29 March, 2009

25. A world of Invisible Walls

How to account for things that are invisible is always tricky, as priests would no doubt testify, and in some respects archaeologists have a comparable problem to religions, in that what we are asking you to comprehend and visualise happened a long time ago. However, archaeologists, unlike priests, are happy to admit (if pushed) that what we are asking you believe is an educated guess, even if, as often the case, we have a story and we’re sticking to it with almost religious zeal.

For British prehistory the problem is that some significant pieces of the visual jigsaw have been destroyed forever, and they have become ‘archaeologically invisible’. I can tell what the woods and the trees looked like -- I have photos that must be fairly close – but the evidence for the human part, particularly the built environment where human life is played out, is somewhat sketchy.

Just as people tend to create God in their own image, so our vision of the past is similarly anthropomorphic; we project our own concerns, interests, and experience onto the past. It could be no other way, since we have no direct shared experience of the past, and unlike God, the idea of the Past revealing itself or communing with us in some special way, would not be regarded as a sound methodology.

Interpreting and visualising the prehistoric built environment has some serious issues. In particular, archaeologists frequently have a problem with the evidence for walls, or rather the lack of it. Once the topsoil has been ploughed by later generations, all trace of walls whose foundations did not cut into the subsoil will disappear, and they will become ‘archaeologically invisible’, leaving only the deeper load-bearing posthole foundations that supported the roof as evidence of a structure.

This contrasts with our contemporary experience, where walls are synonymous with buildings, houses and homes; they define the spaces we inhabit, and have significant symbolic and social meaning. However, we have benefited from two thousand years of architectural innovation, particularly the use of load-bearing walls, which is why posts holding up the roof no longer encumber our homes. These foundations of load-bearing walls will form the archaeological footprints of our homes, but the internal partition walls, small structures such as garages and sheds, together with much of the significant detail, will not necessarily be evident to future archaeologists.

In prehistoric roundhouses above a certain size, a ring beam, positioned at the centre of gravity of the roof, was the main load-bearing component. Although some weight would inevitably have transferred to the wall, because the wall is not intended to carry the load of the roof, the builders could get away with a shallow foundation.
Schematic wall types with shallow foundations

A wall of any kind can become ‘invisible’ if its foundation is eroded, and there was a limited range of natural materials to built walls from in prehistory: timber, stone, and earth in some form, were the only realistic options.

Two examples of roundhouses from Kestor in Devon, with stone walls formed between embedded upright slabs of stone [1]
Stone was used in prehistoric buildings, but without some good method of sticking stones together, it is quite difficult to build a robust wall that is not so thick as to be impractical. The use of stone is very much a local factor. Much of South-East England is devoid of suitable building stone, but many highland areas have an abundance, and circular stone huts are a feature of upland areas. In places like Orkney you have quite easily-worked stone and few trees, making the use of stone in preference to timber a practical solution. In general, timber and daub walls were used, because, until the widespread use of lime mortars, stone was an inferior and far less practical building material, notwithstanding its obvious strength and durability.

This thatched cottage with cob walls (mud brick) at Higher Bockhampton, Dorset, was the birthplace and early home of the novelist Thomas Hardy. (National Trust)
Like many other parts of the world, Southern England has a tradition of building walls from ‘mud brick’, although the unfired bricks made from clay mixed with other material are known as ‘cob’. The cob walls in a storey domestic building will be about 0.6m (2’) thick, and once rendered, often pass unnoticed. However, when cob is used for freestanding walls, it has to have a waterproof capping or ‘roof’ of some kind. [2]
Clay was also commonly used, in the form of daub, applied as coating for a wooden frame. Making daub or cob usually involves mixing material like animal dung, chopped straw, and sand with soil to produce a material, which if kept dry, will set hard, and is simple to maintain with periodic recoating. While, in Britain, mud walls carry somewhat primitive overtones, it must be remembered that in places like ancient Egypt or Mesopotamia mud brick building was the norm, and would have been used or buildings of the highest architectural quality.

Schematic earth-fast timber wall types
The core of most walls would have been made from wood in some form. Solid wooden walls can be created with either horizontal ‘logs’, or vertical ‘staves’. Both techniques can use timber in the round or squared timber created by splitting or adzing. Larger diameter trunks of trees like oak can be readily split with wedges to create squared structural timber, boards, or thin flexible strips, all of which can be used to create walls. The traditional wooden lattice, or wattle, can be made from thin flexible stems like a hurdle, thicker stems, or split wood, depending on the context. The most complex form of walling is a timber frame design; this combines vertical and horizontal jointed timbers, usually with bracing. Timber-frame and posts-and-lintel designs would usually require some form of material like wattle and daub to fill the gaps. It must also be borne in mind that some specialist buildings in the past may have been open-sided, or partially so, just as they are today.

This graph shows the volume of material required to build a 1.5m high, 0.6m thick, wall for roundhouses of differing diameter; it also shows the volume of space enclosed by the wall, and the volume of material require to build a square wall enclosing the same volume.

