A model of ice flow in ancient Britain shows that a glacier could have carried rocks from northeast Scotland to Dogger Bank. From there, humans still had to move the six-ton stone some 400 kilometers – and there is still no direct evidence that this was its route.
By Remy Veness, Lecturer in Glaciology, Sheffield Hallam University
Stonehenge was built from stones weighing between two and 25 tons. The scale of the construction is difficult to grasp, especially considering that the work was done before the wheel was used in Britain. The mystery deepens when it turns out that many of the stones are not made from local rock. Why were these stones chosen, and how did they end up on Salisbury Plain?
Research I conducted with geochemist Anthony Clarke of Curtin University in Australia may provide part of the answer regarding the stone that came from the greatest distance – the Altar Stone.
The Altar Stone is a sandstone slab weighing about six tons and measuring four to five meters in length. It now lies in the center of Stonehenge, parts of it covered by other stones that have fallen on it. It is commonly included in the group of “bluestones” – a collective name for the relatively small stones at the site that are not the more local sarsens.
Previous research has shown that the altar stone came from northeast Scotland, about 700 kilometers from the Salisbury Plain. Its geological origin is in the Orkney Basin, a large area that includes parts of Caithness and the Orkney Islands. In ancient times, there was a large lake in the area, and its sediments over time turned into the sandstone from which the altar stone is made.
The Scottish origin of the stone was identified by the tiny minerals within it. Zircon crystals and other mineral particles preserve a kind of geological fingerprint of the rocks from which they were eroded. Comparison of their age and composition with samples from Scotland pointed to the Orkney Basin, and in particular the Caithness area, as the most likely source known today.
The find solved one question, but created an even bigger mystery: How did Neolithic people move a six-ton rock almost the entire length of Britain?
Glacial conveyor to the North Sea
Northern Britain was covered in ice until about 15 years ago, towards the end of the last ice age. The ice was so thick and continuous that it flowed like a slow river. As it moved, it eroded, lifted and transported rocks and sediments over great distances.
In the new study, we examined whether glaciers could have transported rocks from the Orkney Basin southward, closer to Stonehenge. We used a numerical model I developed over the past five years to predict the trajectory and possible deposition locations of rocks transported by ice.
Large rocks that are far from the geological outcrop from which they came are called glacial erratics. The glacier carries them away from their place of origin, carrying them within or on its surface, and leaving them in the landscape when the ice melts.
The model results do not show a glacial path from Scotland to Stonehenge. On the contrary: most of the ice flowing from the Caithness area headed northeast. However, the model identified a local, narrower path that headed southeast and could have carried rocks towards Dogger Bank, an area now under the waters of the North Sea, east of England.
Such a route would have shortened the distance humans had to move the stone from about 700 to about 400 kilometers. This is still a huge undertaking, but it is more similar to the distance traveled by some of the other bluestones brought from southwest Wales.
Doggerland Rock Museum
Dogger Bank is now known mainly as a fishing area and site for offshore wind farms, but at the end of the Ice Age it was part of Doggerland – the vast plain that connected Britain to mainland Europe.
As the climate warmed and the glaciers melted, people moved north and west from Europe into what later became Britain. Dogger Bank was then a relatively high area within the flat earthscape of Doggerland. It was rich in lakes, rivers, marshes and animals, and therefore attracted groups of hunter-gatherers during the Mesolithic period.
Part of Dogger Bank was formed from a glacial moraine – a ridge of rocks, sand, gravel and debris left behind by the glacier. The moraine effectively marked the end of a glacial “conveyor belt” where rocks from far north accumulated.
A person walking around Dogger Bank at that time might have seen a landscape reminiscent of a combination of a demolition site and a rock museum: stones of various sizes, colors, and compositions, brought to the site by the ice.
We know that people of the Mesolithic and Neolithic periods were well acquainted with rock raw materials and were selective in their choice of stones for toolmaking. Therefore, it is possible that a rock that was unusual in size, color, or texture would have attracted their attention and taken on a useful or symbolic meaning.
Stray rocks, flood and trolls
Throughout history, people have noticed that erratic rocks do not fit with the local geology. In Victorian Britain, they were called erratics – “stray” or unusual rocks – because of their unexpected location. Before the glacial theory was established, some believed that they were carried into place by the biblical flood.
On the other side of the North Sea, such rocks were called “troll stones” in Scandinavia. The inhabitants understood that the rocks were similar to those found in distant mountains, but they did not know how they had come to the plain. One popular explanation was that trolls or giants had thrown them hundreds of kilometers away. The Old Swedish term jättekast literally means “giant throw”.
It is impossible to know what led the early inhabitants of Doggerland to choose the rock that would become the altar stone thousands of years later. In fact, there is still no evidence that this particular rock actually made it to Dogger Bank. The model only shows that rocks of suitable geological origin could have arrived there by ice.
