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According to the established wisdom for some 90 years, many of the smaller rocks making up Stonehenge come from the Preseli Hills in Pembrokeshire, Wales. In 1923, geologist Herbert Henry Thomas first identified an outcrop known as Carn Meini as the source of the spotted dolerite bluestones used to build Stonehenge. (The distinctive markings are the result of elements within the bluestones cooling at different rates after an underwater volcano ejected the stones some 450 million years ago.)

Thomas’ theory was that around 5,000 years ago, during the Neolithic era, humans quarried approximately 80 bluestones (each weighing up to three tons) specifically for Stonehenge. They transported the stones to the site in Wiltshire by moving them south, down from the Preseli Hills, and loading them on rafts to carry them up the Bristol Channel. Other geologists, however, argued that humans had no role in transporting the stones at all. According to this competing school of thought, an Ice Age glacier carried the bluestones to the Wiltshire region some 20,000 years ago, and they were already available locally when Neolithic man began building Stonehenge.

Now, a study published this month in the Journal of Archaeological Science sheds new light on the long-running argument. According to analysis of the Stonehenge bluestones by a team of scientists from Aberystwyth University, University College London and National Museum of Wales, the stones did not come from the Carn Meini outcrop as Thomas concluded. Instead, by using laser mass spectrometry techniques to analyze both the chemical composition of the rock and the microbiology present when it was formed, they traced at least 55 percent of the spotted dolerite bluestones used at Stonehenge to Carn Goedog, a different outcrop several kilometers away.

If their conclusions are correct, they call into question the idea that humans transported the stones to Wiltshire via the Bristol Channel, as both Carn Goedog and Craig Rhos-y-felin (an outcrop his team identified in 2011 as a source for the rhyolites, another type of bluestones, used at Stonehenge) are located on the northern side of the Preseli Hills. According to the new study’s leader, Dr. Richard Bevins of the National Museum of Wales, “The rocks would have had to be dragged up the hills, across the summits and back down again before they even reached the waterways. It’s just not likely.”

Bevins says his team’s analysis will not end debate over how the bluestones made their way to Stonehenge, but may help eliminate some of the unknown variables involved. As he told BBC News: “I’ve been studying the bluestones for over 30 years now, and I’m no closer to finding an answer which convinces me either way. But the one thing which I am increasingly sure of is that each piece of the puzzle we find brings us another step closer to the truth.”

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