22 January, 2009

19. The proper study of mankind is postholes

With all due apologies to Alexander Pope [1], and John Collis [2], postholes are quite important, but it is what they represent, the prehistoric built environment, that gives them their significance to archaeology.

An excavated posthole
The central importance of the built environment to archaeology, to its original inhabitants, and to builders was explored in a series of earlier articles, as was the basic problem of archaeological sites with lots of postholes that, once any roundhouses have been extracted, remain uninterpreted.

Archaeologists have mostly ignored this growing body of uninterpreted evidence, and much information about the prehistoric built environment has been lost in the thousands of unregarded postholes tucked away in reports as ‘unphased’ and others in archives completely unreported. This is where the evidence is to be found, but it is quite complex, and difficult to analyse, and it is little wonder archaeology made up a simpler story that was easier to understand.
Gussage All Saints, Dorset, a typical excavation with plenty of posthole evidence, which produced little structural understanding [3]

Culture is better understood through its built environment than its pots or rubbish, but archaeologists leave the postholes where they found them and take away the pots and the rubbish to study in more detail. If we are lucky, some postholes may surface in a report, but usually only those that conform to the excavator’s preconception of a structure make it into print. It is at this point, somewhere in the post excavation process, that the features are chosen that will be presented as representing the built environment of the site.
This is the point of no return for the rest.

Many archaeological researchers, authors, and teachers, may have not been involved at this sharp end of decision making in writing archaeological reports, and may fail to grasp its significance. The subsequent layers of academic synthesis usually fail to reflect or engage with the doubt, or bias, in the data presented in the report. For an ‘academic’ whose learning comes from reading, the failure of many postholes to make a significant appearance in the peer reviewed literature means, for all intents and purposes, that they do not exist.
This problem is deep down in the engine room of archaeology, somewhere at the dirty end of the business, out of site.

For over sixty years archaeology chose to ignore this aspect of the evidence for the built environment, and invested heavily, in terms of both physical reconstruction and intellectual capital, in a concept of life lived in the dark around a central fire of a one roomed roundhouse, in a world whose ‘simplicity’ is compensated by a rich spiritual and ritual life, now elucidated in surprising detail by many archaeologists.
This simplistic and dysfunctional concept of architecture and the built environment is another result of ignoring the evidence of postholes.

There has been a failure to engage with prehistoric building as a technology, and the need for a robust built environment, something we take for granted, is overlooked, disconnecting the ancient past from traditional timber building, one of most important and significant components of the material culture of England.

A plan of Woodhenge, Wiltshire, a timber circle with concentric rings of postholes.[4]
Timber circles identified as large Neolithic buildings by earlier excavators have more recently been reinterpreted as timber versions of stone circles, where open air rituals and feasting were performed, particularly in the autumn -- that traditional season of outdoor partying! [5] The large posts no longer support a roof, but are now seen as objects in some form of mysterious ritual and symbolic of something (or other).
Woodhenge as it is today, with concrete posts marking the positions of the postholes
It would appear that the idea of communal celebration, social life, and ritual being conducted indoors was, in the eyes these archaeologists, a much later invention, again, perhaps, something introduced by the Romans.
However, what is so extraordinary about timber circles is that the posts are all positioned in the most appropriate places to support a roof using Neolithic technology. Yet it is so typical of our ‘primitive’ ancestors that they could not conceive of putting a roof on top of the posts, or imagined the advantages of a communal life conducted in the warm and dry!
A reconstruction of Woodhenge as a roofed building [4]
From my perspective as an archaeologist who studies structures, I can think of half a dozen sound structural reasons for regarding timber circles as buildings. In fact, to think of them as ‘totem poles’ appears to be more of an article of faith, based on belief rather than evidence.
However, it also is perceived that, two thousand years after timber circles, building technology climaxed in the Iron Age as a ‘mud hut’, a concept in part borrowed from more ‘primitive’ cultures dominated and civilised by our colonial forebears in Africa. Thus, the arrival of the Romans and their civilisation is quite naturally represented as the introduction of ‘architecture’ into our culture.

It doesn’t have to be this way!

If postholes are viewed as structural foundations, building as a technology, and the people who made them as builders, we might just be able to imagine a more appropriate and practical built environment for prehistory. These are important considerations when we return to the real problem: how to detect and recognise the remains of posthole buildings and structures that you don’t yet know to exist?

