Tag: megaliths

Stonehenge: the geologists’ last word?

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A sunset at Stonehenge

The great megalithic structure is the centrepiece of a vast ritual landscape on a 780 km2 plateau known as Salisbury Plain, underpinned by Cretaceous limestone: the largest remaining area of calcareous grassland in northwest Europe. The earliest sign that the Plain was used for ritual purposes dates to ten thousand years ago (8,000 BCE), when Mesolithic hunter gatherers erected large wooden posts to define by an E-W line the Sun’s rise and setting at the equinoxes. The area seems to have been continuously populated until 4,000 BCE when the first Neolithic farmers settled the Plain and began building burial mounds (barrows) to celebrate notable individuals and families.

The Stonehenge monument began as a circular cemetery around 3,100 BCE. Its development to the astonishing structure that remains largely intact today occupied the Neolithic populace and succeeding Bronze Age immigrants for the next 1,600 years. This involved setting up and then repeatedly shuffling around several kinds of boulders or megaliths. The first, around 2,600 BCE, were 2 to 3 tonne blocks mainly of igneous rock (the ‘bluestones’), now known to have originated from outcrops of Ordovician volcanics in Pembrokeshire about 230 km to the west. Next to arrive was a 6 tonne grey-green sandstone slab, now lying flat (hence its being named the ‘Altar’ Stone) beneath a fallen, far bigger megalith,. Once thought to be of Welsh provenance – in the Brecon Beacons 150 km to the west – the Altar Stone is now beyond a shadow of doubt to have come from Devonian strata in northern Scotland, possibly Orkney. The final erection of 30 truly enormous ‘sarsens’ to create Stonehenge’s signature circle and inner ‘horseshoe’ of vertical slabs capped by lintels took place between 2,600 to2 400 BCE. Weighing up to 50 tonnes, the sarsens are locally derived from remnants of Lower Eocene (~55 Ma) sands cemented by chemically precipitated silica (SiO2) that once covered much of southern England.

After 1,600 BCE, serious fiddling with the various stones, the bluestones in particular, ceased. The monument may have remained in some form of use during the Iron Age: it could hardly have been ignored. The first record of antiquarian interest is from the late 17th century and continued sporadically until systematic excavation of archaeological features on the Plain got underway during the 19th century and continues to the present.

Much recent literature has concentrated on what Stonehenge was for and how it was built, leading to a rich eclecticism and a little experimentation. But given the size of its stones and the obviously exotic nature of some of them, there have been disputes between those who consider them to have been brought by natural means and those who suggest collective human endeavour. The latter would have involved vast amounts of labour, shifting the bluestones over 250 km, entire community muscle power to drag the locally occurring sarsens about 25 km from their probable source, and a journey of at least 700 km to get the Altar Stone in place. Since none of the stones could conceivably have been moved by river flow, the only natural alternative for their transport is by glacial action.

Such an ice-transport theory rests on at least one of the several known advances of Pleistocene ice sheets having reached as far south as Salisbury Plain and deposited upon it glacial till that contains material from NE Scotland and South Wales. The most obvious indicators of glacial transport are large erratic boulders strewn far from their source down a previous ice stream that their distribution helps to reconstruct. In Northern Britain a great many megaliths that people erected long ago are glacial erratics of one kind or another. Of course, glacial tills contain grains of all sizes ripped and ground from the course of glacial flow. No so obvious, but equally capable of revealing transportation paths. After ice sheets melt, the till that they dump is eroded so that exotic rock and mineral grains enter drainage systems, some to remain in stream sediments. Two geologists based at Curtin University in Perth, Western Australia collected river sands from four active drainage systems on Salisbury Plain to test the glacial-transport hypothesis for the Stonehenge megaliths (Clarke, A.J.I. & Kirkland, C.L. 2026. Detrital zircon–apatite fingerprinting challenges glacial transport of Stonehenge’s megaliths. Communications Earth & Environment, v. 7, article 54; DOI: 10.1038/s43247-025-03105-3).