While we are dealing with a world were walls have been lost, indirect evidence may, in theory, exist; for every m³ of earth used to create a wall, somewhere, at some time, there was a hole in the ground of similar volume. Large areas of random diggings are found on archaeological sites like Winnall Down,[3] and Bersu referred to the large shallow pits he found at Little Woodbury as ‘working hollows’.[4] A large roundhouse could use 40 m³, or more, of material, which could create a significant ‘borrow pit’, although such holes would be useful for the disposal of rubbish and other unwanted material. At Danebury Hillfort in Hampshire, conical pits were identified as having been used for mixing daub.[5]

Two other things are evident in the graph above: firstly, square walls enclosing the same volume use more material than round ones; and secondly, that the enclosed volume curve rises more steeply than the wall volume, which indicates larger structures are more efficient than smaller ones in terms of walling materials. The figures are for a wall of constant thickness (0.6m), since with a non-load-bearing wall, there is no reason to assume a wall thicker for a larger building. However, in reality, the construction technique, though not necessarily the thickness, would have varied with the size of building.

This graph demonstrates that the load on each meter of a roundhouse wall is increased by making the roof wider.
Having said that walls are not the primary load-bearing part of the structure, some load will inevitably be applied to the wall, due, for example, to the structure's settling. If, in the case of a roundhouse, we imagine that there was no ring beam and that a circular wall carried all the weight, we can draw a graph of the load on each metre of wall. Although the actual loading figures are meaningless, the graph illustrates that the load on the wall would increase as the building got wider.
The important point to understand is that you can increase the size of a rectangular building by making it longer, with no implications for wall loading, but you can only make a roundhouse larger by increasing its width, and therefore the loading on the walls. The practical upshot of all this is that you cannot directly compare small round structures with larger ones, because what works on a small scale might fail under the loading of a bigger roof.

A selection of roundhouse plans from Moel y Gaer, Flintshire, showing the probable position of the wall, and the buildings’ axial symmetry. [6]
The problem of invisible walls has been known for some time, and was discussed by Graeme Guilbert, the excavator of Moel y Gaer, where, apart from rare examples like P10, the majority of his buildings had no trace of walls, surviving only as ring beams. He made two important observations about roundhouse plans: firstly, that the invisible wall probably lined up with the inner posts of the porch; and secondly, that roundhouses usually had a uneven number of posts, and had axial symmetry about their entrances.[6]
This probable positioning of the wall relative to the porch and the general observation that the ring beam is usually at 2/3 of the building diameter, allows for an educated guess as to its likely location.
There is another important point to bear in mind about the design of roundhouses: a strictly circular wall can only contain vertical straight timbers, so any rafter resting on it will bear directly on that part of the wall (a ‘point’ load). This is in contrast to a post-and-lintel wall, which is able to distribute the load between more than one post by using the horizontal lintels; but this forms a polygonal, not a circular, structure.

Roundhouse plans with visible wall foundations: A: Pimperne Down, Hampshire [7]; B: Longbridge Deverill Cow Down, Wiltshire [8]; C: Moel y Gaer P10, Flintshire [6]; D: Orsett S9, Essex [9].
We have seen how there is a tendency to reduce the number of posts in the ring beam during the course of the first millennium.[10] This inevitably makes the shape of the roof more polygonal, and, given the better load-bearing capacity of polygonal walls, it is only natural that this structural evolution would end up with polygonal walls and fewer posts in the ring beam. This can be viewed as resulting both from transferring roof load to the walls and from refining of the design in terms of individual post loading.

This is precisely what we see happening in the Middle and Late Iron Age, exemplified by buildings like Orsett S9.[9] However, due to my own, and many other archaeologists', inability to distinguish between a drainage and a structural feature, the world of invisible walls has become far more confused than was necessary.
In the next article we shall look in more detail at the structural evolution of wall technology, a process that can be seen to reach its climax at the Orsett ‘Cock’ Enclosure. (No giggling at the back!)
Sources & further reading:
Cartoon by B.Kliban from: The Biggest Tongue in Tunisia and other drawings. Penguin 1986, ISBN0140072209
More on the cartoonist B Kliban:
http://www.pbase.com/csw62/kliban http://lambiek.net/artists/k/kliban.htm
[1] A. Fox, 1954: Excavations at Kestor, Reports & Transactions - LXXXVI, Devonshire Association, Exeter
[2] J. McCann, 1983: Clay and Cob Buildings. Shire Library
[3] P. J. Fasham, 1985: The Prehistoric Settlement at Winnall Down, Winchester. Hampshire Field Club and Archaeological Society: Monograph 2
[4] G. Bersu: 1940: Excavations at Little Woodbury, Wiltshire. Part 1, the settlement revealed by excavation. Proceedings of the Prehistoric Society, 6, 30 -111
[5] B. Cunliffe, 1984: Danebury: an Iron Age Hillfort in Hampshire, Volume 1: The excavations, 1969 –1978: the site. CBA Research report 52
[6] Graeme Guilbert, 1981: "Double-ring roundhouses, probable and possible," in Prehistoric Britain Proc Prehist Soc 47 & G. Guilbert, 1982: 'Post-ring symmetry in Roundhouses at Moel y Gaer and some other sites in prehistoric Britain', in Structural Reconstruction - Approaches to the interpretation of the excavated remains of buildings, British Archaeological Report 110, BAR, 67-86
[7 ] D. W. Harding, I. M. Blake, and P. J. Reynolds, 1993: An Iron Age settlement in Dorsett: Excavation and reconstruction. University of Edinburgh. Department of Archaeology Monograph series No. 1
[8] S. C. Hawkes, 1994: "Longbridge Deverill Cow Down, Wiltshire, House 3: A Major Round House of the Early Iron Age." Oxford Journ. Archaeol. 13(1), 49-69
[9] G. A. Carter, 1998: Excavations at the Orsett ‘Cock’ enclosure, Essex, 1976. East Anglian Archaeology Report No 86