The 5,000-year-old mystery of Stonehenge may finally have been solved – with the help of a few small grains of sand, writes British publication Daily Mail. While most scientists believe that Stonehenge's giant stones were brought from Wales and Scotland, another theory suggests that nature helped the builders. According to the so-called glacial transport theory, the ice that once covered ancient Britain easily carried rocks to Salisbury Plain.
However, now, according to an article in the Daily Mail, scientists have found concrete evidence that the megaliths were definitely moved by humans after all. Using advanced mineral fingerprinting techniques, geologists from Curtin University have shown that glacial material never reached Salisbury Plain. If the giant rocks were indeed moved by ice, they would leave behind a trail of debris containing millions of microscopic mineral particles.
But when researchers tested the Wiltshire sand, they discovered that it had not been moved during the last ice age, 20,000 to 26,000 years ago.
Lead author Dr Anthony Clarke told the Daily Mail: “Our findings make it unlikely that megaliths were transported by glaciers and is consistent with existing ideas that megaliths were brought from remote locations by Neolithic people using methods such as sleds, rollers and rivers.”
The Daily Mail notes: One of Stonehenge's most mysterious features is the fact that its stones appear to have come from the most remote corners of the UK. While the large standing stones, or sarsens, lie just 15 miles north of the stone circle, the smaller blue stones and the unique altar stone are not indigenous.
Geologists discovered that bluestones weighing between two and five tonnes have been found in the Preseli Hills in Wales, while a six-tonne altar stone was found at least 750km away in northern Scotland. This, the Daily Mail continues, meant that Neolithic people had to transport specially selected stones over hundreds of miles using only stone and wooden tools. To some researchers, this idea seems so unlikely that the theory of glacial transport seems a more plausible alternative. If ice had indeed covered Salisbury Plain at some point in the distant past, it would have left traces that can still be seen today.
Many larger marks such as scratches in rocks or carved topography are absent or cannot be identified at the Stonehenge site. But the ice will also leave microscopic traces that scientists can see.
Dr Clarke commented to the Daily Mail: “If large glaciers carried bluestone from Wales or northern England to Stonehenge, they would also have carried large quantities of sand and gravel with very distinctive age marks into local rivers and soils.”
Importantly, this sand contains two minerals called zircon and apatite, which can be used as “mini geological clocks,” the Daily Mail points out. When zircon and apatite form by crystallizing from magma, they retain small amounts of radioactive uranium, which decays into lead at a known rate. By looking at the ratio of uranium to lead, scientists can determine how long ago an individual grain of sand was formed.
Because some rocks, such as the megalith Stonehenge, are composed of many types of grains that can be dated, scientists can use this method to create geologic “fingerprints.”
“Because British bedrock is of different ages in different places, the age of a mineral can indicate its origin,” Dr Clarke explains. “This means that if glaciers brought rocks to Stonehenge, then the rivers of Salisbury Plain, which collected zircon and apatite from across a vast area, must still contain clear mineral signatures of that glacial journey.”
Researchers studied more than 700 zircon and apatite grains collected from rivers near Stonehenge. Even though the Earth's age is only half that from 2.8 billion years ago to 300 million years ago, almost none of them match the fingerprints of the bluestone source in Wales or the altar stone in Scotland. Most zircon grains were formed in a dense band 1.7-1.1 billion years ago, when a layer of loose sand called the Thanet Formation covered much of southern England. Meanwhile, the age of all the apatite grains is around 60 million years, which does not correspond to any potential source of the rock in the UK. This is because the same tectonic forces that shaped the European Alps displaced fluid from the chalk and “reset” the uranium apatite clock.
Study co-author Professor Chris Kirkland told the Daily Mail: “The history of the Salisbury Plain sediments appears to be one of reworking over a long period of time, plus a Paleogene ‘shake’ recorded in the apatite, rather than a landscape created by major glacial imports.”
If ice had brought bluestone or altarstone to England, sand would indicate their origin. “However, the material around Stonehenge did not,” Professor Kirkland said. “We therefore conclude that Salisbury Plain remained ice-free during the Pleistocene, making direct transport of megaliths through ice unlikely.”
This provides “convincing, verifiable evidence” that the giant boulders were in fact dragged to Salisbury Plain by hand, the Daily Mail further noted. And this may mean we should be grateful for the ingenuity and determination of those places' ancient inhabitants.
