Bayesian Prehistory

We explore how carbon dates can be statistically analysed in order to look at prehistoric England and causewayed enclosures...
19 July 2011

Interview with 

Alex Bayliss, English Heritage


Diana:: Next up, computer programmers and archaeologists recently used acausewayed enclosure technique known as Bayesian chronological modelling as a window through which to look at the real political and even military events which shaped Britain's pre-historic past.  The new research based on computer refined radiocarbon dates strongly suggests that farming lifestyles were introduced from the continent through Kent and Essex by immigrants and not simply through the transmission of knowledge and ideas.  The work also suggests that large structures known as causewayed enclosures only emerged in Britain once there was enough farming going on for allow leaders to lead ever larger groups of people.  To tell us more, Alex Bayliss.

Alex -   Radiocarbon dating used on its own has quite large error terms.  If you've just got a single radiocarbon date, typically in the 4th millennium BC, it will cover about 250 years.  But because radiocarbon dates is a probabilistic process, if you get 10 radiocarbon dates for a site, they will scatter around the true dates of that site.  So some will be a bit earlier and some will be a bit later, and it will look - even if all those dates are from the same Thursday afternoon in 3000 or 600 BC- you know, the earliest radiocarbon date will 250 years earlier, the latest will be 250 years later, and it will look like that the actual site was in use for 500 years where in reality, it was used for one afternoon.  So, the basic problem when we started was that normal radiocarbon dates kind of put the early Neolithic of Britain into a kind of splodge.  Okay, we know it's early Neolithic but it's this kind of undifferentiated splodge somewhere in the 4th millennium BC.

Diana -   Okay, so how do you go about differentiating that splodge then?

Alex -   So what we do is, instead of just relying on the scientific information - the radiocarbon dating, we put together both the archaeological information with the radiocarbon dating.  Now, I'm the scientist on the team but I've got colleagues at Cardiff University and together, we worked out what that additional information is.  So, for example, if you have a sequence of deposits through a ditch and you have 3 radiocarbon dates, it is very likely from archaeological information that you would know that the sample at the bottom of the ditch is earlier than the sample in the middle, is earlier than the sample at the top of the ditch.  And so, you can actually use that relative dating that you've got in archaeological sequence to refine the radiocarbon dates.

Diana -   That sounds lovely, but what did you get out once you had this amazing sort of level of refinement?

Alex -   Okay.  Well the project that we actually were working on, the group of monuments we were working on and this was a joint project funded by English Heritageand the Arts and Humanities Research Council in Britain was the dating of early Neolithic causewayed enclosures.  Enclosures seem to be gathering places that are used periodically there's evident for really big feasts where several cattle would be killed all at the same time.  You know, they had a massive great barbecue basically.  Now causewayed enclosures are the first big monuments that we have in England.  They can be about 300 metres across.  They have at least one, sometimes four, circuits of ditches.  The ditches can be rock cut, but they have lots of different entrance ways that's why they causewayed enclosures.  Some of them also have palisades and timber lacings in the earth around parts that were formed when the spore from the ditch is coming out.  So these are fairly big things, you would need several people for a month to build one.  

So what we did was we went back to the archives in the museum, no new excavation and we dated all the ones that had holes in from the past.  So we've dated about 40 out of the, roundabout, 90 that are known.  Mostly in southern Britain, although there were few in Ireland, and a few currently on excavated ones further north in England.  And we found that they actually covered a very tight span.  Previously, these things had been thought to be built throughout the early 4th millennium and to be typically in use for maybe 300 or 400 years.  We actually found that they appear very suddenly, roundabout 3700 BC and when I say 'roundabout' I mean we did a generation of within 25 years at 95% probability.  They appear first in Kent and then they spread very rapidly across southern Britain until about 3625 BC. About half of all examples, things that have been built in the first 75 years.  So we have a boom.  We have a real big construction boom and that is of course followed by a bust because then there's evidence that for about the next 50 years, it becomes, for whatever reason, they become very unpopular.  And then in the generation around 3550 BC, again, it becomes popular again, so we have a boom and a bust, and another boom, and then they just go out of use and nobody seems to build them anymore.  So there's a final bust.

Diana -   Right.  So if the initial building just occurs in one generation, what are the implications of that then if that kind of proliferation of building style?  What does that mean?

Alex -   Well at this point, you kind of have to get to the stage where you know, a date is just a number because if you've got no context, you know, we suddenly knew far more about enclosures but we can't - I can't answer that question- without telling you what happens in the rest of the early Neolithic.  So we had to kind of unpick the rest of the splodge.  Okay, so to do that, we just looked at - there's about 1500 existing dates. We got details of all of those, looked at all the archaeology and did a model for that.  And so, we've written basically a completely new story for the early Neolithic in from Britain.  

So what happens is, around about 4050 BC, probably the first pioneer settlers arrive from continent in the Thames gateway.  Then the idea of farming, basically it got Kent to Cheltenham in about 200 years.  Then round about 3850 BC and by 'round about' I mean, within about 25 years of 3850 BC, some kind of critical mass seems to have been reached because suddenly the Neolithic appeared everywhere, bang!  Within the next 50 years, it gets from Cheltenham to Aberdeen.  Then in around 3800 BC, things really start to rock.  It seems like the pioneer phase of Neolithic settlers is over and what they're trying to do is build a society because we have so many innovations in the next hundred years of which enclosures are just the last.  But the point is you've got national exchange networks for high quality stuff being established, and at the end of this process, this century of real dynamic innovation where really everything is changing, that's when causewayed enclosures appear. 

So to now get back to your question:  Why do causewayed enclosures appear?  Well, what's interesting is that both long barriers and causewayed enclosures are typical Neolithic monuments that occur on continental Europe.  So the question is not so much why did it appear in 3700, but why didn't it appear in 4050 BC with the initial colonisation?  You know, this idea was already around on the continent but they didn't bring it with them.  So maybe the idea is that the reason they didn't was because maybe they didn't have the resources to invest in that sort of stuff, they were too busy building their flocks or they were too busy setting out their fields, and actually, maybe what we're seeing in the 3700s and with enclosures is an attempt to have control of what happens at the festival when they had their enclosure meeting.  Who could get their hands on all these new exotic stuff that was being traded.  So maybe it's actually about the emergence of power is perhaps our idea.  Now, we've emerged from the splodge, there's this story and we've actually got a dynamic narrative and because, what have I been talking about?  I mean, I've been describing a political narrative of southern Britain 5000 years ago and we simply couldn't do that before without the dating.

Diana -   So from this study, it looks like farming was brought in by specific group or groups of people rather than as a spread of knowledge through indigenous populations which marries really well with the genetic data, and ultimately, it's their fault we have big community buildings.  You can read more about Alex's team's work in the book "Gathering Time" dating the early Neolithic enclosures of southern Britain and Ireland.


Add a comment