Using standard mineral-separation techniques – removal of low-density minerals (mainly quartz and feldspar) and those that are magnetic – Anthony Clarke and Christopher Kirkland mounted and polished samples of the remaining high-density grains embedded in resin. Using automated X-ray spectroscopy they identified grains of two minerals, zircon and apatite, that can be dated using uranium and lead isotopes. Zircons are virtually absent from the underlying Chalk although phosphorus-rich horizons in that rock sometimes contain apatite, a complex calcium phosphate. Both minerals are commonly found in igneous and metamorphic rocks and, being chemically resistant and hard, are often present in sediments derived by erosion of such parent rocks. The authors analysed U-Pb isotopes using laser ablation plasma mass spectrometry of suitable grains of each mineral. The U-Pb data from 250 apatite grains revealed a dominant age peak at 60 Ma, roughly the base of the once overlying Palaeogene sediments. Far fewer grains hint at older ages (175, 215, 300 and 625 Ma) in the Mesozoic, Palaeozoic and Neoproterozoic. The 550 analysed zircons span an age range from the Silurian to Palaeoproterozoic (432 to 1870 Ma), with a few outliers as young as 285 Ma and as old as 3396 Ma.

These data seem to suggest that they can support virtually any glacial transport hypothesis, including that of the Altar Stone, let alone the Stonehenge bluestones. However, that would be to misunderstand the complexity of sediment transport in relation to their original provenance. Erosion from a bedrock source leads to transport and deposition in sedimentary rock. Later uplift and erosion of that secondary host rock is followed by later sediment transport to another rock repository and so on and so forth through the entire geological history of Britain, across  its jumble of many tectonic terranes and the effects of numerous orogenic episodes! The Salisbury Plain chalk lands were covered by Palaeogene sedimentary rocks of the London Basin. And, lo and behold, one of those younger sediments, the Thanet Formation sandstones, tell much the same U-Pb story as do the modern river sediments of Salisbury! Those Palaeocene sands elsewhere directly overlie the Chalk and, in some localities on Salisbury Plain, still do today in the form of the chemically cemented sarsens. About 50 Ma ago (early Eocene) the Palaeocene rocks and those beneath were broadly buckled by the outermost ripples of the Alpine orogeny. Once eroded from above the Plain they would certainly have delivered that signature to the mercy of subsequent back and forth river transport. And indeed the sarsens, hard to miss in that landscape, perhaps still do so. Yet no one has thought to examine their content of heavy-mineral grains.

It does seem to me that the authors, perhaps inadvertently, walked into a sedimentological minefield in a vain attempt to put the lid on the fractious debate about human- versus glacial-transport of the Stonehenge megaliths. It is not their data that fling down a ‘challenge’ to the latter hypothesis (see their Conclusions), but the widely accepted absence of even the tiniest nugget of bluestone or Devonian sandstone on the vast and heavily excavated ritual landscape of Salisbury Plain, or indeed in the gravels of the streams that currently drain the Plain. But this where the plot thickens. A recent paper by one of the proponents of the glacial hypothesis (John, B.S. 2024. A bluestone boulder at Stonehenge: implications for the glacial transport theory. E&G Quaternary Science Journal v. 73, p. 117-134;DOI: 10.5194/egqsj-73-117-2024) describes a small piece of bluestone (22 × 15 × 10 cm) that was found during excavations at Stonehenge in 1924 and mysteriously ‘rescued’ by a Robert Newall and stored in his attic for almost 50 years, eventually examined by geologists and then returned to the attic. In 1976, two years before his death Newall passed it to the curator of Salisbury Museum ‘for safe keeping’. Brian John claims that its shape and surface texture suggests glacial transport. It also has several percussion scars suggesting that it had been worked, perhaps by someone hoping to make a stone tool. Unsurprisingly, Johns succeeded in provoking a storm of criticism from archaeologists largely of the human-transport wing of the controversy. And then there is the Mumbles Erratic, found at the eponymous Mumbles headland to the west of Swansea Bay. It too looks like a ‘bluestone’, but is it an erratic or from a Neolithic ship wreck carrying bluestones from Pembrokeshire?

