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.
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.
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.
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.
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, and Bersu referred to the large shallow pits he found at Little Woodbury as ‘working hollows’. 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.
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 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.
This is precisely what we see happening in the Middle and Late Iron Age, exemplified by buildings like Orsett S9. 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.
More on the cartoonist B Kliban: http://www.pbase.com/csw62/kliban http://lambiek.net/artists/k/kliban.htm