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”.