Maximum extent of glaciation in SW Britain during the Anglian Stage 478 to 424 ka ago (Credit: Wikipedia Commons)

A great deal of work by British glaciologists has established the flow patterns and extent of major ice sheets, but only for four onshore, even though there is offshore evidence for repeated glaciation back as far as 2.5 Ma ago. The most extensive of these was the Anglian Stage between 478 and 424 ka ago. The figure above shows that the Irish Sea Glacier did not reach the Stonehenge area, but it did cross Pembrokeshire to reach Somerset on the eastern side of the Bristol Channel. Bluestone erratics may have been much more easily available than blocks hewn at their source in SW Wales, an hypothesis that is currently in vogue. Nope, the quest is not over …

Provenance of the Stonehenge Altar Stone: a puzzling development

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 Curiously, two weeks after my previous post about Stonehenge, a wider geochemical study of the Devonian sandstones and a number of Neolithic megaliths in Orkney seems to have ruled out the Stonehenge Altar Stone having been transported from there (Bevins, R.E. et al. 2024. Was the Stonehenge Altar Stone from Orkney? Investigating the mineralogy and geochemistry of Orcadian Old Red sandstones and Neolithic circle monuments. Journal of Archaeological Science: Reports, v. 58, article 104738;   DOI: 10.1016/j.jasrep.2024.104738). Since two of the authors of Clarke et al. (2024) were involved in the newly published study, it is puzzling at first sight why no mention was made in that paper of the newer results. The fact that the topic is, arguably, the most famous prehistoric site in the world may have generated a visceral need for getting an academic scoop, only for it to be dampened a fortnight later. In other words, was there too much of a rush?

The manuscript for Clarke et al. (2024) was received by Nature in December 2023 and accepted for publication on 3 June 2024; a six-month turnaround and plenty of time for peer review. On the other hand, Bevins et al. (2024) was received by the Journal of Archaeological Science on 23 July 2024, accepted a month later and then hit the website a week after that: near light speed in academic publishing. And it does not refer to the earlier paper at all, despite two of its authors’ having contributed to it. Clarke et al. (2024) was ‘in press’ before Bevins et al. (2024) had even hit the editor’s desk. The work that culminated in both papers was done in the UK, Australia, Canada and Sweden, with some potential for poor communication within the two teams. Whatever, the first paper dangled the carrot that Orkney might have been the Altar Stone’s source, on the basis of geochemical evidence that the grains that make up the sandstone could not have been derived from Wales but were from the crystalline basement of NE Scotland. The second shows that this ‘most popular’ Scottish source may be ruled out. To Orcadians and the archaeologists who worked there, long in the shade of vast outpourings from Salisbury Plain, this might come as a great disappointment.

Cyclical sediments of the Devonian Stromness Flagstones. (Credit Mike Norton, Wikimedia)

The latest paper examines 13 samples from 8 outcrops of the Middle Devonian Stromness Flagstones strata in the south of the main island of Orkney close to the Ring of Brodgar and the Stones of Stenness, and the individual monoliths in each. On the main island, however, there is a 500 m sequence of Stromness Flagstones in which can be seen 50 cycles of sedimentation. Each cycle contains sandstone beds of various thicknesses and textures. They are fluviatile, lacustrine or aeolian in origin. So the Neolithic builders of Orkney had a wide choice, depending on where they erected monumental structures. Almost certainly they chose monolithic stones where they were most easy to find: close to the coast where exposure can be 100 %. The Ring of Brodgar and the Stones of Stenness are not on the coast, so the enormous stones would have to be dragged there. There is an ancient pile of stones (Vestra Fiold) about 20 km to the NW where some of the mmegaliths may have been extracted, but ancient Orcadians would have been spoilt for choice if they had their hearts set on erecting monoliths!

