Humans produce waste: Large concentrations of humans produce large concentrations of waste. Kitchen waste, broken objects, and similar material 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 these new settlements but would rarely be taken out again.[clarification needed]
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.
The City of London serves as an example, for urban excavations have been performed there since the late 1900s. These excavations, performed in populated areas of the city, revealed historical evidence of events unforeseen previously by historians. London sits on a tel, which preserves a layer of dark material, attributed to the burning of the city by Boudica in 60 AD. It was only by excavation of the urban areas that these revelations could be made, as the city has long since outgrown its borders after its rebuild, some years after the Boudican revolt.
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. The trend of urban excavation in areas such as Europe, the East Coast of the United States, and other western world cities, has grown since the war. Due to the bombing of landscapes during the war, the possibility of losing evidence connected to early civilizations became realized. The idea of settlement follows a principle, that settlement is made where resources are conveniently accessed. Prosperous cities, such as Boston and London, began as settlements, and grew quickly due to the convenient access of resources. However, the fact that these cities still exist today does not dismiss the fact that, at some point in history, some other civilization had once settled there.
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 5-metre x 5 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.