The secrets of Stonehenge: how was it built?

Where on Earth did the stones come from?

New technology is pinpointing the sources of the huge monoliths

Stonehenge shouldn’t be there. Its great standing stones – their total weight originally equivalent to that of 13 blue whales – loom starkly from a landscape without rock outcrops. So where were they quarried?

The 12th-century historian Geoffrey of Monmouth wrote that giants had carried the megaliths from Africa to Ireland, and that Merlin then took them on to Salisbury Plain. Others such as the 16th-century lawyer John Rastell thought the stones had been moulded on the spot out of cement.

The 18th-century antiquary William Stukeley observed that not all of the stones were of the same type. He agreed with an earlier suggestion that the largest – sandstone blocks known as sarsens – hailed from near Marlborough, about 20 miles north of Stonehenge. But what of the other, smaller monoliths known as bluestones?

Africa and Ireland continued to be suggested as possible sources. One theory posited that they had arrived with ancient Greek tin traders, used as ballast in their ships. Perhaps they came from Brittany or Finland; British sources mooted included Dartmoor and Edinburgh. Then, in 1921, geologist Herbert Thomas established that Stonehenge’s bluestones had been quarried from the Preseli Hills in Pembrokeshire. But how did they get to Stonehenge? Were they transported by Neolithic people, or could they have been carried east by glaciers?

Thomas had no doubt that humans were responsible, and most experts since have agreed. However, the glacier theory lingers, supported less by solid geological evidence than by disbelief that anyone would have travelled so far – 150 miles in a straight line – to source these megaliths some 5,000 years ago when they were quarried.

Over the past few years, new lab technologies have enabled geologists Richard Bevins and Rob Ixer to confirm that Pembrokeshire was indeed the main source of the bluestones. They have identified two outcrops in the Preseli Hills – Craig Rhos-y-felin and Carn Goedog – that were the sites of megalithic quarries for two specific types of bluestone known at Stonehenge.

Yet not all of Stonehenge’s bluestones were sourced from Pembrokeshire. Bevins and Ixer believe that the Altar Stone, a unique greenish sandstone megalith lying flat near the centre of Stonehenge, came from elsewhere. Their research suggests that it’s probably from eastern Wales or the Marches, though they haven’t ruled out another, more distant source.

Stone cold certainty

But what of Stonehenge’s sarsens? These were, it seems, collected around 2,500 BC – five centuries later than the bluestones. Sarsen is almost pure silica, making examples from one location all but identical to another – so defeating traditional geological attempts to use variations within the stones to help identify sources. Now, though, science has again come to the rescue, thanks to the work of David Nash of Brighton University and a technology called portable X-ray fluorescence (pXRF).

Nash’s team analysed the surfaces of the sarsens at Stonehenge, firing radiation from a gadget the size of a hair dryer. By studying X-rays reflected back, they created a geological “fingerprint” that showed all the stones to be very similar to each other.

A further breakthrough arrived when a metre-long rod, or core, of sarsen unexpectedly turned up in Florida. This core was originally drilled from a Stonehenge trilithon during conservation work back in 1958; Robert Phillips, who’d been on the job, had taken it with him when he moved to the United States. In 2018, Phillips offered it back to English Heritage – and Nash and his team stepped in. Now they could compare a Stonehenge megalith with sarsen boulders they had sampled across southern England, crushing up the rock and applying sophisticated geochemical analyses.

The only match for the megalith was in West Woods, on the edge of the Marlborough Downs. So this is the source of the Florida core – and, by implication, all the other Stonehenge sarsens, which pXRF had shown to be so alike.

How were the stones transported to their current site?

Sea and river voyages have been proposed – but human-powered overland treks seem most likely

Moving stones weighing up to 4 tonnes from the hills of west Wales to the plains of southern England would be a mighty undertaking even in the 21st century. That our Neolithic ancestors achieved the feat seems mind-blowing.

