How are archaeological sites formed?

Some of the first questions that people tend to ask when they look in an archaeological trench for the first time are: 'How come it's so deep in the ground?' and 'Why is it like that?' The answers are to be found in the particular processes that take place once a site falls out of use and eventually decays into ruin.

The crumbling and rotting decay of any structure can be quite rapid. In a tropical climate the rainforest has been known to reclaim a deserted building in a matter of weeks. In temperate climates like that of the UK it takes a bit longer. But it's still the same continual processes of new growth of plants and fungus within decaying materials, together with often rampant animal activity, natural erosion, weathering, and human intervention in the form of scavenging and thievery, that combine to turn a standing building into the type of archaeological remains that Time Team finds today.

To illustrate what can happen we've built a fictional cottage and in the length of this web page we're going to speed up time and let it crumble to the ground.

Buildings with their roofs still intact can last unoccupied for many decades. However, as soon as the weather can get to the roof structure a series of wet and dry, damp and warm conditions soon breed bacteria, which can then start rotting the roof timbers. Constant exposure to water and oxygen, combined with a variety of salts in the air and regular changes in temperature, also break down the structure of wooden roof timbers. And any opening to the outside world invites insects and wildlife to start burrowing and eating away.

Figure 1 (Illustration: Nick Pearson)

Once a critical stage of decay is reached the roof collapses under its own weight into the building. For our illustration (Figure 1) we've caused the first major event in the decay of the structure to take place: the roof framework has rotted and collapsed and the cottage is now completely exposed to the elements. Archaeological remains are now rapidly being made.

Figure 2 (Illustration: Nick Pearson)

Figure 2 shows the house about 10 to 15 years down the line. With the weather protection gone, rainwater has been allowed to saturate the walls. Rainwater contains carbonic acid and so it's naturally corrosive. Once the water has settled between the courses of stone it starts to erode any mortar.

In the case of fine stone, such as limestone or sandstone, the water gets absorbed by the stone itself. As soon as a frost occurs it freezes into ice crystals, which carry an enormous amount of energy as they expand. (If you've ever seen a burst water pipe you'll know what we mean.) This action literally forces walls apart and pops render off surfaces.

Environmental forces also corrode any iron fixtures and fittings as iron oxides and carbonates encrust the metal and eat into its structure. The continual weathering of the building means that tiny particles of dust, grit and grime accumulate: soil is being made. As a result plants start to seed and grow within the building, causing even more damage.

Figure 3 (Illustration: Nick Pearson)

By Figure 3 you're probably wondering where the house has gone. We've helped it along with some virtual robbing. You've probably heard the people on Time Team talking about material being robbed out. Well, reclamation (as it's now called) is an ancient business. One person's rubbish is another's gold, and it's never been truer than when it comes to building materials. Why waste hours and hours dressing stone when loads of ready-prepared stuff is waiting for you in a derelict building?

Time Team often finds 'robber trenches' where building stone has been excavated out of the ground by ancient brickies. Often, when archaeologists say that they have found 'walls', we are actually looking at the tops of foundations as the stone used in the original wall has been reclaimed.

Figure 4 (Illustration: Nick Pearson)

Our cottage has been completely overgrown in Figure 4. This has happened because of the continual build-up of soil and the growth of plants and grasses overrunning the site. The plants themselves decay and the soil is then naturally reseeded. This seasonal cycle of growth and death, together with different weather conditions changing the fabric and texture of the soil surface, provides us with the stratigraphy, like a layer cake, that archaeologists are always talking about.

You can still see the 'lumps and bumps' where the building used to be – the stuff that Stewart Ainsworth enthuses about – and under that lumpy soil are traces of the original interior features. These may include pottery fragments, fittings and personal belongings, together with remains of the roof and floors, all ready and waiting to be discovered by Phil Harding or other diggers.

Now, where would you put your first trench?

Find out more

If you would like to find out more about the intriguing subject of how archaeological sites are made, try checking out the following list of publications. Some of them are archaeological journals. If your local library doesn't hold them the chances are that your local or county museum or records office will. Some of the titles are a bit longwinded, but persevere; it's all interesting stuff.

Archaeology is Rubbish by Mick Aston and Tony Robinson (Channel 4, 2002)

Some Aspects of the Excavation of Timber Buildings by Philip Barker, in World Archaeology 1.2, 1969

Techniques of Archaeological Excavation by Philip Barker (Routledge, 1993)

Seeing Beneath Soil by Anthony Clark (Routledge, 1996)

The Elements of Archaeological Conservation by J M Cronin (Routledge, 1990)

Geoarchaeology in Action: Studies in soil micromorphology and landscape evolution by Charles French (Routledge, 2002)

Soil Science and Archaeology by Susan Limbrey (Academic Press, 1975)

Natural Formation Processes and the Archaeological Record by D T Nash and Michael Petraglia (British Archaeological Reports, 1987)

Archaeology: Theory, methods and practice by Paul Bahn and Colin Renfrew (Thames and Hudson, 2000)