People, not glaciers, transported rocks to Stonehenge, study confirms

Humans — not glaciers — transported Stonehenge's megaliths across Great Britain to their current location in southern England, a new study confirms.

Scientists have believed for decades that the 5,000-year-old monument's iconic stones came from what is now Wales and even as far as Scotland, but there is still debate as to how the stones arrived at Salisbury Plain in southern England.

Now, an analysis of microscopic mineral grains from rivers around Stonehenge has revealed that glaciers didn't reach into the region during the last ice age (2.6 million to 11,700 years ago), refuting an idea known as the "glacial transport theory" that suggests Stonehenge's bluestones and 6.6-ton (6 metric tons) Altar Stone were delivered by ice sheets to Salisbury Plain. The new study was published Jan. 21 in the journal Communications Earth and Environment.

"While previous research had cast doubt on the glacial transport theory, our study goes further and applies cutting-edge mineral fingerprinting to trace the stones' true origins," study authors Anthony Clarke, a research geologist at Curtin University in Australia, and Christopher Kirkland, a professor of geology also at Curtin University, wrote in The Conversation.

Stonehenge's bluestones, so called because they acquire a bluish tinge when wet or freshly broken, are from the Preseli Hills in western Wales, meaning people likely dragged them 140 miles (225 kilometers) to the site of the prehistoric monument. More remarkable still, researchers think the Altar Stone inside Stonehenge's middle circle came from northern England or Scotland, which is much farther away — at least 300 miles (500 km) — from Salisbury Plain and may have required boats.

The glacial transport theory is a counterproposal to the idea that people moved the stones from elsewhere in the U.K. to build the monument on Salisbury Plain, instead using stones that had already been transported there by natural means. However, as Stonehenge’s rocks show no signs of glacial transport, and the southern extent of Great Britain’s former ice sheets remain unclear, archaeologists have disputed the idea.

To investigate further, the researchers behind the new study used known radioactive decay rates to date tiny specks of zircon and apatite minerals left over from ancient rocks in river sediments around Stonehenge. The age of these specks reveals the age of rocks that once existed in the region, which, in turn, can provide information about where these rocks came from.

Different rock formations have different ages, so if the rocks that became parts of Stonehenge were dragged across the land by glaciers, they would have left these tiny traces around Salisbury Plain that could then be matched with rocks in their original locations.

The researchers analyzed more than 700 zircon and apatite grains but found no significant match for rocks in either western Wales or Scotland. Instead, most of the zircon grains studied showed dates between 1.7 billion and 1.1 billion years ago, coinciding with a time when much of what is now southern England was covered in compacted sand, the researchers wrote in The Conversation. On the other hand, the ages of apatite grains converged around 60 million years ago, when southern England was a shallow, subtropical sea. This means the minerals in rivers around Stonehenge are the remnants of rocks from the local area, and hadn’t been swept in from other places.

The results suggest glaciers didn't extend as far south as Salisbury Plain during the last ice age, excluding the possibility that ice sheets dropped off the megaliths of Stonehenge for ancient builders to subsequently use.

"This gives us further evidence the monument's most exotic stones did not arrive by chance but were instead deliberately selected and transported," the researchers wrote.

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