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For centuries, Stonehenge has attracted all sorts of theories—varying widely in credibility—as to how it cropped up some 5,000 years ago.
Some explanations have leaned outlandish, pointing to aliens or Merlin the wizard, while scientists have squabbled over the more probable stone origin stories. For example, some teams have proposed that glaciers pushed some of these stones to the oft-visited site at Salisbury Plain in southern England because their makeup reflects far-off locales. But most researchers think that people brought the stones over via sea or land.
The stones at this iconic monument fall into three groups: the hulking, nearly 28-ton sarsen stones, most of which seem to have come from around 15 miles away; bluestones that appear to originate more than 140 miles away in Wales; and the Altar Stone, a 6-ton slab that may have been brought from an area of northeast Scotland that’s more than 400 miles from the site.
The bluestones and the Altar Stone in particular have sparked lively discussions on glacial versus human transport. Some claim that glaciers shepherded these foreign stones over hundreds of miles during the last Ice Age, which was more than 20,000 years ago. Until now, no studies have harnessed state-of-the-art geological techniques to test this glacial transport theory.
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Along with these stones, ancient glaciers would have deposited millions of microscopic grains of minerals including apatite and zircon. When these two minerals form, they bottle up small concentrations of radioactive uranium that turns into lead at a specific rate. So studying the ratios of these elements enables the researchers to measure the age of each of these minerals bits. This is a crucial hint, as rocks throughout Britain vary widely in age.
“If glaciers had carried rocks all the way from Scotland or Wales to Stonehenge, they would have left a clear mineral signature on the Salisbury Plain,” explained Anthony Clarke, a geologist at Curtin University in Australia, in a statement.
Clarke and his colleagues inspected sand at rivers near Stonehenge for these telltale mineral grains, and found no evidence that the bluestones came from Wales or that the Altar Stone hailed from Scotland. Most of the zircon gains they studied were 1.7 to 1.1 billion years old, suggesting they came from ancient sheets of sedimentary rocks once covering that area of England. And the apatite seems to have lingered at Salisbury Plain for tens of millions of years. The findings were recently reported in Communications Earth and Environment.
“That makes the alternative explanation—that humans moved the stones—far more plausible,” Clarke said.
How exactly people schlepped these incredibly heavy stones, however, remains a mystery, much like the speculation over statues on Rapa Nui. The Stonehenge builders may have wielded rolling logs, “but really we might never know,” Clarke added.
It’s not even known precisely why people constructed Stonehenge, but researchers have some ideas: The site seems to have been used in a variety of ways, serving as an observatory, burial site, and even a party spot. With all this ancient intrigue, it’s no wonder that the monument continues to draw more than a million visitors each year. 
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Lead image: Rachelle Haun / Wikimedia Commons