If the hypothesis is correct, humans would have had to move the stone at least twice: first from Dogger Bank to a safer place on land, and then to Stonehenge.
The sea rises and the landscape disappears.
Doggerland was gradually submerged as sea levels rose due to the melting of the great ice sheets. Initially a broad land bridge connecting Europe and Britain, it then shrank to a marshland and islands, and finally disappeared beneath the North Sea.
Parts of Doggerland remained above sea level until about 8,000–9,000 years ago, and perhaps as late as 7,000 years ago. Stonehenge only began to be built later, about 5,000 years ago. This means that if the altar stone was indeed found at Dogger Bank, it would have had to have been moved to an intermediate site before the area was flooded, long before it was finally placed at Stonehenge.
It is possible that the stone already held significance for the people of Doggerland, and that rising sea levels prompted them to move it west. However, this is only speculation: there is currently no direct archaeological evidence that people “rescued” the stone from the sea, or that the stone was preserved for thousands of years until it reached the Salisbury Plain.
Humans have still come most of the way.
Once the stone reached the east coast of England, if it did arrive there by glacier, there was still about 400 kilometers to go to Stonehenge. The study clearly indicates that there was no glacial route leading it from the southern North Sea to the Salisbury Plain.
Therefore, humans still had to make most of the journey. The stone may have been dragged overland, transported by boat along the coast or in rivers, or passed through several stages that combined different modes of transport.
One possible route was the Berkshire Ridgeway, an ancient road running over relatively high ground from east to west towards Salisbury Plain. It is sometimes described as one of the oldest roads in Europe, and evidence suggests that it was used at the time Stonehenge was built.
Even after the possible shortcut provided by the glacier, dragging a six-ton rock hundreds of miles would have required planning, cooperation, detailed knowledge of the landscape, and the provision of food and equipment for a large group. This reinforces the understanding that prehistoric communities in Britain were not isolated groups, but maintained contacts, exchanged materials and ideas, and knew how to organize large-scale operations.
What the research proves – and what it still doesn’t
The study suggests a possible route, but does not reconstruct the proven journey of the Altar Stone. The stone itself, a sample of it, or a known staging post where it was deposited has not been found at Dogger Bank. The model suggests that a glacier could have carried rocks from certain parts of the Orkney Basin to Dogger Bank, but does not prove that the Altar Stone was one of them.
The exact origin of the stone within the Orkney Basin has also not been identified. Samples from the Caithness area provide the best known match to the zircon crystal fingerprint, but the main direction of ice flow from there is not southerly. Only a more limited path has been found towards Dogger Bank.
So the cautious conclusion is that a glacier may have helped on the first part of the journey, but humans would have had to move the stone hundreds of miles. Further research will need to locate the exact sandstone outcrop in northeast Scotland and see if there is archaeological or geological evidence for stations along the way.
Either way, the story of the Altar Stone points to a world where glaciers, rising sea levels, and human communities all worked together to shape the journey of a single rock. The glacier may have started the journey, but it was humans who brought the stone to the heart of Stonehenge.
Scientific article and sources:
- The article in the Journal of Quaternary Science
- Original article in The Conversation
- The study that determined that the altar stone originated in northeastern Scotland
- Sheffield Hallam University Research Review
More of the topic in Hayadan:
- Most of the large stones of Stonehenge have one source: West Woods, Wiltshire
- Two wooden pillars preceded the Stonehenge stones and marked the days of the solstice
- A stone circle was discovered that surrounded Stonehenge about two miles away.
- Stonehenge builders' village discovered
- Summer Solstice – the day when the sun is at its northernmost point
Questions and Answers
Did a glacier carry the altar stone to Stonehenge?
No. The model does not identify a glacial route from northeast Scotland to the Salisbury Plain. It suggests that rocks from the region could have reached Dogger Bank, from where the stone would have had to be transported another 400 kilometers by man.
How do you know that the altar stone came from Scotland?
The ages and compositions of zircon crystals and other minerals in a rock create a geological fingerprint. The best match so far is found in sandstones from the Orkney Basin in northeastern Scotland.
Was the altar stone found in Dogger Bank?
No. Dogger Bank is currently submerged under the North Sea, and no direct evidence of the Altar Stone has been found there. This is a possible route derived from the ice flow model.
Who carried the stone the rest of the way?
If the glacial hypothesis is correct, prehistoric communities would have had to transport the stone from Doggerland to the coast of England and then to Salisbury Plain. They may have used a combination of land haulage, rivers and sea routes.
For the original publication: Opening the original publication
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You're right. I've corrected it. I translated a quote without the quotation marks.
The article is written in the first person, for example, "Research I conducted," and creates the impression that Avi Blizovsky himself discovered the source of the stone. The opening of the original publication shows that it was not he who wrote the article. It would have been appropriate to note this at the beginning of the article.