Stonehenge’s massive central Altar Stone, a gift from Scotland. - Rob Ixer and Peter Turner

 

Stonehenge’s massive central Altar Stone, a gift from Scotland.

Rob Ixer and Peter Turner

Dr Rob Ixer indicates the Altar Stone

A new analysis of Stonehenge’s central six-tonne Altar Stone indicates that it likely to have come from Northeast 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 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, more so in acidic/granitic rocks than basic/basaltic ones. As they are chemically inert and quite resistant to weathering therefore

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

b) they can be a significant detrital component in clastic sediments such as sandstone (recognising that their radiometric age is usually earlier than that of the enclosing sediment retaining the date of their origins from those 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. 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 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 (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 Welsh origin on the banks of Milford Haven.

The Australian team was able to determine that within the Altar Stone these detrital grains had a range of ages suggesting their formation by a number of different igneous events. Some gave Ordovician ages of between about 470 to 444 million years ago, slightly older than the formation age of the sandstone. However, mixed in were also grains that were far far older, greater than 1000 million years which must have been eroded from much more ancient Archaean rocks. Their ages suggested that these eroded grains came from an ancient landmass/terrane Laurentia (now forming most of North America and Greenland) rather than from Ganderia, Meguma or East Avalonia terranes (these three from north to south, now forming the underlying basement to most of England and Wales and the Eastern coastal strip of North America). The very old Altar Stone  zircon dates matched igneous events in Laurentia; events that did not occur in (then) the far distant probable Gondwanan Ganderia, Meguma, or East Avalonia terranes. Hence, no Old Red Sandstone sandstones in England and Wales can carry, any Laurentian mineral grains, as English/Welsh Old Red Sandstone lithologies are essentially sourced from rocks with non-Laurentian basements.

Tectonic processes slowly (taking almost 100ma) brought these land masses together. Their join is now marked by the Iapetus Suture. This is a geological feature that runs roughly along the border between England and Scotland. It associated with the mountain building event 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 and is shown in figure 1. Parochially but importantly for this study it caused the abutting of Scotland against England; crucially only British Palaeozoic rocks north of the Iapetus Suture can show an abundance of Laurentian characteristics, hence the Altar Stone must be Scottish.

Although more sampling is needed 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 northeast Scotland. These rocks are quite unlike the Old Red Sandstone of the Anglo-Welsh basin and comprise a thick (2000m+) sequence of cyclical sandstones, limestones and shales deposited in a large (lacustrine) lake system. The basin is flanked on all sides by 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’s tractor beams, there are two alternative methods of transport for the Altar Stone: glacial dumping on Salisbury Plain or physical manhandling by Neolithic people, either overland or by boat. Despite vociferous, special and cyclical pleading from a lone living glacial proponent there is no evidence of any glacial erratics on Salisbury Plain, the nearest accepted glacial deposits that travelled from the west occur close to the Somerset coastline (but no further) and to the north of Stonehenge they are more than 100kms distant and carry no Scottish rocks. It has been anthropogenically moved.

This breath-taking result now raises many archaeological puzzles, notably how the Altar Stone was transported and more significantly why. These should be thoroughly formulated before thinking of supplying answers and choosing one option and are not questions for geologists to answer, for that would be hubris!, but for 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 transhipped/rafted from there along the Severn Estuary to Somerset and then punted down rivers to Salisbury Plain, together with the Preseli bluestones. Major 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 the provenanced igneous Preseli bluestones come from the northern slopes of the Preseli Hills not from its Milford Haven accessible southern slopes. The current belief is that the Bluestones were manuported overland along a proto- A40, and it has even been suggested 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 “A lithologically unremarkable, grey-green, micaceous sandstone is perhaps the secondmost famous Scottish lithic export in the world (after the Stone of Destiny/Scone) for it is stone 80 (numbering after Atkinson, 1979) namely the fallen and much travelled ‘Altar Stone’ from Stonehenge”.