Stonehenge, where were the stones from? Geochemical fingerprinting research reveals origins of the sarsen stones

Typically weighing over 20 tonnes and standing up to seven metres tall, the largest stones at Stonehenge are called the sarsen stones. These form all fifteen stones of Stonehenge’s central horseshoe, the uprights and lintels of the outer circle, as well as outlying stones such as the 30-ton Heel Stone, the Slaughter Stone and the Station Stones. Fifty-two of the original 80 or so sarsens remain at the monument on Salisbury Plains in Wiltshire and form one of Britain's most visited heritage sites. For centuries though, where these stones came from has been a matter of speculation.

Described by English Heritage as a “masterpiece of engineering”, Stonehenge was erected in the late Neolithic period about 2500 BC. Archaeologists and geologists have for years been debating the source of the two distinct stone types used in the prehistoric structure. The smaller ‘bluestones’ have attracted more attention from scientists, and their origins have been traced to Pembrokeshire in west Wales. Yet the original sites of the giant sarsen stones, which are found naturally across many parts of southern England, has been a mystery until very recently, when a team led by Professor David Nash from the University of Brighton undertook a study as part of a project funded by the British Academy and the Leverhulme Trust.

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Geochemical fingerprinting helps discover where the Sarsen stones originated

Professor David Nash is an expert in geochemical sediments and environmental change. He was joined by his Brighton colleagues Dr Jake Ciborowski and Dr Georgios Maniatis and partners that included archaeologists and Stonehenge experts Professor Timothy Darvill (Bournemouth University) and Professor Mike Parker Pearson (University College London), and heritage specialists Susan Greaney (English Heritage) and Katy Whitaker (Historic England).

The most accurate means of determining the provenance of any stone artefact is geochemical fingerprinting, whereby the elemental chemistry of the artefact is matched against that of potential source areas. For Stonehenge, this required two stages: an initial analysis of the sarsen stones at the monument, followed by equivalent analyses of sarsen boulders across their range of natural occurrence (south of a line from Devon to Suffolk).

To determine the elemental chemistry of the sarsen stones at Stonehenge, the team used a device called a portable X-ray Fluorescence Spectrometer (or pXRF). All 52 of the remaining sarsens were analysed using this non-invasive instrument, with six chemical readings taken per stone. Statistical analyses, performed by Dr Georgios Maniatis, were used to identify any clustering within the dataset.

To identify the most likely source of the sarsens, the team compared chemical data from a core drilled through Stone 58 in the central horseshoe at Stonehenge against equivalent data from 20 sarsen areas across southern England. These analyses used a high-precision analytical technique called inductively-coupled plasma mass spectrometry (ICP-MS). Standard geochemical provenancing approaches were used to define any chemical similarities between the Stonehenge core and sarsen outcrop samples.

Researcher with Stonehenge stones in the background unpacks geological research observation tools.

Dr Jake Ciborowski prepares to undertake pXRF analyses at Stonehenge.

Stonehenge sarsen stones traced to West Woods on the edge of the Marlborough Downs

From their pXRF data, Professor David Nash's team discovered that 50 of the 52 sarsen stones at Stonehenge share a consistent chemistry, pointing strongly to a common source. More excitingly still, they were able to use their ICP-MS data to identify the most likely source of the sarsen stones as West Woods, on the edge of the Marlborough Downs, around 15 miles north of the famous stone circle.

Susan Greaney, Senior Properties Historian for English Heritage, the charity that cares for Stonehenge, said: “This research provides a fantastic leap forward in our knowledge about Stonehenge, as we can finally answer the question of where the iconic sarsen stones were brought from. We’re so pleased that the core from Stone 58, which the Phillips family returned to Stonehenge, has enabled the team to undertake a small amount of destructive sampling, adding a crucial piece of evidence to the jigsaw.”

The ‘Philips Core’ she refers to was drilled from Stone 58 during conservation work at Stonehenge in 1958. The location of the core remained a mystery until Robert Phillips, a representative of the company who did the drilling work, returned it to the UK from his home in Florida. Stone 58 was one of several stones at the site that had toppled over in the distant past. During conservation work in the 1950s, a longitudinal crack was discovered running through the stone. To conserve the stone, three cores around 2.5cm in diameter were drilled through its full thickness (around 1m) to insert metal rods.

Two of the cores then disappeared, though part of one was rediscovered at Salisbury Museum in 2019. The third core was given to Robert Phillips, who worked for the drilling company, and went with him to the USA when he retired. Phillips returned the core to English Heritage in 2018 to provide material for research, before he passed away in 2020.

Further to the pioneering research using the ‘Phillip’s Core’ and other samples to show where Stonehenge's large sarsen stones were likely to originate, in a paper published in the journal PLoS ONE, an international team led by David Nash were able to reveal that the geological structure of the stone made the sarsen ideal for building a monument made to last. Analysing a small section of the 1950s core, David Nash's team found that the sarsen's structure of sand-sized quartz grains cemented tightly together by an interlocking mosaic of quartz crystals was what made the stone so impervious to crumbling or erosion.

Dr Jake Ciborowski working on a Sarsen

Dr Jake Ciborowski analysing a sarsen lintel stone at Stonehenge using a pXRF instrument.

"... probably the most analysed piece of stone other than Moon rock."

David Nash said: “It is extremely rare as a scientist that you get the chance to work on samples of such national and international importance. Stonehenge is part of a World Heritage Site and is subject to the strictest legal protections, so it would be highly unlikely that we would be able to access this type of material today. Getting access to the core drilled from Stone 58 was very much the Holy Grail for our research. Thanks to help from organisations such as the British Geological Survey and the Natural History Museum we have been able apply a suite of state-of-the-art techniques to it. This small sample is probably the most analysed piece of stone other than Moon rock!“ The research was given a nomination for The Current Archaeology Awards 2020, which celebrates projects covered by the magazine, highlighting those deemed to have made the most outstanding contributions to archaeology.

Where next for geological research on the Stonehenge stones? There are still many mysteries to be uncovered. “Although we now understand where most of the sarsen megaliths at Stonehenge originated, we still don’t know where two of the 52 remaining sarsens at the monument came from,” says David Nash. “These are upright Stone 26 at the northernmost point of the outer sarsen circle and lintel Stone 160 from the inner trilithon horseshoe. It is possible that these stones were once more local to Stonehenge, but at this stage we do not know. We also don’t know the exact areas of West Woods where the sarsens were extracted. Further geochemical testing of sarsens and archaeological investigations to discover extraction pits will be needed to answer these questions.”

Professor Dave Nash analysing the sarsen core extracted from Stone 58 at Stonehenge. Photo by Sam Frost, English Heritage

Professor David Nash with the core sample drilled from Stone 58. (Image courtesy of Sam Frost, English Heritage.)