The Altar Stone Story
Lying prostrate in the centre of Stonehenge under the fallen stone (55b) of the Great Trilithon and its lintel (156) is a 5-metre long slab of six tonnes of grey–green, micaceous sandstone known as the Altar Stone (80). Debate has raged over the years whether the Altar Stone originally stood upright and was knocked down to its current position when the Great Trilithon fell or if it has always been recumbent across the axis of the monument.

The Altar Stone lying under the fallen stones of the Great Trilithon

Most plans of Stonehenge show the Altar Stone at 90 degrees to the main solstice axis; convenient, neat and symmetrical. But this is not correct as the Altar Stone actually lies at 80 degrees across the axis. This orientation means that its long dimension points precisely to the Winter Solstice Sunrise and Summer Solstice Sunset directions.

In 1923 the British geologist Herbert Thomas suggested that the Stonehenge bluestones came from the Mynydd Preseli, in north Pembrokeshire, Wales. He also proposed that the single grey-green micaceous sandstone at Stonehenge had a strong similarity to either sandstones from the Cosheston Subgroup from the shore of Milford Haven in west Wales or from the Senni Formation, outcropping between Kidwelly and Abergavenny in south Wales.

Milford Haven was suggested as the preferred source of the Altar Stone. This location sat well with a transport route proposed in the 1950’s by Richard Atkinson (Stonehenge, 1956) who envisaged the bluestones were brought to Stonehenge by sea, loaded into rafts at Milford Haven, sailed along the coast, then across the Bristol Channel and up the river Avon, before a final land crossing to Salisbury Plain.

However, detailed petrographic examinations of the Altar Stone and samples from Old Red Sandstone outcrops in west Wales cast doubt on the Cosheston Subgroup as a source for the Altar Stone and a source further to the east, such as the Senni Formation of the Brecon Beacons, was suggested. The implication being that if the Altar Stone was not sourced at Milford Haven then the sea route for the bluestones seemed increasingly unlikely, resulting in a totally land-based route following a natural corridor leading from west Wales and beyond the Severn estuary, the preferred option.

The proposed land route is supported by the identification of two bluestone quarries on the northern flanks of the Mynydd Preseli, at Craig Rhos-y-felin and Carn Goedog, avoiding Milford Haven altogether, which has led to suggestions to the possibility that the Altar Stone was collected en route from the eastern section of the Senni Formation which lies on a natural routeway, the line of the modern A40 road, along the valleys which may have been significant in prehistory.

When a ‘bluestone’ is not a ‘bluestone’
The term ‘bluestone’ was used by early excavators at Stonehenge for all the ‘foreign stones’, being exotic to Salisbury Plain, as opposed to the more local sarsen stones. Therefore by definition a ‘bluestone’ is any non-local stone at Stonehenge, comprising over twenty different lithologies, including the Altar Stone.

For many this has always been an unsatisfactory categorisation and consider only the finely worked smaller stones of the inner bluestone horseshoe (oval) and the (unworked) stones of the bluestone circle (between the five sarsen trilithons and the outer sarsen ring) as the ‘true’ bluestones. But even this is a varied group containing spotted and unspotted dolerites, various rhyolites and rhyolitic tuffs.

The majority of these ‘true bluestones’ have been sourced to the Mynydd Preseli area in west Wales. Recent studies have refined the Preseli sources of some of the bluestone lithologies; at least one pillar was taken from the rhyolite source of Craig Rhos-y-Felin (the source of the main rhyolitic debitage at Stonehenge and possibly the buried stump of Stone 32d); Carn Goedog has been identified as the main source of the spotted dolerites where at least five bluestone pillars (stones 33, 37, 49, 65, 67) were taken; Garn Ddu Fach, is the source of the non-spotted dolerite Stone 62.