In a nutshell, the geological case made by Bevins et al. (2024) for rejecting Orkney as the source for the Stonehenge Altar Stone (AS) is as follows: 1. Grains of the mineral baryte (BaSO4­) present in the AS are only found in two of the Orkney rock samples. 2. All the Orcadian sandstone samples contain lots of grains of K-feldspar (KAlSi3O8) – common in the basement rocks of northern Scotland – but the AS contains very little. 3. A particular clay mineral (tosudite) is plentiful in the AS, but was not detected in the rock samples from Orkney. Does that rule out a source in Orkney altogether? Well, no: only the outcrops and megalith samples involved in the study are rejected.

To definitely negate an Orcadian source would require a monumental geochemical and mineralogical study across Orkney; covering every sedimentary cycle. Searching the rest of the Old Red Sandstone elsewhere in NE Scotland – and there is a lot of it – would be even more likely to be fruitless. Tracking down the source for the basaltic bluestones at Stonehenge was easy by comparison, because they crystallised from a particular magma over a narrow time span and underwent a specific degree of later metamorphism. They were easily matched visually and under the microscope with outcrops in West Wales in the 1920s and later by geochemical features common to both.

But all that does not detract from the greater importance of the earlier paper (Clarke et al., 2024), which enhanced the idea of Neolithic cultural coherence and cooperation across the whole of Britain. The building of Stonehenge drew people from the far north of Scotland together with those of what are now Wales and England. Since then it hasn’t always been such an amicable relationship …

See also:  Addley, E. 2024. Stonehenge tale gets ‘weirder’ as Orkney is ruled out as altar stone origin. The Guardian 5 September 2024.

Geology cracks Stonehenge mysteries

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High resolution vertical aerial photograph of Stonehenge. (Credit: Gavin Hellier/robertharding/Getty)

During the later parts of the Neolithic the archipelago now known as the British Isles and Ireland was a landscape on which large stone buildings with ritual and astronomical uses were richly scattered. The early British agricultural societies also built innumerable monuments beneath which people of the time were buried, presumably so that they remained in popular memory as revered ancestors. Best known among these constructions is the circular Stonehenge complex of dressed megaliths set in the riot of earlier, contemporary and later human-crafted features of the Chalk downs known as Salisbury Plain. Stonehenge itself is now known to have been first constructed some five thousand years ago (~3000 BCE) as an enclosure surrounded by a circular ditch and bank, together with what seems to have been a circular wooden palisade. This was repeatedly modified during the following two millennia. Around 2600 BCE the wooden circle was replaced by one of stone pillars, each weighing about 2 t. These ‘bluestones’ are of mainly basaltic igneous origin unknown in the Stonehenge area itself. The iconic circle of huge, 4 m monoliths linked by 3 m lintel stones that enclose five even larger trilithons arranged in a horseshoe dates to the following two-centuries to 2400 BCE coinciding with the Early Bronze Age when newcomers from mainland Europe – perhaps as far away as the steppe of western Russia – began to replace or assimilate the local farming communities. This phase included several major modifications of the earlier bluestones.

It might seem that the penchant for circular monuments began with the Neolithic people of Salisbury Plain, and then spread far and wide across the archipelago in a variety of sizes. However, it seems that building of sophisticated monuments, including stone circles, began some two centuries earlier than in southern England in the Orkney Islands 750 km further north and, even more remote, in the Outer Hebrides of Scotland. A variety of archaeological and geochemical evidence, such as the isotopic composition of the bones of livestock brought to the vicinity of Stonehenge during its period of development and use, strongly suggests that people from far afield participated. Remarkably, a macehead made of gneiss from the Outer Hebrides turned up in an early Stonehenge cremation burial. Ideas can only have spread during the Neolithic through the spoken word. As it happens, the very stones themselves came from far afield. The earliest set into the circular structure, the much tinkered-with bluestones, were recognised to be exotic over a century ago. They match late Precambrian dolerites exposed in western Wales, first confirmed in the 1980s through detailed geochemical analyses by the late Richard Thorpe and his wife Olwen Williams-Thorpe of the Open University. Some suggested that they had been glacially transported to Salisbury Plain, despite complete lack of any geological evidence. Subsequently their exact source in the Preseli Hills was found, including a breakage in the quarry that exactly matched the base of one of the Stonehenge bluestones. They had been transported 230 km to the east by Neolithic people, using perhaps several means of transport. The gigantic monoliths, made of ‘sarsen’ – a form of silica-cemented sandy soil or silcrete – were sourced from some 25 km away where Salisbury Plain is still liberally scattered with them. Until recently, that seemed to be that as regards provenance, apart from a flat, 5 x 1 m slab of sandstone weighing about 6 t that two fallen trilithon pillars had partly hidden. At the very centre of the complex, this had been dubbed the ‘Altar Stone’, originally supposed to have been brought with the bluestones from west Wales.

