Stonehenge and the Scope of Uncertainty

Today we're headed back in time, to the green sloping plains of southern Great Britain, where a great circle of ancient standing stones has fascinated us since before recorded history. Stonehenge is not only one of the most recognized landmarks in the entire world, it is also the focus of pseudoscience, pseudohistory, and New Age mysticism. Today we're going to look at Stonehenge myths — not in the context of debunking them, but from the perspective of characterizing the scope of what remains unknown about this most iconic of all monuments.

Questions about Stonehenge that we often hear are who built it, when, and why. You may well have heard the popular legend that Merlin the magician directed magical giants to lift the mighty stones to create it — a story that comes from a fictional text written by Geoffrey of Monmouth around 1136 CE, a time when Stonehenge had already been well known to the Britons as an ancient enigma for at least 1200 years. Later, the prevailing archaeological theory was that it was built by the druids — and we'll talk about that shortly. The question that looms over all of this, and to which we still don't have a solid answer — is what was the purpose of Stonehenge?

When pondering this, a rational question to ask is why don't we know? People obviously lived there when it was built, and people still live there now; it would seem to follow that its purpose would have always been known via this continuous thread of history. And that's where the breakdown occurs: it turns out there was no continuous habitation. In fact, there have been at least five known distinct periods of human habitation of what we now call the Salisbury Plain, each probably separated by periods of disuse.

Although prehistoric humans have occupied Great Britain for much longer, evidence of farming on the Salisbury Plain goes back to around 3500 BCE, in the form of trees having been cleared for agricultural use. But the first evidence of a monument at the Stonehenge site is considerably older; a series of three to five postholes where it appears timbers may have been placed upright by an unknown stone age culture. This dates to an incredible 10,000 years ago.

It's worth noting that all these dates given are approximate and have error bars, as they come mostly from radiocarbon dating of artifacts from the site: mostly timber, bones, and antlers, which were used as digging implements. Various sources give slightly different dates for most of these events, and those that I give here are my own synthesis from multiple sources.

The earliest known people at Stonehenge were those of the Windmill Hill tradition, consisting of various neolithic stone age cultures. These were the creators of the original Stonehenge, who made its first earthworks over about two centuries circa 3000 BCE. Deposits of bone indicate the Windmill Hill cultures were there for about five centuries. Stonehenge then stood abandoned.

The next contributors were the bronze age people of the Beaker culture, approximately 2500 - 2000 BCE, who began their work using wooden posts but soon switched to stones. By the end of their stay at the site, the iconic large stones, called sarsens, were in place. It was during the Beaker period — named for their ceramic vessels — that Stonehenge appears to have been most widely used. Isotopic analysis of bone fragments reveals that some of the animals brought there had come from all over Great Britain, and a village just a few kilometers away shows evidence of occasional mass occupation. Once the Beaker culture finished this main body of work, creating the most familiar face of Stonehenge, they too appear to have abandoned the site.

The people of the Wessex culture were the next, and last, builders of the great monument. Their most notable contribution were what's called the Y and Z holes, two concentric rings of pits outside the main sarsen circle. Their intended purpose may have been to hold upright stones or timbers, but they appear to have never done so, and were filled in not deliberately, but by natural accumulation of wind-blown soil.

Centuries after the departure of the Wessex culture, the fifth and final habitation of the Salisbury Plain began. By the time of the Romans, permanent structures like the iron age hillfort at modern-day Salisbury (about 10km south of Stonehenge) had begun to appear, and the towns that grew around them continue thriving today. It's only from that point that a consistent, continuous thread of known history in and around Stonehenge has been recorded.

The Romans had no knowledge of the Wessex culture, let alone any insight into what they'd used Stonehenge for. Similarly, we've no evidence that the Wessex had any idea what the Beakers had used it for, or that the Beakers knew the reason the Windmill Hill culture had created their earthworks. It's likely that humankind didn't just lose the history of Stonehenge once, but possibly as many as four separate times. None of those four earlier cultures left any written histories; we know them only from their archaeological telltales.

Given this history, it's clear that the actual druids had nothing to do with Stonehenge, as it predated them by thousands of years. But it wasn't until the twentieth century that archaeological science had progressed far enough to make this determination. Ironically, this was about the same time that modern followers of Neopaganism and the New Thought movement began calling themselves druids and holding their solstice celebrations there. The original hypothesis that Stonehenge was the creation of druids had come from one eighteenth century archaeologist, who worked at a time before we had useful tools for dating sites.

