New Mineral analysis indicates Stonehenge’s Altar Stone originated from Scotland

[aurelianus]

https://www.megalithic.co.uk/article.php?sid=2146415628
Submitted by Andy B on Wednesday, 14 August 2024
By Dr Rob Ixer and Prof. Peter Turner, two key Altar Stone researchers. A new analysis of Stonehenge’s central six-tonne Altar Stone indicates that it is likely to have come from north east Scotland, at least 750 kilometres away from its current site in Wessex, and perhaps more than 1000 kilometres if it travelled following the present-day coastline. Plate tectonics and precise radiometric age dating are the keys to this discovery.

Almost exactly 60 years ago a series of papers convinced the geological world that the disputed idea of continental drift was correct, with the concept of plate tectonics (a continual process of crust being created and destroyed) being the mechanism for this movement. In the succeeding years the movements of landmasses since the Proterozoic (2.5 billion years ago) have been and are being reconstructed (mainly based on palaeomagnetic data) to show cycles of break-up, coalescence and recombining of super-continents.

Zircon, rutile and apatite are small rare minerals found in igneous rocks. They are found more in acidic/granitic rocks than basic/basaltic ones. Zircon, rutile and apatite are chemically inert and quite resistant to weathering, so

a) they are ideal for obtaining radiometric ages to date their creation within their parent body of igneous rock;

b) they can be a significant component in the make-up of clastic sediments such as sandstone - bearing in mind that their radiometric age is usually earlier than that of the enclosing sediment, and retains the date of their origins from those original landmasses.

This study, published in Nature, analyses the chemical and isotopic composition of detrital zircon, rutile and apatite grains within the Altar Stone – an assumed Palaeozoic Old Red Sandstone - to determine their isotopic ages and so help to pinpoint the sediment’s origin.

Two samples of the Altar stone were used - one excavated in the 1920s by Lieutenant-Colonel William Hawley and a sample known as Wits 277 - a thin section of a piece directly knocked off the Altar Stone in the 1840s and labelled as such. Both are from the Salisbury Museum collection. You can read more about these here in Current Archaeology magazine.

An earlier study in 2023, by a team of eight researchers led by Richard Bevins and published in the Journal of Archaeological Science demonstrated that the Altar Stone was not Welsh but probably from northern England and perhaps Scotland: the authors had noted (in 2020) the presence of a few extremely old and so intriguing zircons.

Quite by chance researchers in Australia contacted the British Stonehenge research group and were invited to confirm the presence of these old zircons and to provide additional data. This they have done - with breath-taking results based on uranium-lead and lutetium-hafnium age radiometric dating, derived from these zircon, apatite, and rutile mineral grains.

It is ironic that every time new researchers are asked to join and contribute to the Altar Stone studies, from the original duo of ourselves (Rob Ixer and Peter Turner) in 2006 to over 12 contributors by 2024, the Altar Stone appears to move further away from its originally proposed origin on the banks of Milford Haven in South Wales.

The Australian team was able to determine that within the Altar Stone these detrital grains have a range of ages suggesting their formation by a number of different igneous events. Some give Ordovician ages of between about 470 to 444 million years ago, slightly older than the formation age of the sandstone. However, also mixed in are grains that are far, far older - greater than 1000 million years which must have been eroded from much more ancient Archaean rocks.

Their ages suggest that these eroded grains originated from the ancient landmass/terrane of Laurentia - which now forms most of North America and Greenland - rather than from Ganderia, Meguma or East Avalonia terranes, which from north to south, now form the underlying basement to most of England and Wales and the Eastern coastal strip of North America.

The very old dates from the zircon in the Altar Stone matche igneous events in Laurentia; events that did not occur in the Gondwanan, Ganderia, Meguma, or East Avalonia terranes which were then thousands of miles away.

This means that no Old Red Sandstone sandstones in England and Wales can carry mineral grains from Laurentia, as English/Welsh Old Red Sandstone lithologies are essentially sourced from rocks with non-Laurentian basements.

Tectonic processes slowly (taking almost 100 million years) brought these land masses together to their current locations. Their join is now marked by what is known as the Iapetus Suture. This is a geological feature that runs roughly along the border between England and Scotland. It is associated with the mountain building event called the Caledonian Orogeny and marks the collision site between Laurentia and Ganderia (with Meguma and East Avalonia) due to the closure of the Iapetus Ocean.

Importantly for this study, this event caused the incredibly slow, and continuing collision of the rocks of Scotland with England. Crucially, only British Palaeozoic rocks north of the Iapetus Suture can show an abundance of Laurentian characteristics, these results indicate that the Altar Stone has to have originated from Scotland.

Although more sampling is needed to identify the source more exactly, these extraordinary results - using all of the age dates - suggest that the Altar Stone most closely matches Old Red Sandstones from the Orcadian Basin, which includes both the Orkney and Shetland Islands plus much of north east Scotland.

