Urban archaeology
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Urban archaeology is a sub discipline of archaeology specialising in the material past of towns and cities where long-term human habitation has often left a rich record of the past.
Humans produce waste. Large concentrations of humans produce large concentrations of waste. Faeces, kitchen waste, broken objects etc. all need to be disposed of. Small numbers of people can dispose of their waste locally without encouraging vermin or endangering their health. Once people began to live together in large numbers, around five thousand years ago, such methods began to become impractical. Material would be brought into the these new settlements but would rarely be taken out again.
Up until the nineteenth century when organised rubbish disposal became widespread in urban areas people invariably threw their waste from their windows or buried it in their gardens. If their houses fell down, a common enough occurrence when planning laws were non-existent, owners would pick out what they could reuse, stamp down the remains and rebuild on the old site.
The effect of this is that even a moderately sized settlement of any antiquity is built on top of a heap of refuse and demolished buildings and is therefore raised up from its original height on a plateau of archaeology. This is most apparent in the tel sites of the Near East where towns that have been occupied for thousands of years are raised up many metres above the surrounding landscape.
In walled towns such as those in medieval Europe the effect of the encircling defences was to hold in the waste so that it could not slip outwards, magnifying the effect.
Historic development
Archaeological excavation within historic cities therefore often produces a thick stratigraphy dating back to the original foundation and telling the story of its history. The City of London for example, sits on a tel which preserves a layer of dark material attributed to the burning of the city by Boudicca in AD 60.
The dense stratigraphy of such cities posed problems for the archaeologists who first excavated them. Earlier excavations were generally limited to rural areas, or towns which had been long abandoned. Open area excavation was feasible as there was plenty of space and the archaeology could often be exposed just in plan. In working cities however, space for excavation is usually limited to the size of the open plot and one layer of archaeology needs to be excavated before the next one can be exposed.
Issues such as this had appeared before, at Pompeii or at multiphase rural sites but the move towards the investigation of cities, which began in Europe following the Second World War when bomb damage left areas open for investigation, meant that there was a necessity in finding a new method of excavating.
Methods
The resulting solution revolved around the method of single context recording. The practice involves drawing each feature individually in plan and then relating its position to the site grid rather than planning large areas at once. Each drawing is made on a square piece of translucent film representing a 25 square metre grid square. The site is excavated down to the first significant layer of archaeology and features excavated and recorded as normal but also planned as single contexts. The site is then reduced to the next layer of archaeology and the process begins again. The excavation and recording can continue until natural deposits are reached. A small, deep trench known as a sondage is often excavated at first to provide a view of the entire stratigraphy at once and give an indication of the quantity of material to be excavated.
Once the work is finished, the square sheets can be overlaid onto one another to provide a picture of the site. By identifying which features cut others and using information from dateable artefacts and ecofacts an archaeologist can isolate various phases of activity and show how the use of the site developed of periods of hundreds or even thousands of years. Context record sheets produced by the individual excavators provide further information on each context's nature and relationship with its neighbours. Such interpretation would be impossible using open area excavation where numerous overall site plans would soon seem inflexible.