula of South Wales. They are largely igneous rocks (similar to lava) which have experienced a small amount of metamorphism due to their deep burial in the Earth before they were brought back to the surface by erosion in the last few million years.
How to move the stones
The People Method: people quarried or collected the bluestones in Pembrokeshire, then brought them to Salisbury Plain by boat across the sea and pulled them the last part of the journey.
The Ice Sheet Method: the bluestones were broken off by an ice sheet (a kind of very large glacier) in a cold period around 400,000 years ago. As the ice sheet grew it spread to Salisbury Plain, or at least quite near. When the ice sheet melted it left the bluestones behind as what are known as glacial erratics.
For and Against
The arguments for the People Method are that the bluestones are similar in size and many are similar in composition. Most appear to have come from the Preseli Mountains and there is little evidence for other Welsh rocks on Salisbury plain.
The arguments for the Ice Sheet Method are that the stones are, in fact, quite diverse and even include a stone from far to the east. If there were an ice sheet, modelling suggests that it would be travelling in about the right direction to end up at Salisbury Plain. Finally, and most importantly, it’s an absolute bastard to carry a stone as big as a bluestone across the sea by boat, let alone drag it the last bit of the way to Salisbury Plain.
Now from a pure sense argument I’m very much inclined to support the Ice Sheet Method. It involves a simple natural process. It doesn’t ask people to do something that they haven’t done at almost any other stone circle, which is use non-local stone. It doesn’t demand the impossible or our poor feeble ancestors in their hide covered boats. Whilst the romantic in me loves the epic effort involved, the engineer wipes oily stains from his hands and says “Naa, not a chance, mate.”
But… something is niggling. It’s a small thing, but I can’t help thinking about it.
Glaciers and ice sheets are renowned for their lack of selectivity when it comes to stone collecting. Due to their bigness and force they can pick up any size of rock, from grain to boulder, on their frozen undersides. And they do. Also, because ice sheets and glaciers behave like one solid (well sort of solid) mass they can carry the small and the large with equal ease.
But when the ice sheet finally melts it dumps everything, from grain to boulder, in the same place. Unlike many other geological processes, ice sheets are astoundingly untidy. However, this jumble of rubble will tend fit tidily in one thing though – the statistical bell curve of size distribution by weight.
To explain: An ice sheet could pick up a really large boulder but it wouldn’t pick up many because their aren’t that many around. But with smaller rock chunks the ice sheet picks up more of them because there are more of them. So the ice sheet will be picking up thousands of tiny pebbles at the same time as it picks up one large boulder. So you get a graph of number of stones vs stone size like the one in Fig.1.
On the other hand those millions of tiny pebbles, when you weigh them, don’t weigh that much. Medium sized stones are heavier but the largest boulders are much heavier still. So with that you get a graph of the weight of each stone versus its size, as shown in Fig. 2.
You can combine this information by calculating the total weights of all the sizes of stones collected. This involves multiplying the number of stones of each size by the average weight of each stone of any size. What you get is the bell curve graph of the weight of all the stones in one size fraction vs stone size like that shown in Fig. 3.
Ice sheets lose
“So what?” I hear you say. Well it’s simple. If you plot the distribution of all the bluestones or related stones on Salisbury plain it would have one big spike. This represents the large, and
relatively similar in size, bluestones of Stonehenge. There may also be a couple of small spikes for those smaller stones found in local burial sites. Importantly, it doesn’t look like the ice sheet dump distribution. (Fig. 4)
So perhaps someone cleared the rest of the bluestones away. There are two clear arguments against this.
If the bluestones were the biggest stones deposited by the ice sheet (A on the bell curve) then there should be absolutely thousands of smaller bluestones lying around. Their aren’t. Even if farmers had subsequently collected the medium size stones for use in walls and houses (and there’s no evidence of this) there would still be masses of pebbles lying around. Their aren’t.
Alternatively, if the bluestones were the “average” stone size (B on the bell curve), then there should be much larger stones lying around. There aren’t. Perhaps the Stonehenge builders chopped these larger stones into bits. Maybe. Whatever, even in this cas you’d still find a large number of pebbles lying around. You don’t.
Notably, the Neolithic long barrows of Salisbury Plain, which are older than Stonehenge, do not include one single one of these theoretical large bluestone boulders in their construction. For that matter they include very few small “Welsh” stones. Indeed, for that matter, they don’t include any sarsens either.
In North Wiltshire and the Coltswolds, similar long barrows always used local stone in their construction if it was available. It seems odd that the builders of Salisbury Plain perversely chose not to use the handy bluestones of Salisbury Plain. But that’s probably because they weren’t there yet.
People win … at least partly
Reluctant as I am to say it, it’s time to give up on the idea of a glacier reaching Salisbury Plain carrying its bluestone bounty. People must have been involved in getting the stones to Salisbury Plain. I have no idea how they did it but just fact that they did makes the people of Neolithic Britain pretty smart (and unbelievably determined), in my opinion.
But whether an ice sheet carried carried the stones part of the way from Pembrokeshire, maybe to the Somerset plain… hmm. That’s another argument that I’ll leave alone for the moment.
Atkinson, R.J.C. 1956 Stonehenge, Pelican, pp221
Burl, A. 2006 A brief history of Stonehenge, Robinson, pp368
John, B. 2008 The Bluestone Enigma – Stonehenge, Preseli and the Ice Age, Greencroft, pp160
ARTICLE REPRODUCED BY KIND PERMISSION OF EDWARD PEGLAR. YOU CAN READ THE ARTCLE AT HIS OWN BLOG HERE.
Stonehenge graces the front cover of National Geographic this month and is the subject of a 24 page article within. As usual, the photography and graphics cannot be faulted and if you’re a stonehead of any shape, sort or description it’s definitely worth a trip to the newsagents.
On the other hand, the article is at available here complete with photo gallery and other stuff if you’d rather not move from the desk right now.
I have not read the article as yet, so I’m not going to say anything more about it right now, except that there seems to be some interesting stuff about the bluestones coming from the Preselis and some interesting detail about the ‘Amesbury Archer‘ – to quote:
“The man was footloose and fabulously (my italics) rich. Isotopic tooth analysis shows that the ‘Amesbury Archer’, unearthed in 2002, grew up in the Alps. He was buried around 2400 B.C. with metalworking tools, a quiver of fine arrows, and the earliest gold yet found in Britain”.