Built from stones weighing between 2 and 25 tonnes, the structure of Stonehenge demonstrates a scale of construction hard to imagine before the invention of the wheel. The mystery deepens when you consider that the stones are not from the local bedrock. So why these stones, and how did they get there?
Recent work I carried out with geochemist Anthony Clarke from the University of Curtin, Australia, might have an answer to these two questions for the most far travelled stone of Stonehenge – the Altar Stone.
The Altar Stone was sourced from 700km away in north east Scotland, from a region of bedrock geology known as the Orcadian Basin that was once a lake called Lake Ocradie. The sandstone of the Altar Stone was formed from these lake sediments. Now, new work by our international team has looked to test the possibility it was transported by glaciers.
It is the largest and heaviest of the so-called bluestones (Stonehenge’s travelled rocks), weighing 6 tonnes and measuring 4 metres long. The bluestones were the first stones of Stonehenge predating the larger stones which form the arches by 2,000 years.
Nash DJ, Ciborowski TJR, Darvill T, Parker Pearson M, Ullyott JS, Damaschke M, et al., CC BY-NC
We know the north of the UK was covered by ice until approximately 15,000 years ago at the end of the last ice age. This ice was so thick and persistent that it flowed like a slow moving river, picking up and carrying rock and debris with it. The team looked to test if it was possible for the rocks from the Orcadian Basin to travel closer to Stonehenge if transported by this body of ice.
We studied this by using a new, numerical model, which I have been developing over the last five years for predicting the possible deposit locations of the glacially transported rocks (known as erratics). This showed that rocks transported from the Orcadian Basin could be transported to a region off the east coast of England called Dogger Bank.
Although today Doggerland is known for cod fishing and wind farms it is thought to have once been a neolithic site of significance as early humans moved from southern Europe north and west into the UK as the climate warmed at the end of the last ice age approximately 12,000 years ago. Dogger Bank represented a local topographic high point within the Doggerland region, rich in archaeological evidence.
What is interesting about Dogger Bank is that it is formed in part by a large glacial moraine (a ridge of rock and debris deposited by a glacier). This moraine marks the end of a glacial “conveyor belt”, where a range of glacially transported rocks and boulders from the north would have been deposited during the last ice age.
Walking around Dogger Bank at the time, the site would probably have looked like something in between a building demolition site and an interesting rock museum. We know Neolithic people were really knowledgeable about rocks, and were selective about the stones they chose for tools. Which suggests they might have deliberately chosen the Altar Stone for some of its attributes.
More recently there are countless examples of people realising the landscape’s glacial erratics were out of place. The Victorians in Britain for example chose the name “erratic” due to the rocks’ unusual locations (at the time they were thought to be the result of the biblical flood). On the other side of the North Sea, Scandinavians know erratics as “troll stones”, having realised these boulders came from the upland mountains. They used to think the only explanation was that they had been thrown hundreds of kilometres by playful trolls with the early Swedish term for erratics (jättekast) literally translating to giant throw. It’s hard to say what exactly made the Neolithic people of Dogger Bank choose the Altar Stone. But if it was transported by ice to Dogger Bank they would have needed to move it at least once before taking it 400 km to Stonehenge.
Doggerland was gradually flooded as large ice sheets around the world melted at the end of the last ice age. We know it stayed above sea level until around 8-9,000 years ago (perhaps even as late as 7,000 years ago). However, throughout that time the landscape saw dramatic change. It transitioned from a land bridge connecting Europe to Britain, to a small island, where the Altar Stone stood. This island eventually slipped below the sea too.
This is interesting because Doggerland was inundated before the construction of Stonehenge. Therefore, the Altar Stone would have had to be transported to at least one intermediate site before finally being incorporated into Stonehenge.
Once the Altar Stone was safely on the east coast of England, the idea that it was moved by humans to Salisbury Plains becomes more plausible. This distance would be roughly the same distance as the other bluestones, the next most far travelled stones, but still ten times the distance of the more locally sourced larger Sarsen Stones. Around the time of the construction of Stonehenge, we know that east-west travel was happening along the Berkshire Ridgeway, a neolithic high route often touted as the oldest road in Europe, leading directly to the Salisbury Plains.
Our recent study opens up a new hypothesis for how the Altar Stone could have travelled south to Stonehenge, and why people might have been motivated to first move the stone in response to rising sea levels threatening its initial site.
This work suggests that neolithic people not only had the agency and coordination to move a large rock hundreds of kilometres, but also to potentially realise that their landscape was changing and that they needed to act.
