How researchers discovered that the giant altar stone of Stonehenge came from northeast Scotland

No one knows exactly why Stonehenge was built. This world-famous monument on Salisbury Plain in Wiltshire is meant to commemorate the dead and is aligned with the movements of the sun and moon.

It consists of an outer ring and inner horseshoe made of large ‘sarsen’ and ‘trilith’ stones, and an inner circle and horseshoe made of smaller ‘bluestones’. It was built in several phases between 5,000 and 4,200 years before the Common Era.

The Altar Stone is one of the most enigmatic stones at Stonehenge and is generally considered to be a bluestone. Despite its name (it is believed to have been used by the architect Inigo Jones in 1620), its function is unknown.

The six-ton, five-meter-long rectangular altar stone lies flat in the heart of Stonehenge. It is a gray-green sandstone that is much larger and different in composition from the other bluestones. So where did it come from?

In our new paper, published in Nature, we have traced the Altar Stone’s origins back to north-east Scotland, meaning it travelled at least 700km to Salisbury Plain. This is an incredible distance for the Neolithic period, before the wheel is thought to have arrived in Britain. This astonishing discovery sheds new light on the skills and long-distance connections of Britain’s Neolithic inhabitants.

Let’s recap what we know and how we narrowed down the region where the Altar Stone came from. The great stones of Stonehenge (sarsen stones) come from tens of kilometers away, but moving these 30-ton monsters was no easy task in the Neolithic period.

The smaller, exotic bluestones are a different story. They do not come from Stonehenge, usually weigh 1-3 tons and are up to 2.5 meters high. The Altar Stone, which also does not come from Stonehenge, is twice as large as the largest other bluestone. It is not known when it arrived at Stonehenge or whether it ever stood upright.

It was not until 1923 that geologist HH Thomas realised that most of the igneous bluestones came from the Mynydd Preseli in Pembrokeshire, south-west Wales. Our ongoing work has narrowed down the sources of these igneous bluestones to isolated rocky crags on the northern slopes of the Preseli Mountains.

Thomas also suggested that the Altar Stone probably came from ancient red sandstone rocks found south and east of Mynydd Preseli, on the presumed transport route of the bluestone to Stonehenge. This suggestion persisted and remained unchallenged for 80 years.

In the early 2000s, we began to look again at alleged altar stone fragments in museum collections. Some fragments were obviously misidentified, so the time-consuming process of clarifying the situation began.

The Altar Stone was originally thought to have originated in West Wales, near Milford Haven. However, in the late 2010s we subjected its fragments to a series of geological analyses. These results pointed to East Wales or the Welsh border as the location and ruled out a West Wales origin.

But how can we be sure that the museum fragments are genuine without taking a sample of the altar stone? Today, we are no longer allowed to chip away at pieces of Stonehenge, as was done in the past.

New technology

In the early 2020s, we began using X-ray fluorescence analysis, a non-destructive chemical analysis method, on the bluestones of Stonehenge – in particular the many alleged altar stone fragments found in older archaeological excavations. We then compared these with X-ray fluorescence analyses of the surface of the altar stone itself.

The sediment grains in the Altar Stone are held together by the mineral barite, giving it an unusual chemical composition with a high proportion of the element barium. Some museum fragments were identical to the Altar Stone – proof that an inscribed fragment removed from the Altar Stone in 1844 was genuine was crucial. These few, valuable fragments could be used for our study, so we did not need to collect new samples directly from the Altar Stone.

By now, our scientific team included geologists from England, Wales, Scotland, Canada and Italy. We had analyzed a number of ancient red sandstone samples from across Wales and the Welsh borders to find a chemical and mineralogical match for the Altar Stone. Nothing looked similar. In autumn 2022, we concluded that the Altar Stone could not have come from Wales and that we had to look further afield for its source.

At the same time, a chance encounter with Tony Clarke, a PhD student at Curtin University in Perth, Western Australia, gave us the opportunity to go even further. We asked the Curtin group to determine the age of a number of minerals in two of the Altar Stone fragments, in the hope that this would provide us with information about their age and possible origin. This method can be used to date mineral grains in the rock and create an age “fingerprint” that assigns the grains to a specific region.

Our new study was published in Nature shows that the age fingerprint of the altar stone identifies it as coming from the Orkney Basin in north-east Scotland. The results of this age determination are truly astonishing and turn assumptions made for centuries on their head.

It is exciting to know that the culmination of our work over nearly two decades has revealed this mystery. We can say with certainty that this iconic stone is Scottish, not Welsh, and more specifically, that it comes from the ancient red sandstones of north-east Scotland.

The Altar Stone comes from the Orkney Basin and has had a remarkably long journey – a straight-line distance of at least 430 miles. This is the longest known journey for a stone used in a Neolithic monument.

Our analyses cannot answer how the Altar Stone got to Stonehenge. Forests were one of several physical barriers to land transport. A journey by sea would have been equally daunting. Nor can we answer why it was transported there.

Whatever archaeologists discover in the future, our findings will have enormous implications for understanding Neolithic communities, their connections with each other and the way they transported things over distances. In the meantime, our search for an even more precise source of the Altar Stone continues.

This article is republished from The Conversation under a Creative Commons license. Read the original article.