Did Scientists Just Unravel One Mystery of Stonehenge?
A rock sample from the Neolithic structure Stonehenge made its way to a science lab after 60 years in a private collection. Here's how one rock told us from whence it rolled.
Sabrina Stierwalt, PhD
Ask Science
The Quick And Dirty
A rock sample extracted from Stonehenge 60 years ago recently resurfaced and allowed for detailed analysis of its composition
The Stonehenge sample best matches other sandstones in West Woods, Wiltshire, in the southern UK, suggesting the massive stone megaliths originated there
Why the prehistoric monument, which pre-dates the Egyptian pyramids, was erected over 4000 years ago remains a mystery
Evidence suggests Stonehenge may have been an astronomical observatory, a royal burial ground, or a sacred healing site
About eight miles north of Salisbury in the southern United Kingdom sits the collection of massive, carefully place stones known as Stonehenge. Sometime around 3000 BCE, the area was dug out in a circular ditch known as a henge. A few centuries years later, around the time the earliest pyramids in Egypt were also being constructed, the large iconic stones were added to the earthwork henge.
Henges themselves are not uncommon. Have you heard of Woodhenge?
Henges themselves are not uncommon. Have you heard of Woodhenge? It's not far from Stonehenge and also dates back to the Neolithic time period. But Stonehenge is certainly something special. (There is also, of course, Carhenge outside of Alliance, Nebraska, but that one was built in 1987.)
What is Stonehenge?
The large circle of what was originally thought to be 80 stones is meticulously aligned so that someone standing at the center of the circle will see the sunrise on the summer solstice pass through the entrance to the monument marked by an archway of gigantic stones. Outside the main circle, separated from the rest, sits a large, relatively rough stone known as the Heel Stone. While standing inside the monument and looking to the northeast, that same summer solstice sun rises over the Heel Stone on the longest day of the year.
When I visited Stonehenge in the early 2000s, I saw a procession dancing to drum beats marking the start of a neo-Druid ceremony.
Thousands of people gather at the monument on the summer solstice to mark this first sunrise of summer in the northern hemisphere. Due to the global pandemic, 2020 marked the first year the event was streamed live online. Before 1977, visitors to the stones were allowed to walk through and even climb on them. But to save the stones from erosion, a rope now keeps visitors relegated to the perimeter a short distance away. Entrance is reserved for religious observers including modern Pagans. When I visited Stonehenge in the early 2000s, I saw a procession dancing to drum beats marking the start of a neo-Druid ceremony.
Archeologists have puzzled and debated for centuries how the gigantic rocks at Stonehenge came to be placed there at a time long before tractor-trailers and cranes. The larger stones are called sarsen megaliths. Megalith is a term for a large stone that is part of a prehistoric monument and sarsen describes the hard sandstone material of the rock. The sarsen megaliths that make up Stonehenge are typically 6 to 7 meters long (including portions of each rock that are buried in the ground) and weigh 20 metric tons. That's the equivalent of about 10 SUVs. The largest of the stones stretches 9 meters tall and weighs more than 30 metric tons.
Even the smaller stones near the center—called "bluestones" for the blueish hue they take on when cut or wet—weigh in around 4 metric tons. These bluestones are a variety of rock types including dolerites, tuffs, rhyolites, and sandstones and are not local to the area which is mostly chalk bedrock. Archaeologists have pinpointed the origin of the bluestones as the Preseli Hills in southwest Wales, an astounding 200 kilometers (that's about 125 miles) on the other side of the Bristol Channel.
New information on where the Stonehenge megaliths came from
The origin of the sarsen megaliths, however, had not been confirmed until a recent article published in July 2020. Since writings from the sixteenth century, the massive rocks that make up the recognizable outer circle of upright stones and the center trilithon horseshoe (that's two vertical stones with a stone placed across the top like an altar) were assumed to come from Marlborough Downs. That's about 30 kilometers (or about 18.7 miles) north of Stonehenge and the River Kennet. The new work confirms that at least 50 of the 52 stones came from West Woods in the Marlborough region and just south of the river.
In the new study, researchers first performed X-ray fluorescence spectroscopy to learn what the stones were made of. In this technique, scientists bombard the rock with X-rays and measure the light that then gets emitted. Different elements will produce different signatures in the resulting emission. They found that the stones were 99 percent silica and one percent other trace elements.