Yet achieve it they did. Herbert Thomas suggested that the bluestones had been hauled along an overland route of 170–180 miles, passing north of Gloucester to avoid the Severn estuary – the crossing, he thought, would have been too dangerous.

However, many archaeologists disagreed, favouring the theory that the stones had been transported by water. They argued that the Altar Stone had been quarried at Milford Haven, a natural harbour a day’s walk south of the Preseli Hills, and that this would have been the departure point for transport by sea.

The voyage might have hugged the coast to the Bristol Channel, then taken one of two routes: following English rivers to Stonehenge, sledging the short land sections; or continuing south-west along the coast, around Land’s End, east to Christchurch harbour and then up the river Avon.

The scale of this transport operation must have been unmatched in Europe at the time

On the face of it, transporting the sarsens would have been far easier. They had to travel only 15–20 miles south overland from the Marlborough Downs. But sarsens are far bigger than bluestones, weighing in at between 4 and 40 tonnes compared with the 1–4-tonne bluestones. What’s more, the sarsens were artificially shaped, and some of that carving would have been carried out on site, so they would have been heavier when they were transported.

Many archaeologists have envisioned men with ropes pulling stones across rolling logs. This idea has featured in TV programmes and numerous experiments, and is enshrined today in a full-scale model at Stonehenge. Yet research carried out by Barney Harris, an archaeologist at University College London, has cast doubt on this theory. Experiments, along with observations of people moving megaliths in Indonesia, demonstrate that rollers are uncontrollable; stationary sleepers or fixed tracks would be required.

Due to the stones’ vast size and irregular shapes, a sledge would have been essential. The biggest loads would have weighed more than 45 tonnes, which could be pulled only along a fixed trackway. The scale of this operation, particularly the volume of timber required, must have been unmatched in Eu- rope at the time.

It’s easier to envisage the transport of the smaller bluestones, dragged on sledges over sleepers or perhaps even carried with poles or wooden frames; such methods have been recorded in north-east India.

• Listen | Mike Pitts answers listener questions on Britain’s most famous prehistoric monument, Stonehenge, on this episode of the HistoryExtra podcast

The road to ruins

This is what I propose as a possible route taken by the bluestones. Following river valleys and avoiding frequent crossings and steep slopes, we traverse Wales beside the rivers Taf and Tywi, rise to near the source of the Usk north of the Brecon Beacons, and follow that river to the sea. We then paddle across the Severn estuary – a quite feasible crossing that saves us from having to climb the Cotswolds (which a more northerly crossing would necessitate). We continue along the Bristol Avon, perhaps still by boat, and reach Stonehenge via rivers and land, with a total journey of 220 miles.

By the same principles – track river valleys, avoid steep climbs – there is only one option for the sarsens: head south to the edge of the Marlborough Downs, drop into the Vale of Pewsey and follow the Avon Valley (which avoids Salisbury Plain’s forbidding escarpment), reaching Stonehenge in 17 miles.

How did the sarsens get their distinctive shape?

Incredibly hard stones were shaped without the use of metal tools

Get close to Stonehenge and you’ll see that the stones are carved. Other megaliths across north-west Europe have artificially smoothed faces, but at Stonehenge the builders went further, removing quantities of rock to shape the stones to a more or less standard pattern. Not only that, but several of the sarsens are connected one to another with mortice-and-tenon or tongue-and-groove joints – techniques we can only assume were borrowed from timber construction.

Squared lintels and, in one case, stone joints are prominent in medieval illustrations of Stonehenge, but it wasn’t until 1901 that anyone seriously addressed how they were carved. In that year, the first scientific excavation at the site determined that the stones had been raised before iron tools were available; it also found stone tools that had been used in the work.