The Altar Stone has frequently been referred to as an ‘anomalous bluestone’, both in its lithology and in its size and weight, an Old Red Sandstone (ORS) apparently from west Wales its source conveniently close to the site of the majority of the bluestones in the Mynydd Preseli area in west Wales some 225 km west of Stonehenge. The west Wales source of the Altar Stone has remained unchallenged for almost a century.

In 2022 a study concluded that a notable feature of the Stonehenge Altar Stone is the presence of baryte (Ba) in both the Stonehenge debitage (buried debris) and the Altar Stone (Bevins, Pearce, Ixer, Hillier, Pirrie, and Turner, 2022). These high Ba contents were in marked contrast with those from a small set of Old Red Sandstone field samples, raising the possibility that the Altar Stone may not have been sourced from the Old Red Sandstone sequences of Wales.

This raised questions over the potential source of the Altar Stone, and any possible transportation route, as there are not currently any reports of baryte-bearing sandstones in the Old Red Sandstone sequences of Wales or the Welsh Borderland.

Future Study
A recent study (Bevins, Pearce, Ixer, et al, 2023) has shown that samples from the Altar Stone yield a high Baryte (Ba) content, yet baryte-bearing sandstones in the Old Red Sandstone (ORS) sequences in Wales, the Welsh Borderland and the West Midlands and Somerset in England, the so-called Anglo-Welsh Basin, do not contain these same high levels of Ba; the geologists have therefore concluded that it is extremely unlikely that the Altar Stone was derived from the ORS of the Anglo-Welsh Basin.

No other ORS locations with comparable Ba concentrations as observed in the Altar Stone are known in outcrops of the Anglo-Welsh Basin, suggesting that this geographic area should be excluded as a possible source and look elsewhere in the search for the source of this enigmatic stone.

There is no doubt that considering the Altar Stone as a ‘bluestone’ has influenced thinking regarding the long-held view to a source in Wales. If the Altar Stone is no longer ‘classified’ as a bluestone, it is freed from the century old connection to the Mynydd Preseli-derived bluestones.

Long Meg, Cumbria

Studies will now turn to ORS in Northern Britain. We can expect sampling from monuments containing ORS monoliths such as the remains of the megalithic tomb comprising six red sandstone orthostats known as the Calderstones in Allerton, Lancashire. Long Meg, in Cumbria, a 3.8m high monolith consisting of red sandstone, generally thought to have been cut from cliffs of the Permian Penrith Sandstone Formation bordering the River Eden close by.

Further north, in Scotland, on the west coast of Arran there are the remains of a number of Neolithic stone circles at Machrie Moor. Circles 2 and 3 are of pebbly red sandstone. Orkney contains among the finest Neolithic settlements and monuments in Britain, including the Ring of Brodgar and the Stones of Stenness, both constructed using ORS age sandstone quarried at Vestra Fiold and Staneyhill just a few kilometres from the stone circles.

Ring of Brodgar, Orkney

They Came Bearing Gifts from the North
It is the long-distance transport of the bluestones that makes Stonehenge particularly fascinating for so many. Monoliths used in the construction of stone circles are usually sourced locally (such as the sarsens at Stonehenge); yet the Welsh bluestones represent one of the longest transport distances known from source to monument in the construction of a megalithic monument anywhere in the world. If sourced to Northern Britain then the transportation distance of the Altar Stone may well exceed this.

The Stones of Stenness, Orkney

We now know that the Altar Stone did not come from Wales, yet, it is not known when the Altar Stone arrived at Stonehenge.

Current wisdom considers that Stonehenge was first erected in the Late Neolithic period around 3,000 BC. This initial phase of construction was followed by four further re-modelling phases (Parker Pearson, 2023), the last being in the Middle Bronze Age, around 1600 BC before the monument appears to have gone out of use. This is an immense period of time for continued development of a single monument. Perhaps we should consider this long period of continuous development against the reconstruction of a major cathedral on the sacred site of an old wooden church over a similar period time; for example St Paul’s Cathedral. However, by comparison it is significant that the cathedral was rebuilt following natural disasters, whereas we see no evidence of this at Stonehenge where the monument appears to have been remodelled by successive generations.