So why wasn't this study done long ago? The use of X-ray fluorescence spectroscopy was first proposed in the 1920s! Well, the X-ray technique is good for a first pass, but to get a more detailed account of the geochemical composition of the stone, the scientists further performed two kinds of mass spectrometry. A sample of rock is again bombarded but this time by electrons and the resulting ions are then grouped by their mass and charge which will be different for different elements. For this technique you need an actual sample of the rock.
A sample extracted from the core of the ancient stone resurfaced in 2018 after sitting in someone's home collection for over 60 years.
By chance, a sample extracted from the core of the ancient stone resurfaced in 2018 after sitting in someone's home collection for over 60 years. The piece of rock was removed in the late 1950s when metal rods had to be put in place to stabilize a cracked megalith. The owner, Robert Phillips, worked for the company involved in preserving the stones and received the sample as a gift. He brought the piece of stone to the United States from Britain in 1977 when he emigrated and then returned it in 2018 for science.
Scientists tested the Stonehenge piece and other sarsens across the south of Britain. The more detailed analysis of the chunk of megalith revealed that it best matched sandstones in West Woods, Wiltshire, about 25 kilometers north of Stonehenge. West Woods is also known as a "hive of Early Neolithic activity" with a large ancient burial site, prehistoric cultivated fields, and a polissoir—a rock known to be used for sharpening stone axes.
Why the Stonehenge mystery remains
Even with this important piece to the Stonehenge puzzle, the prehistoric monument still offers up more mysteries than it solves. For starters, sharp-eared listeners may have noted that I said only 50 of the 52 stones have similar chemical signatures that tie them to an origin in West Woods. What about those other two stones? The compositions of Stone 26 and Stone 160 did not match the rest of the group nor did they match each other. These stones could be unique outliers that were brought in separately or they could perhaps be remnants of a larger group of stones that were once part of the monument.
Only 50 of the 52 stones have similar chemical signatures that tie them to an origin in West Woods. What about those other two stones?
And now that we know where they came from, how were the stones transported over 4000 years ago? Some scientists originally proposed that such large stones may have been moved not by humans but by shifting glaciers. Now geologists suspect the stones were either dragged by large teams of oxen and people or moved on rollers and by boat. Now that the chemical makeup of the stones is better understood, researchers can look for chips of similar rock along suspected transport routes to see if the arduous move left any evidence behind.
Why was Stonehenge built?
And perhaps the biggest mystery is why Stonehenge was built at all. As noted by the authors of the new mass spectrometry study, the builders' "choice of stone used to construct Stonehenge was far from pragmatic or based simply on local availability," so clearly the site serves an incredibly special purpose. Was it an astronomical observatory? Clearly the stones were carefully placed to align with the motions of the sun, so perhaps the monument was used to mark the passing of the seasons and as a guide for when to sow crops and to predict animal mating habits.
A suspected 150 people were cremated and buried there including men, women, and children.
Archaeologists have also found evidence that Stonehenge could be a royal burial ground. Human remains found at the site date as far back as 3000 BCE, before the large outer circle of megaliths was erected, according to radiocarbon dating studies. A suspected 150 people were cremated and buried there including men, women, and children. Given the special nature of the site, Stonehenge may be an ancestral memorial to the prehistoric elite.
Others find evidence that Stonehenge may have served as a sacred place of healing. The thousands of person-hours required to move the bluestones across such large distances suggest they must have been a key part of the monument. Such stones were once considered to have mystical properties, so they could have been intended for healing the injured or the sick. Alternatively, the builders may have themselves originally come from the Preseli Hills and so took the stones from the same place.
The immense scale of the stones and their careful placement makes it overwhelmingly clear that this place is special.
Whatever its purpose, the power of the site is unmistakable. The immense scale of the stones and their careful placement makes it overwhelmingly clear that this place is special. When will we unlock the next Stonehenge mystery? I guess you never know when a piece of 5,000-year-old prehistoric megalith will turn up in someone's private rock collection.
Please note that archive episodes of this podcast may include references to Ask Science. Rights of Albert Einstein are used with permission of The Hebrew University of Jerusalem. Represented exclusively by Greenlight.
Dr Sabrina Stierwalt earned a Ph.D. in Astronomy & Astrophysics from Cornell University and is now a Professor of Physics at Occidental College.
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