Archaeologists then looked at how sarsens on the Marlborough Downs had been shaped in more recent times: with fire and water in the 17th and 18th centuries, and with steel tools in the 19th and early 20th. They assumed that natural sarsen slabs of more or less megalithic shape would have been available to our Neolithic predecessors on site, and thus that the main tasks they would have been confronted with were edge-trimming followed by selective smoothing. The former might have been achieved by pouring cold water onto lines of burning wood on the stone; the sudden change in temperature would crack the stone. (There is no evidence that this process was used, though.) The stones would have been shaped by hammering with mauls – small boulders of very hard sarsen, up to the size of a football. Finishing and jointing, said the archaeologists, would be done by laboriously pounding to remove stone dust.

High-tech revelations

Our understanding of Neolithic techniques has improved with the development of our own technology, which has enabled the first comprehensive examination of the megaliths. By bouncing laser light off the stones and converting the data into precise distance measurements, scientists have built 3D digital models of the megaliths – and these have revealed that the sarsens were almost totally dressed. Archaeologists Marcus Abbott and Hugo Anderson-Whymark distinguished between four stages in the dressing process, employing progressively smaller hammers: first, heavy shaping at source, then coarse preparation, finishing and jointing on site. (These stages must have created large amounts of silica dust in the air, making silicosis a health hazard for those working on the stones.)

Though most of the bluestones were undressed, those now in the central horse- shoe arrangement (see aerial view opposite), plus a few in an outer ring, were finely carved. This was probably done using similar techniques to those used in the dressing of the sarsens.

Another aspect revealed by the laser study is just how much damage was inflicted on the stones by 18th and 19th-century souvenir hunters. When first carved, most megaliths would have had sharper corners than they do today.

• Read more | Stonehenge: a prehistoric tourist trap

How did Neolithic engineers erect these huge stones?

Logs and levers were probably used to raise sarsen stones

So you’ve accomplished the unimaginably arduous task of transporting the stones to their new home in what’s now southern Wiltshire. Now you need to get them in place and upright. Most modern theories on how this was achieved were shaped by a 1924 book by the aptly named Herbert Stone.

A retired engineer, Stone described in detail the creation of the sarsen circle, demonstrating his ideas with working models. Stone’s workmen tipped a fully carved megalith down a ramp into a pit. They then tied its top to a set of sheerlegs (a timber A-frame) and hauled it upright using ropes attached to the top of the frame. This process is depicted in English Heritage’s Stonehenge guidebook, and has been used in several experiments, notably for a 1990s BBC/PBS film involving two full-scale concrete megaliths. Yet it would have been impossible.

Let’s start at the beginning. We know that Neolithic people built large oak structures, some comparable to Stonehenge, so would have applied lessons learned from working with timber. In the stone monument, the sides of 30 lintels in a perfect circle curve to fit the ring. Such carving could only have been done when the lintels were still on the ground, and the ring must have been made first, assembled and then moved out of the way before pits were dug.

All erection experiments, from Stone’s models to the concrete giants that appeared in the TV documentaries, have been carried out in a field. But Stonehenge was a building site. The largest stones – the horseshoe of five trilithons (10 huge megaliths and five lintels) at the centre of the site – had to go up first; the stones were too big to pass through gaps in the outer circle. When it came to raising that circle, there was no space on the inside for sheer legs and gangs of pullers, nor to lie megaliths on the ground inside the circle to lift using sheerlegs on the outside. There had to be another way.

Polynesian precedent

I found a clue in a demonstration conducted on Rapa Nui (Easter Island) in 1986 for Norwegian adventurer Thor Heyerdahl. A horizontal stone moai (monolithic human figure) was rocked with levers, and then rubble was packed in space created beneath the head end. This growing mound raised the moai to a near-vertical position, from where a light pull on ropes finished the job.

William Gowland, the archaeologist who directed the 1901 excavation at Stonehenge, proposed something similar here: a process using long wooden levers to lift the sarsens, piling up logs until each megalith slipped upright into its pit.

Herbert Stone thought that the lintels had been dragged up mounds heaped against the megaliths, but there are signs of neither mounds nor the necessary quarry pits. So Gowland’s method also appears more plausible for the installation of the lintels. One end of a lintel is raised with levers, and logs placed underneath. The same is done to the other end, then again at the first, repeat- ing until the lintel is level with the top of standing megaliths, when it is slid across.