These rocks are quite unlike the Old Red Sandstone of the Anglo-Welsh basin and comprise a thick (2km wide) sequence of cyclical sandstones, limestones and shales deposited in a large (lacustrine) lake system. The basin is flanked on all sides by the Laurentian basement, and the sediment was locally sourced, quite consistent with the radiometric dating of the zircons and other minerals.

Putting aside Merlin’s magic or space alien tractor beams, there are only two alternative methods of transport for the Altar Stone from Scotland to Stonehenge - the dumping by an ancient glacier on Salisbury Plain, or physical manhandling by Neolithic people, either overland or by boat.

Despite vociferous, special and cyclical pleading from a lone proponent of the glacial hypothesis, there is no evidence for any glacial erratics on Salisbury Plain - the nearest accepted glacial deposits travelled from the west, and occur close to the Somerset coastline, but no further than that. And to the north of Stonehenge evidence of glacial action is more than 100km distant and carries no Scottish rocks. It must have been anthropogenically moved.

This breathtaking result now raises many archaeological puzzles - notably how the Altar Stone was transported from Scotland, and more significantly why. This should be thoroughly formulated before thinking of supplying answers and choosing one option, and are not questions for we geologists to answer, for that would be hubris! It is over to the archaeologists to solve.

Not Milford Haven but perhaps Scapa Flow.

Here it might be useful to be reminded of past and present assertions about the movement of the other Stonehenge bluestones from their outcrop origins to the Wessex circle.

In 2006 Ixer and Turner with absolute confidence wrote: “A lithologically unremarkable, grey-green, micaceous sandstone is perhaps the most famous Welsh lithic export in the world for it is stone 80 (numbering after Atkinson, 1979) namely the fallen ‘Altar Stone’ from Stonehenge”.

The prevailing almost century old belief was that the Altar Stone and a companion sandstone now known as the Lower Palaeozoic Sandstone were collected from the shores of Milford Haven (the exact outcrop for the Lower Palaeozoic Sandstone on the shore-line was identified by Sir Kingsley Dunham the leading geologist of the day). They were said to have been shipped or rafted from there along the Severn Estuary to Somerset and then punted down rivers to Salisbury Plain, together with the Preseli bluestones. Claimed proofs of this route included dropped/lost bluestones found on Steep Holm and rumoured orthostats resting on the bottom of Milford Harbour.

In the two decades since, piece by piece, detailed petrographical and geochemical work has shown all this to be unlikely. The Steep Holm rocks are nothing like any rock associated with Stonehenge or even Salisbury Plain, they may even be ship’s ballast.

The Altar Stone and Lower Palaeozoic Sandstone origins are separate, and neither is from the Milford Haven area - the Lower Palaeozoic Sandstone is not Devonian in age but older (Ordovician perhaps Silurian) and comes from north or northeast of the Preseli Hills; and it is now clear that the igneous Preseli bluestones come from the northern slopes of the Preseli Hills not from its southern slopes accessible from Milford Haven.

The current proposal is that the Bluestones were transported overland along a proto- A40, and it has even been suggested the transportation was accompanied by a succession of communal celebrations - something more difficult to do on the high seas.

How ironic it would be, were the same process needed to be repeated when dealing with the transhipment of a Scottish Altar Stone.

In 2024 a wiser Ixer and Turner suggest with some confidence that “A lithologically unremarkable, grey-green, micaceous sandstone is perhaps the secondmost famous Scottish lithic export in the world, after the Stone of Destiny/Scone. This is stone 80, using Atkinson's numbering from 1979, namely the fallen and much travelled ‘Altar Stone’ from Stonehenge”.

The incremental search for the Altar Stone's origin

We have detailed a a retrospective of 21st century Altar Stone research that you can follow here - it highlights the incremental ‘journey’ taken by both researchers and the stone as they moved conceptually away from Wales.

This is a genuinely shocking result, but if plate tectonics and atomic physics are correct - and there is no indication they are not - then the Altar Stone is Scottish. What we don't know is how or why it travelled the length of Britain to its current location as a significant part of the Stonehenge monument.

This beautiful bit of 21st century science is truly breathtaking and will send seismic shocks through the bedrock of current interpretations of the British Neolithic.

Now that the geologists have done the heavy lifting (if you will) - it is over to the archaeologists...

Dr Rob Ixer is a specialised mineralogist by training, having worked on metal ores. He graduated from Manchester University with a BSc (Hons) in geology with subsid. in Northwest European Prehistory. He stayed at Manchester for his PhD on "controls on mineralisation in Derbyshire".

Rob taught mineralogy/petrography for decades at Aston University, then at Birmingham and Leicester universities, but retired in 2000. He is now an honorary senior research fellow in the Institute of Archaeology at UCL and a Fellow of the Society of Antiquaries of London.

Prof. Peter Turner is a geoscientist with interests in sedimentary mineralogy, palaeoclimatology, and palaeomagnetism. His Ph.D was from Leicester and he went on to lecture at the Universities of Aston and Birmingham. He was a pioneer in palaeomagnetics - awarded a DSc by the University of Wales for research on the magnetisation of sedimentary rocks. His current interests (apart from the provenance of the Altar Stone) include remagnetisation processes in sedimentary rocks.