The Stonehenge stone circle is always good for a surprise – and this is no different now: New analyses reveal that the altar stone of the megalithic sanctuary was neither of local origin nor came from Wales like the other bluestones. Instead, this monolith comes from a rock formation in the north of Scotland. However, it is a good 750 kilometers away from Stonehenge. How the builders of Stonehenge managed to get this six-ton stone all the way to southern England is still a mystery.
The megalithic complex of Stonehenge in the south-west of England is one of the most famous prehistoric buildings in the world. The first version of this stone circle was built around 5,000 years ago, and over the next two millennia or so the complex, which was probably used as a place for rituals and festivals, was expanded and modified several times. Through comparative analyses, scientists have reconstructed the origin of most of the megaliths in the stone circle. According to this, the sarsen stones from which the gate-like trilithons and the outer stone circle were built come from a local rock formation. The large bluestones inside the complex, however, were brought from Wales, around 250 kilometers away.
The centre of the stone circle is the Stonehenge altar stone, a five-metre long and one-metre wide block of sandstone with greenish inclusions. It lies flat on the ground today, but may once have been upright. Until now, archaeologists assumed that this six-tonne megalith must also be of Welsh origin. But mineralogically, the sandstone is very different from the bluestones from Wales, which are mostly made of volcanic rock. “The actual origin of the altar stone therefore remains an open question,” explain Anthony Clarke from Curtin University in Australia and his colleagues. To clarify this question, they subjected the altar stone to a detailed mineralogical and isotopic analysis.
Ancient minerals from northeast Scotland
The focus of the investigation was on grains of the minerals zircon, rutile and apatite, whose age and isotopic composition can provide clues about the origin and time of formation of this sandstone. “Our analyses showed that some mineral grains in the altar stone are between one and two billion years old, while others are around 450 million years old,” reports Clarke. Together with the mineralogical and isotopic characteristics, this suggests that the oldest grains must have been formed in the area of the ancient continent of Laurentia, before the younger mineral components of this sedimentary rock were added in the Middle Ordovician.
This combination of minerals allowed the team to determine the origin of the altar stone. “This gives us a clear chemical fingerprint – and this suggests that the altar stone comes from rock formations in the Orcadian Basin in Scotland,” reports Clarke. This is because there, in the north-east of Scotland, there are old Laurentian formations with sprinklings of Ordovician minerals. The composition of the altar stone, however, does not match an origin in Wales or other regions of Great Britain. “This result is really remarkable – it refutes assumptions that have existed for a century,” says co-author Richard Bevins from Aberystwyth University in Wales. “We have thus deciphered the age and chemical fingerprint of one of the most famous stones of this world-famous Stone Age monument.”
How did the Stonehenge builders transport the stone?
But the Scottish origin of the Stonehenge altar stone also raises questions: “This stone has travelled over 750 kilometres. This is the longest documented journey of a stone used in a monument of this period,” says co-author Nick Pearce from Aberystwyth University. Senior author Robert Ixer from University College London adds: “This raises the question of why and how the altar stone was brought from the north of Scotland to Stonehenge – a distance of more than 750 kilometres.” As the team explains, a journey over land with a stone weighing several tons like this would have been almost impossible in the Neolithic period: “Even with the use of pack animals, rivers and topographical barriers such as the Grampian Mountains, the southern highlands and the Pennines, as well as the densely forested landscape of prehistoric Britain, would have posed enormous obstacles to the overland transport of the megalith,” say Clarke and his colleagues.
They therefore suspect that the Neolithic builders of Stonehenge brought the altar stone by sea to the vicinity of Stonehenge. Such transport by water is also assumed for the bluestones of Stonehenge and other construction projects of the megalithic culture. Nevertheless, such a sea voyage along almost the entire east coast of Great Britain would have been risky and costly. “This suggests that there were long-distance trade networks and a higher level of social organization than previously assumed for the Neolithic period in Great Britain,” says co-author Christopher Kirkland of Curtin University. The researchers’ next step is to find the exact place of origin of the altar stone in Scotland.
Source: Aberystwyth University, Curtin University; Article: Nature, doi: 10.1038/s41586-024-07652-1