How was Stonehenge built?

If ever there was a construction that prompts questions, it is Stonehenge. Who built it? Why? When? How?

We get a glimpse of The world of Stonehenge at the British Museum exhibition, a fabulous opportunity to think about the people who created this globally famous monument. We can wander through the gallery, surrounded by treasures and everyday objects of the time from across northern Europe, and feel how different things were from our own times. Knowing the exhibition was coming, I decided I should finally write the book I'd been telling myself I should for nearly 40 years – ever since my first excavation at Stonehenge, and the discovery that there were so many untold stories. But what to say?

There are many, many books about Stonehenge, and most of them do the same thing. They tell us about archaeology and archaeologists – they describe what we know about the landscape and the people four or five thousand years ago, and all the different constructions that came and went on the site. They also explain, especially and often with great confidence, why Stonehenge was built. It seemed to me that the last question, apparently the hardest to answer, is actually the easiest. Within the limitations of ordinary human endeavour and stone age times, almost anything could have been possible – and so long as we properly represent the site's archaeology, whatever we come up with can never be proved wrong. That makes it, for me, the least interesting question.

But how? Now there was a subject! How were the stones brought to Salisbury Plain from different parts of Britain? As Stonehenge may be the only megalithic monument with jointed stones in the world, apart from a small, and totally unconnected structure in Tonga in the South Pacific, how were the stones shaped? And how were stones weighing many tons raised into the air with nothing but ropes and timbers, by people who had never done anything like it before? These are not easy questions to answer, but there is plenty of hard evidence, including new research and much that has been done over the past century, often to be found in obscure reports. No one had published a book just about the stones since 1924. In this blog I'm going to share with you some of the things I learnt as I wrote mine.

For archaeologists like me engaged with Stonehenge, it's easy to get drawn into debates about the small details – puzzling over exactly what happened at a monument that changed and grew, poring over century-old excavation diaries and lifting the lids on dusty museum boxes – and lose sight of the stones. The great, weathered blocks that have been there for thousands of years, once fresh in their new arrangements and now a familiar ruin, have been studied more by artists than archaeologists – the first proper analysis of their shapes and surfaces was made as recently as 2012.

Thanks to that study, we know there to be more signs of original surface dressing, and more Bronze Age carvings into those Neolithic surfaces, than any of us had realised. The stones themselves have much to tell. They also have an emotive power that comes partly from their lost stories, and partly from their sheer presence as forms. Every stone is different, from small shapeless boulders to massive, variously dressed slabs. Carved joints and natural hollows and wrinkles are unique to every megalith and lintel. All are covered with lichens which, in the four decades since daily visitors have been excluded from the central part of the monument, have grown into delicate and fragile gardens, pouring in streaks down faces and painting them with splashes of translucent pale greens, spattered with purple and dark brown. And every sight is different, as rain darkens and emphasises, sun animates, and times of day and seasons bring their own distinctive light and shade.

Stonehenge is very unusual in the ancient world for the distances over which its materials were transported to the site, especially those megaliths we know as bluestones. Most of these, made from different types of igneous rock, were quarried in south west Wales – I estimate their journey at 220 miles. However, the really big stones, and the ones that give Stonehenge its distinctive silhouette, were found in southern England. These are the sarsens, made from hard sandstone, and none had to travel further than from the Marlborough Downs, 20 miles to the north.

After mapping precise routes for these different stones, and calculating weights for them all, I now believe the distinction between bluestones and sarsens is even more important than I had originally thought. If we were to think about the total amount of weight moved for each distance, in pure ton-miles the bluestones (35,000–40,000) sound more impressive than the sarsens (25,000). But that ignores two significant factors.

First, we think the original bluestone structure was a large circle of 56 stones, raised five centuries before anything else. People could, if they'd wanted to (who knows?), have brought one stone to Wiltshire every year for 56 years - raising one could have been done entirely independently of any others. By contrast, the sarsens were carved and engineered to work together, with a variety of joints and shapes. Moving and dressing one stone would have been part of a larger project, needing more people to work together. And then there's size.

An average bluestone weighed two tons, an average sarsen 20 tons – and the largest approached twice that. Bluestones could have been carried across streams (people carried stones of similar size, in a wooden frame, on the border of Myanmar and India in the last century). Sarsens would have needed strong sledges, adding to the weight – as would the fact that some of the dressing would have been done at Stonehenge after arrival. A large sarsen on an oak sledge, following a route taken by over 70 other stones, would have broken the soft ground, making a wooden track necessary. A route of 15 to 20 miles from the Marlborough Downs to the north would alone have been a monumental undertaking. For generations, people would have known that the bluestones came from - far away over the horizon to the west. But the really memorable feat would have been the journeys, not the distance, made by the sarsens.

As a postscript, archaeologists in the last century found lumps they called Chilmark ragstone in some of the pits holding up the megaliths. The nearest source for this is 12 miles to the south-west, but no modern geologist has yet been able to find a specimen to examine, and the stone's presence remains a mystery.

Behind almost every believable proposal for how the large Stonehenge stones were raised from laying on the ground, is the image of crowds hauling on long ropes, aided by the leverage of sheerlegs – a tall A-frame of oak poles. You can see it in the English Heritage guidebook, and the BBC featured it in a 1990s film in a spectacular re-enactment with full-scale concrete 'megaliths'. The trouble is, it couldn't have worked.

Models, diagrams and experiments have all imagined one or two stones in an empty field. But the would-be Stonehenge was a building site. The largest stones, known as trilithons (two uprights, one horizontal lintel) must have been erected before the circle that was to surround them, as they were too big to pass through gaps in the ring. This made it impossible to raise circle stones using long ropes, whether you wanted to lay stones down on the inside and pull outwards (there would have been no room for the stones) or outside and pull in (with no room for sheerlegs, ropes and pullers). So it's clear most of the upright stones at Stonehenge could not have been erected this way.

I found inspiration for a solution on Rapa Nui (Easter Island), in a demonstration islanders put on for Thor Heyerdahl in the 1950s. They raised a fallen statue, similar in weight to a large Stonehenge megalith, by repeatedly rocking it from side to side with levers, each time carefully placing small stones underneath the lifted side. Slowly a rubble mound grew beneath one end of the rising head, until, when it was nearly upright, a gentle tug on some ropes finished the job. The same thing could have been done on Salisbury Plain – not with stones, but wood, slowly adding tied and perhaps jointed timbers to a growing tower as 30 tons of megalith gently and safely rose to an upright position.

By the early 20th century, five large stones had recently fallen and 10 were propped up with wooden poles. Another fell in 1963. Such dilapidation was stopped by major restoration work in the 1920s and, the 1950s and '60s – without which there would now be few stones standing. Unfortunately during these works considerable disturbance was done to archaeological layers underground which was not always well recorded. Nonetheless, there is still much to excavate at Stonehenge for the future, and more stories to learn.

How to Build Stonehenge by Mike Pitts is published by Thames & Hudson.

The world of Stonehenge was open ran from 17 February to July 2022.

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