Lead archaeologist Mike Parker Pearson considered that the bluestones (56 in number) were erected in the set of stone holes known as the Aubrey Holes during the first construction phase, c.2,950 BC. The larger sarsen stones were then thought to have been brought to Stonehenge during construction Phase 2, at the end of the Late Neolithic, c. 2,500 BC.

If the Altar Stone was sourced from west Wales, as initially thought, we could have expected it to have arrived with the other bluestones in the first phase of construction, i.e. 2,950 BC.

However, ‘declassifying’ the Altar Stone as a bluestone releases the stone from that first phase and the date of its arrival at Salisbury Plain is yet to be determined.

Old Red Sandstone map UK

But there may be clues in the proposed long-distance links between Stonehenge and other regions of Britain. Results from the study of Late Neolithic cattle (Bos taurus) teeth from Durrington Walls (Viner, Evans, Albarella, Parker Pearson,, 2010) suggested that cattle were brought to the site from a variety of grazing areas in different parts of Britain to the site in Wiltshire. Strontium isotope analysis provided evidence that cattle and pigs consumed during feasting at Durrington Walls were brought from western and northern areas, including Scotland.*

This long-distance connection occurred during the Stage 2 construction phase c.2,500 BC; it is possible that an Altar Stone (with a high Ba level) sourced from Northern Britain also arrived during this period, perhaps along with the cattle that hauled it, long after the arrival of the bluestones. Perhaps marking a significant occasion to celebrate the erection of the sarsens?

The next phase of the investigation for the source of the Altar Stone will further explore these long-distance connections in which the team will hope to match the distinctive lithology, mineralogy and geochemistry of the Altar Stone to Old Red Sandstone sequences across the other regions of Britain described above.

Sources:
Linking derived debitage to the Stonehenge Altar Stone using portable X-ray fluorescence analysis – Richard E. Bevins, Nick J.G. Pearce, Rob A. Ixer, Stephen Hillier, Duncan Pirrie, and Peter Turner. Mineralogical Magazine (2022), pp.1–13.

The Stonehenge Altar Stone was probably not sourced from the Old Red Sandstone of the Anglo-Welsh Basin: Time to broaden our geographic and stratigraphic horizons? – Richard E. Bevins, Nick J.G. Pearce, Rob A. Ixer, et al, Journal of Archaeological Science: Reports, Volume 51, October 2023.

Cattle mobility in prehistoric Britain: strontium isotope analysis of cattle teeth from Durrington Walls (Wiltshire, Britain) – Sarah Viner, Jane Evans, Umberto Albarella, Mike Parker Pearson, Journal of Archaeological Science, Volume 37, Issue 11, November 2010.

Mike Parker Pearson, Stonehenge: A Brief History, Bloomsbury Academic, 2023.

Note
*Previous research had assumed that samples with high strontium isotope values (over 0.714) were likely to have originated from Scotland or abroad, because values above this level appear to be extremely rare in areas of England and Wales.
However, it is now recognised that there are localised places in southwest England and Wales where strontium values also reach these levels.
Recent research from samples collected in the southwest of England has found levels of strontium over 0.714 which suggests that previous strontium isotope research should be reconsidered. For example, the livestock remains found at Durrington Walls may NOT have originated from Scotland; it may be more likely that the animals had come from the south-west of England, or possibly Wales. Clearly further research is required to clarify the origins of the livestock remains from Durrington Walls before we can be certain of prehistoric mobility.
See: Putting South-West England on the (strontium isotope) map: A possible origin for highly radiogenic 87Sr/86Sr values from southern Britain
Gundula Müldner, Delphine Frémondeau, Jane Evans, Alexis Jordan, Steven Rippon
Journal of Archaeological Science, Volume 144, August 2022
(Edited 26/12/23 – note added)