While it is important to keep this long-term goal in mind, we must start with the basics: What is a posthole?
The difference between postholes and stake holes
A posthole is a hole dug to accommodate a timber post, which would typically used to support a load acting vertically upon it, usually as part a as larger structure with multiple post foundations. For this reason, the top of the post would be narrowed to form a ‘tenon’ so it can be jointed into a corresponding hole, or mortice, in a horizontal timber. The horizontal timber will link it to other posts, and the weight of the structure will be spread evenly between several posts.
Basic mortice and tenon joints
Posts are least suited to loads' being applied from the side, so in this situation, driven stakes, which fit tightly into the ground, would be used. This produces a feature archaeologists refer to as a ‘stake hole’, the ancient equivalent of a hole left by a modern sharpened fence post driven into the ground. Fences are not very heavy, but they do get pushed sideways by animals and even by the wind, making stakes more appropriate in this circumstance. Stakes are designed to sink into the ground under pressure, whereas posts are not so designed; the terms, and the usage, are not interchangeable. Usually, the builder does not want a post to sink, since it is supporting the structure. However, pointed timbers driven a very long way into soft ground like marshes or lake bottoms can be used as foundations; in this context they are known as piles.

Posts are all about vertical loading, because it is only when perpendicular to the earth surface, aligned with the Earth’s gravitational field, that posts will work properly. The simple and intuitive notions of ‘perpendicular’, ‘level’, and ‘true’ are absolutely essential attributes for any successful earthbound structure. Once a post or structure starts to go ‘out of true’ or leans, collapse may not be far off. The rigidity and design of the timber structure will ensure that, whatever forces are applied to the post, it is not pushed over and remains vertical.

A posthole represents civil engineering just as much as a piece of stone wall, or a brick pier, and has a ‘supply chain’ of equal complexity. The post itself started out as a tree, probably grown specifically for this purpose, and may have taken more than one generation to produce. This tree was cut down and trimmed to size, and a timber weighing several hundred kilos was extracted from woodland and transported to site. The post may have been charred at the base to resist rotting. Then it would have been measured and cut to a precise length, with a tenon formed at the top, before being placed in a pit of a particular depth and joined with the rest of the structure. This logistical chain may contain two generations of woodmen, carters, labourers, carpenters, and most importantly, someone to decide the size of the post and the location of the posthole -- an architect.

The section through theoretical posthole F42
Quite a lot of information can be extracted from a good posthole. There is no typical posthole, so we will consider a theoretical posthole. Archaeological features are often numbered and prefixed with the letter ‘F’ (for "feature"); ours is F42.

F42 is a posthole about 0.5m wide and 0.65m deep, cut into gravel subsoil. It has a ‘post pipe’ that is the impression left in the soil by the post, either as the result its rotting in situ or by fresh soil having filled the void left by removal of the post. The original post was about 0.3m in diameter and would probably have been oak, a ubiquitous tree and the best native species for posts. We can consult the appropriate tables for English oaks growing in this environment; this suggests that the original tree was about 60 years old and roughly 23m tall [6]. About 10m of the trunk was probably usable and it would have weighed approximately 250 kilos.
Tall oaks, approximately 60 years old
The actual load that can be carried by F42 will depend on the nature of the gravel soil it was dug into. This can be calculated; however, it is far more useful to observe that, as a rule of thumb, prehistoric builders used one post per 10-12 square metres of roofed building. This conception of "load per post" is a key concern of this research as well as of prehistoric builders, and will be explored more fully later articles.

If the figure of one post per 10-12 m2 of floor space is accepted as representing the safe limit for prehistoric builders, then ignoring scores or even hundreds of postholes, as many reports do, has an tremendous potential to upset any interpretation that may have been put on the built environment.

Of course, F42 may have held a post representing the spirit of somebody’s late great-aunt Sally, or a even of a god, and may have been something to worship, make offerings to, or dance around; the choice is yours -- it’s all a question of belief.