The stones of Stonehenge colour-coded by lithology. The sandstone ‘Altar Stone’ lies beneath fallen blocks of a trilithon at the centre of the circle. (Credit: Clarke et al. 2024, Fig 1a)

A group of geologists from Australia and the UK, some of whom have long been engaged with Stonehenge, recently decided to apply sophisticated geochemistry at two fragments broken from the Altar Stone, presumably when the trilithons fell on it (Clarke, A. J. I. et al.2024.  A Scottish provenance for the Altar Stone of Stonehenge. Nature v.632, p. 570–575; DOI: 10.1038/s41586-024-07652-1). In particular they examined various isotopes and trace-elements in sedimentary grains of zircon, apatite and rutile that weathering of igneous rocks had contributed to the sandstone, along with quartz, feldspar, micas and clay minerals. It turned out that the zircon grains had been derived from Mesoproterozoic and Archaean sources beneath the depositional site of the sediment (the basement). The apatite and rutile grains show clear signs of derivation from 460 Ma old (mid-Ordovician) granites. The basement beneath west Wales is by no stretch of the imagination a repository of any such geology. That of northern Scotland certainly does have such components, and it also has sedimentary rocks derived from such sources: the Devonian of Orkney and mainland Scotland surrounding the Moray Firth. Unlike the lithologically unique bluestones, the sandstone is from a thick and widespread sequence of terrestrial sediments colloquially known as the ‘Old Red Sandstone’. The ORS of NE Scotland was deposited mainly during the Devonian Period (419 to 369 Ma) as a cyclical sequence in a vast, intermontane lake basin. Much the same kinds of rock occur throughout the sequence, so it is unlikely that the actual site where the ‘Alter Stone’ was selected will ever be known.

To get the ‘Alter Stone’ (if indeed that is what it once was) to Stonehenge demanded transport from its source over a far more rugged route, three times longer than the journey that brought the bluestones from west Wales: at least 750 km. It would probably have been dragged overland. Many Neolithic experts believe that transport of such a large block by boat is highly unlikely; it could easily have been lost at sea and, perhaps more important, few would have seen it. An overland route, however arduous, would have drawn the attention of everyone en route, some of whom might have been given the honour of helping drag such a burden for part of the way. The procession would certainly have aroused great interest across the full extent of Britain. Its organisers must have known its destination and what it signified, and the task would have demanded fervent commitment. In many respects it would have been a project that deeply unified most of the population. That could explain why people from near and far visited the Stonehenge site, herding livestock for communal feasting on arrival. Evidence is now pointing to the construction and use of the ritual landscape of Salisbury Plain as an all-encompassing joint venture of most of Neolithic Britain’s population. It would come as no surprise if objects whose provenance is even further afield come to light. It remained in use and was repeatedly modified during the succeeding Bronze Age up to 1600 BCE. By that time, the genetic group whose idea it was had been assimilated, so that only traces of its DNA remain in modern British people. This seems to have resulted from waves of immigrants from Central Europe, the Yamnaya, who brought new technology and the use of metals and horses.

See also: Gaind, N. & Smith, R. 2024. Stonehenge’s enigmatic centre stone was hauled 800 kilometres from Scotland. Nature, v. 632, p. 484-485; DOI: 10.1038/d41586-024-02584-2; Addley, E. 2024. Stonehenge megalith came from Scotland, not Wales, ‘jaw-dropping’ study finds. The Guardian, 14 August 2024.