For several centuries, modern people have been trying to figure out how prehistoric farmers in southern Britain moved multi-ton blocks of stone into a pair of concentric circles at Stonehenge. Chemical studies on the stones have revealed their origins: the smaller bluestones hailed from two quarries in Wales, and the larger sarsen stones came from 30km north of Stonehenge.

We still don’t have direct evidence of the engineering behind the famous prehistoric monument, but one popular idea suggests that people dragged the stones on wooden sleds, using pig grease to make the sleds easier to move. A new look at potsherds from a nearby village may lend some support to the idea, but it’s still not direct evidence.

Pigging out at Stonehenge

Durrington Walls, a large village near Stonehenge, often hosted feasts associated with ceremonies at the nearby stone circle. Microscopic traces of ancient fats left behind on potsherds provide some clues about what people ate at those feasts. Animal fats are all pretty similar, but each species has its own unique set of molecules (called lipids), which make it possible to tell which animal an organic residue came from. More than a third of the pots at Durrington Walls had held mostly pig fat.

Archaeologists have found pig bones in abundance at Durrington Walls and at sites near other monuments, like Avebury. In fact, a recent study found that people were bringing pigs from quite far away, perhaps as far as Wales, for feasting at Stonehenge. But the condition of most of those bones suggests that the pigs were roasted whole, probably on spits—not cooked in pots. Most pig bones were discarded more-or-less intact, with some charring that suggested roasting rather than boiling. So if pig fat ended up in a container, someone had probably put the container under the roasting pig to catch the drippings on purpose.

Archaeologist Lisa-Marie Shillito suggests that purpose may have included greasing slipways or rollers beneath wooden sledges that hauled the massive stones to Stonehenge.

Nothing went to waste

Archaeologists found the pig lipid residues mostly in a type of pottery called Grooved Ware: bucket-shaped vessels up to 0.4m (1.3ft) wide. These are big vessels, ideal for storing large quantities of something, and they seemed more likely to be found buried in pits. Based on ethnographic evidence, we know that pig tallow lasts for long periods if it’s stored in a pit with a relatively stable temperature. And pig tallow is softer than the harder tallow that comes from rendering cattle or sheep fat, so it would be better for greasing a sled (or a slipway for a sled).

Of course, there are a lot of things you could use pig fat for if you lived in a Neolithic village. The lard or tallow that comes from rendering the fat could provide fuel for lamps, tallow for candles, or material for conditioning leather and hides. People in Durrington Walls would probably have needed most of those things.

In fact, Shillito points out that modern people have a bad habit of assuming that anything potentially edible in the archaeological record was meant to be eaten. “Interpretations of pottery residues are often overly simplistic,” she wrote. The pig fats stored at Durrington Walls could have been intended for all sorts of non-food purposes, even if they were a byproduct of food preparation. Pigs were probably a multi-purpose resource. But could that purpose have included megalithic engineering projects?

Ancient pork barrel projects?

If we're asking "could be," then the answer is yes, according to experimental archaeology. In this context, that means “several archaeologists tried different ways of moving very large rocks,” so now you understand why Indiana Jones always seemed to be in such good shape. In a 1997 study, a team led by Julian Richards and Mark Whitby found that dragging the rocks on wooden sledges worked best, especially if they pulled those sledges along a timber slipway greased with a synthetic version of pig fat. A team of 20 sturdy archaeologists could pull the smallest of the monument’s stones, the 1-2 metric ton bluestones, at about 1.6kph (about 1mph).

The archaeologists weren’t the first to come up with that idea. A painting from a 4,000-year-old Egyptian government official’s tomb showed a team of workers moving a colossal stone statue on a wooden sledge, with someone pouring out liquid from a jar ahead of the sledge. It’s not clear whether the liquid was supposed to be grease or water (and it’s possible that it was just a ritual offering made along the way), but other images from ancient Egypt and Mesopotamia also show workers using lubricants in construction projects.

And in Indonesia, on the island of Sumba, people still build tombs with massive megaliths that take teams of a few hundred people to pull to the gravesite on wooden sledges. They accomplish the task by laying logs across the path and using them as rollers to pull the sledge over. Once the sledge is past, of course, part of the team has to grab the logs and hurry them to the front again. It’s hard work, and the trip can take days or weeks, but it gets the job done.

“The so-called ‘greased sled’ hypothesis is an interesting idea and worthy of consideration,” Cardiff University archaeologist Richard Madgwick (who recently studied isotopic signatures in pig bones at Durrington Walls but wasn’t involved with Shillito’s study) told Ars. “However, there is little direct evidence to support it.”

Verdict: We still don’t know

The fact that people were collecting pig fat at feasts in a village nearby doesn’t prove that they used pig-greased sleds to haul multi-tons tones from quarries to the monument, but it certainly shows that they could have. We already know that it would have worked well and that several cultures around the world thought of the idea. However, nothing directly links the containers of pig fat to the process of building Stonehenge, and there’s no physical evidence of sledges or slipways left.

And there are some serious questions about why fat for stone-moving would be stored at Durrington Walls, so close to Stonehenge itself.

“If the grease was used in moving the stones to the area, it doesn’t make sense for it to be produced at their final destination. It would be more likely to be produced at the point or origin or perhaps staging posts along the route,” Madgwick told Ars. “The wide-ranging origins of the animals also makes the grease production hypothesis seem a little less likely, as it would mean the animals being brought to the site, tallow extracted, and then the produce sent back over distance.”

On Sumba Island, people slaughter pigs and water buffalo in huge quantities all along the route to feed the crews hauling stones to build tombs, and it’s easy to imagine people in Neolithic Britain taking a similar approach to feeding people along the way to Stonehenge.

But it’s also possible that the greased sledges were sort of a last-mile technology, used for moving the sarsen stones from relatively nearby or for bringing the bluestones on the final leg of their journey. (It’s been suggested that they travelled on boats along rivers for the bulk of the trip from Wales.) For now, there’s just not enough evidence to say for sure.

“The archaeological record at Stonehenge is complex, and making sense of how people achieved some of the feats that are in evidence will long remain a challenge,” Madgwick told Ars.

Meanwhile, archaeologists definitely do have enough evidence to know that although this was an impressive feat, it was hardly miraculous or unprecedented. “There is ample ethnographic and experimental evidence to demonstrate how megaliths can be moved with ‘pre-industrial’ technology,” wrote Shillito.

Antiquity, 2019. DOI: 10.15184/aqy.2019.62. (About DOIs).