References and further reading:
[1] From Alexander Pope: An Essay on Man. As reproduced in Poetical Works, H. F. Cary, ed.: London: Routledge, 1870, 225-226: “Know then thyself, presume not God to scan; The proper study of Mankind is Man.”
[2] This reference is to J. Collis, 1977: “The proper study of mankind is pots”, in The Iron Age in Britain: a review. Sheffield University Press
[3] J.V.S. Magaw & D.D.A. Simpson, 1979: Introduction to British Prehistory. Leicester University Press. Fig. 7.12; & G. J. Wainwright, 1979: Gussage All Saints: an Iron Age settlement in Dorset. Dept of Environment, Archaeological reports No 10
[4] http://www.english-heritage.org.uk/stonehengeinteractivemap/sites/woodhenge/01.html
[5] A good general account of the ”Stonehenge ritual landscape” can be found in F. Pryor, 2004: Britain BC: Life in Britain and Ireland Before the Romans. Harper Perennial. pp. 235–238
[6] N. J. James, The foresters companion 1982, 3rd ed. Oxford, Basil Blackwell. pp. 355.


Mr Puffy's Knitting Blog: said...

Carter do you post your pics on Flickr? It's a vibrant community of photogs and has lots of groups that would love to see your pics.

alice said...

Hi Geoff,
You blog on the prehistoric timber environment is exceedingly interesting! I'm an archaeiologist making reconstructions of precolumbian timber roundhouses in the dominican republic for my PhD, and often turn to the work done on British prehistory for inspiration...and also because I was also once a British Bronze Age student. My chellenge is, I have wonderful houseplans (external and internal structures, monumental entrances etc, all set in bedrock, so very clear), but I don't have the physical knowledge of vernacular architecture to understand what characteristics of the plan imply for the real building as it was in the past. your blog really tried to address this for arcaheologists, and I like that. alice samson

Geoff Carter said...

Hi Alice,
Thank you very much for your comments;

This precisely what this blog is about, I feel that ‘academic’ archaeology has failed to address these problems in its research, and in the education and training it provides.

I have just started to address these problems in the blog, so hopefully in the next few months I will help provide a way of looking at structures and understanding them as buildings, that may be helpful. It is not very difficult, just a different, and a slightly more forensic way of looking at the structural evidence.

It is important to understand any structure in terms of its local environment, materials, and culture. Should you wish to discuss any structures, I am always interested to learn more about the subject, so feel free to contact me.

Thank you for your visit and spread the word

edward pegler said...

Dear Geoff

This is a bit of a bizarre question but have you ever thought of having a go at putting a wooden lid on Stonehenge? I understand that the post-holes of Stonehenge are a mess and the central arrangement of trilithons at different elevations is immensely awkward. Indeed it's probably entirely wrong but it would be interesting to see if such a thing could work. The point is that if the pair of us are going to argue for a lid on Woodhenge and the two structures are similar in design then, well...

I suppose my thoughts originally turned to this when I saw the Maltese megalithic temples and tried to imagine what they would have used to span the roofs there. It's pretty likely that they had roofs as the mortuary caves of Hal Saflieni imitate the interiors of the temples including the roof detail. The span you would have needed there, about 10m, is just about manageable with thick beams or tree trunks, although they probably corbelled the roof in part.

Sorry. I suspect that these are just random thoughts.


Geoff Carter said...

Hi Ned
I've done some work on it already and I know roughly how it works. It's just another Annular building [Ei]; but it's a couple of months work; [which I may to do next, I'll see how Woodhenge is received].

It is important to understand that most of the 'stone-holes' are postholes; the presence of a stone- chip in the fill of feature does not make it a stone-hole, any more than a piece of antler makes it a deer-hole. Besides, having all these phases with stones wandering round the site is just silly.

It is only some weird idea that it must involved bearded people waving things at the sky that requires it to be open.
From a structural point of view the stones are an architectural feature - interesting - but possibly not supporting the roof. It is mixing two forms of foundations; it would be easier to understand without them. However, it is useful to have one 'real'level to put on the building section.

PS. The 'stone-holes' at Woodhenge are part of the stairs [2 sets]. Stairs in large ancient timber buildings tend to be very robust.

Yvonne Osborne said...

I find your reconstruction of Woodhenge as a roofed building fascinating and I thought at first you were referring to Stonehenge and who would think that postholes could tell more about a people than their pots and rubbish? We saw a crumbling section of Hadrian's Wall when we were in Northern England several years ago. How could this have originally been timber?? Cool stuff you have here, readable for a commoner.

Geoff Carter said...

Hi Yvonne, thanks for commenting; Most the prehistoric world was built of timber, and had been for thousands of years. I'll tell you a secret; 'Stonehenge' is an unusual stone element in an otherwise timber building.