Several layers from different periods can be represented on an archaeological site. Archaeologists have to make sense out of it by working out the sequence. In general the lower layers are earlier and the upper layers are more recent. Therefore, Roman layers are lower than Saxon ones; Saxon ones are lower than medieval ones and so on. However, not every period is represented on a site. Roman layers may be covered by medieval ones if there was no Saxon activity on the site. The principle of stratigraphy can give a relative date to each layer (one layer is earlier than another) but not an absolute date, which is arrived at by looking at artefacts or by taking samples for scientific dating.
Stratigraphy gets quite complicated, however, when humans dug pits or ditches through earlier layers, or piled middens (rubbish dumps), banks or mounds up over them. Archaeologists number each layer or feature as they dig through it. Afterwards they can create a diagram, called a Harris matrix, to show the sequence of activity on a site.
In this diagram the topsoil has been numbered 1, the pit 2 and the natural soil into which the pit is dug is 3. The Harris matrix would be:
This expresses the idea that the natural soil through which the pit is dug is the oldest, the pit is later in date and the topsoil is the most recent feature.
If another pit cut through the first one the matrix gets a little more complicated.
The intercutting pit is number 4. Although this cuts into the natural soil directly, it also cuts through pit 2. This means that pit 4 must be more recent than pit 2. The Harris matrix would therefore look like this:
Note that the numbers do not follow each other in order. Features are numbered as they are found, but this does not necessarily reflect the original sequence of digging.
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