Why is archaeology always buried?

Sometimes, of course, the archaeology isn't buried deeply, but is still at the surface. This is often true in rural areas, where it is at constant risk from ploughing. And bear in mind too that the stone walls you see in excavations are often only the lowest layers of the original building. Some of them would have been under the surface in foundation trenches (or only just above) anyway.

That said, it's true that most archaeology requires at least some excavating. So why is this so?

First, you have to remember that there were no refuse collectors in the past, so that rubbish simply accumulated close to occupied sites rather than being taken away. The same was true of rubble when buildings were demolished or rebuilt. Generally this would be spread out, flattened, and built on again, so that layers of archaeology built up over the years.

Where urban archaeology is concerned, sites are often very deep because of this continuous process of demolition and rebuilding. Urban rubbish was also disposed of in garden pits during much of the medieval period, and this too helped to raise soil levels, so that internal floor levels were built up to compensate. Sites that are occupied over long periods can build up to considerable heights in this way: many ancient urban sites are first identified as mounds or small hills rising above the surrounding landscape. The ancient biblical site of Jericho, for example, stands at the foot of a 15-metre-high tell (occupation mound) representing the various layers of later occupation.

Sometimes the build-up is intentional. This is the case with many waterfront sites, where material was deliberately tipped into riverside areas to extend the land out into the river. The same can be true of badly drained or swampy areas.

In rural areas, the natural process of vegetation growth and decay gradually raises the level of the ground. Farmers also used to add 'nightsoil' (human faeces) and other materials to the land to act as fertiliser, improve drainage or retain moisture. The nightsoil often contained other waste material as well – including pieces of pottery that can confuse the archaeology in the fields in which they are found.

Research into soil formation shows that layers can accumulate at a rate of 2.5cm a year. It doesn't take long, then, for past occupation layers to be buried very deep. In Canterbury, for example, it is estimated that the soil overlaying the original Roman temple precinct floor in the Blue Boy Yard site, which Time Team excavated for the 2000 Live formed in a period of just 50 years.