The tops of the megaliths needed to be perfectly level to support the lintels. Stone argued that the correct heights would have been achieved by digging pits at different depths according to the size of each stone – but, with each stone typically weighing 20 tonnes, it would have been impossible to do this with any level of precision. Instead, the megaliths could have been erected to a slightly greater height than needed, and the tops then cut level with one other.

How long did Stonehenge take to build?

Construction of the monument may have been surprisingly rapid

The site we now call Stonehenge evolved over several hundred years. The first ditch-and- bank earthwork henge, together with a large ring of bluestones, was created around 3000 BC. These stones were later arranged nearer the centre. Around 2500 BC they were removed, the sarsens were installed, and the bluestones were replaced among them. But how long did each period of construction actually take? Is it possible to calculate how many hours this massive Neolithic construction project swallowed?

Such intangibles are hard to pin down. No one has carved a megalith, nor moved a bluestone to Wiltshire, since Neolithic times. Even so, thinking on the time and effort that went into creating the world-famous monument has evolved over the past few decades.

Archaeologist Richard Atkinson, who directed excavations at Stonehenge in the 1950s and 60s, estimated that it would have taken 1,500 men 10 years to fetch the sarsens from the Marlborough Downs. Fifty masons must then have worked 10-hour days non- stop for two years and nine months just to shape them, he believed.

In 1961, Canadian geologist Patrick Arthur Hill was less daunted by the logisti- cal challenges. He calculated that 25 men could have pulled a 50-tonne sarsen to Stonehenge in less than a week – if they dragged it over snow. Hill’s idea remains popular, but it’s worth noting that he was inspired by Inuit sledges crossing Arctic sea ice, not a frosty crust in an English winter.

• Read more | Your guide to Britain’s prehistoric stone circles

More recently, engineers interested in efficiency have taken up the case: you can watch YouTube videos showing people competing to move, erect and lift stones with as few hands as possible. But was efficiency even a consideration in the construction of Stonehenge – a monument that, let’s not forget, defies all economic sense? Recently, archaeologists have been looking at practices in places such Madagascar, north-east India and the Indonesian island of Sumba, where people have been raising stones for genera- tions. In those areas the goal – or, at least, the effect – seems to be to have as many people on the job as possible. A good crowd brings status for organisers and sponsors, and participants share in the fun, accumulating credits for their work.

Ancient efficiency

The difficulties of creating Stonehenge are sometimes exaggerated. Megaliths weighed less than Atkinson thought, and excavations reveal that Neolithic people were skilled engineers, carpenters and stoneworkers.

On the other hand, we suspect they liked to party, as people do today when moving a stone on Sumba. Bluestones would have passed through villages to be seen and celebrated. There is evidence for feasting at Durrington Walls near Stonehenge, where it is thought as many as 4,000 people gathered for the great build.

Taking all that into account, here’s a plausible scenario for the timescale of construction. Setting out with a bluestone from the Preseli Hills in late summer, a team of 25–30 (picking up others en route) could have reached the Bristol Channel in 45 days – 30 if using a boat down the lower river Usk. Crossing the sea, paddling upriver and moving to a sledge as necessary, they’d arrive on site by mid-October, two months after departure.

Moving a sarsen demanded more effort: perhaps 200–300 people would take a few weeks to haul one to Stonehenge (assuming a track was in place). So a thousand workers could have moved all of the sarsens in 6–18 months – or perhaps twice that long, depending on rituals and extraneous events.

Dressing-time is almost impossible to estimate. The required shape of one megalith depended on that of another, and the project had to maintain momentum. Erecting trilithons and fitting lintels could be done in one season; with 1,000 people working in 15 teams of 60–70, the circle could be constructed the next season. The whole thing might, then, have been completed in five spectacular, never-to-be-forgotten years.

This article first appeared in the March 2022 issue of BBC History Magazine