So with this basic framework in hand of what we do know about Stonehenge, we can begin to make reasonable assessments of some of these alternative history claims about it. Generally the alternative theorists leverage the fact that much remains unknown about Stonehenge as support for their new or fringe idea — a logical progression that's hard to support. We can assert this because Stonehenge, like many ancient mysteries, has a relatively limited scope of uncertainty.

What is meant by the scope of uncertainty? Often, when we discuss any ancient mystery, some detail remains uncertain. For example, in Skeptoid #202 about Puma Punku, an ancient megalithic site at Tiwanaku in Bolivia, there are certain stones that could have been fashioned and moved using any of a variety of methods that we know about. But in some of the cases, there wasn't enough evidence left to tell us for sure which was used. So archaeologists might say something like "We don't know how the ancients did this," a statement which often has the unfortunate effect of communicating that we have no idea how it could have been done, leading some to wrongly infer that it wasn't possible by any earthly means. In fact, the scope of uncertainty is quite small. We're familiar with the methods used by the Tiwanaku culture, but don't always have enough evidence to determine which method was used on a given stone; yet all too often, this uncertainty is misinterpreted as ancient aliens being as likely as anything else. In my experience, this disconnect is a widespread problem. It applies to Stonehenge as well.

A specific example of this is the question of how the mighty sarsens weighing 20 tons were lifted 8 meters or more into the air. We've done geochemical analysis on the stones and we can tell where they were quarried, and there are a variety of perfectly serviceable methods to transport them, but we still don't know how the ancients lifted them. Does that mean we must invoke giants? No. The scope of uncertainty in this case is very small. There are at least two methods we know the ancient Britons used to lift megaliths: sliding them up timber ramps, and also levering them upward by rocking them back and forth, adding a bit more structure to the fulcrum with each tip. Understanding the scope of the uncertainty is crucial to anyone who wishes to avoid chasing some fanciful hypothesis like giants or alien influence.

What of the modern claims, stemming mainly from alternative science theorists, that Stonehenge was an advanced solar observatory? These claims are based on creative interpretations of precise measurements of the stones' positions and alignments. However, Stonehenge as we see it today can best be described as a "reasonable facsimile" of the main sarsen stone arrangement constructed by the Beaker culture. Ever since it was first studied from an archaeological perspective in the late 1600s, Stonehenge has gone through renovations, reconstructions, cleanups, and repairs. The Stonehenge we see today does not match what's depicted in the earliest photographs, and even those photographs differ from pre-photographic illustrations and paintings. There's hardly a stone on the site that hasn't been adjusted, straightened, picked up from where it had fallen, or repositioned onto a stronger concrete footing. More than anything else, all this restoration is what puts the alternative theorists' claims of ultra-precise astronomical alignments at risk of fatal error.

But even more dramatic than the restorations made in recent centuries were the destructive activities in the preceding two thousand years. The reason Stonehenge appears incomplete is that many of its great stones were toppled over and stolen by locals who needed big rocks for whatever reason. Anyone trying to read anything into the precise position of the stones does so at his peril.

Neither should we read too much into the hyper-popular belief that Stonehenge is aligned with the summer solstice. It's not, exactly; and the claimed alignment depends on stones that are not in their original positions. However, this is not to say that any alignment is nonexistent or accidental. It's easy to pinpoint the direction of the sunrise or sunset on the solstices, and many ancient structures throughout the world incorporated these headings, just as many others are built to the equally easily-determined ordinal directions. Chichen Itza, Chaco Canyon, and Newgrange in Ireland are just a few examples. What we can't say is that this means Stonehenge was used as a calendar or an observatory. There's no evidence either way.

The scope of uncertainty surrounding Stonehenge is reasonably small. The uncertainty of who built its various stages is very small, limited to the known margin of error of the radiocarbon dating. The uncertainty of its purpose is also manageable. Today, we build courthouses, churches, and community centers, even festival sites like Burning Man — all traditional ceremonial structures, to handle the same cultural needs as most societies worldwide have always faced. We've no evidence that Stonehenge had any darker or more mysterious purpose than this. When we say we don't know what its purpose was, we don't mean that we haven't any idea. We have a very good idea, tempered with a modest and reasonable degree of uncertainty that we fully expect to fall within a predictable range. "We don't know" does not mean "It was probably giants and druids and Merlin."

And so, through all the oddball theories and pseudohistories, Stonehenge itself abides. It is evidence of great ingenuity, and above all, great passion, whatever the inspiration may have been. From the fortitude of those who hauled the mighty stones and erected them, today's visitors can draw a sense of pride in the longevity and gravity of some of our greatest creations — no druids needed.

By Brian Dunning
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