The
Altar stone is towards the middle of Stonehenge. Credit: Nash DJ, Ciborowski
TJR, Darvill T, Parker Pearson M, Ullyott JS, Damaschke M, et al., CC BY-NC
Built
from stones weighing between 2 and 25 tonnes (2 to 28 tons), 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 Curtin University in Australia might have an answer to these two questions
for Stonehenge's most far-traveled stone—the Altar Stone.
The Altar Stone was sourced from 700 km
(435 miles) away in northeast 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 tested the
possibility that it was transported by glaciers.
It is the largest and heaviest of the
so-called bluestones (Stonehenge's traveled rocks), weighing 6 tonnes (6.6
tons) and measuring 4 meters (13 feet) long. The bluestones were the first stones at Stonehenge,
predating the larger stones that form the arches by 2,000 years.
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 tested whether rocks from the Orcadian Basin could have traveled closer to
Stonehenge if transported by this body of ice.
We studied this using a new numerical
model, which I have been developing over the past five years to predict the
possible deposit locations of glacially transported rocks (known as erratics).
This showed that rocks transported from the Orcadian Basin could have been
carried to a region off England's east coast called Dogger Bank.
Although today Dogger Bank 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.
To
someone walking around Dogger Bank at the time, the site would probably have
looked like something between a building demolition site and an interesting
rock museum. We know Neolithic people were highly knowledgeable about rocks and
selective about the stones they chose for tools. This suggests they might have deliberately chosen
the Altar Stone for some of its attributes.
More recently, there are countless
examples of people realizing that the landscape's glacial erratics were out of
place. The Victorians in Britain, for example, chose the name "erratic"
because of 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 realized these
boulders came from the upland mountains. They used to think the only
explanation was that they had been thrown hundreds of kilometers by playful
trolls, with the early Swedish term for erratics (jättekast) literally
translating to "giant throw." It's hard to say exactly what 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 (250 miles) 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 8,000–9,000 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
England's east coast, the idea that it was moved by humans to Salisbury Plain
becomes more plausible. This distance would be roughly the same as that of the
other bluestones, the next most far-traveled stones, but still 10 times the
distance of the more locally sourced larger Sarsen Stones. Around the time of Stonehenge's construction, we
know east-west travel was happening along the Berkshire Ridgeway, a Neolithic high route often touted as the oldest
road in Europe, leading directly to Salisbury Plain.
Our recent study opens up a new
hypothesis for how the Altar Stone could have traveled 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
kilometers, but also potentially realized that their landscape was changing and
that they needed to act.
Provided by The Conversation
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