Stratigraphy

Understanding the way in which deposits have accumulated to form the layers of an archaeological site requires an awareness of stratigraphy. This is the study of the build-up of soil, refuse, building debris and other material in the ground: the 'strata'.

Normally we dig from the most recent (usually the uppermost) layers down to the earliest (which tend to be the lowest) ones. However mere depth of burial is not by itself a reliable guide to age. For instance a Victorian cellar could be cut through the foundations of a Roman building. In this case, the Victorian deposits in the cellar would be at a lower depth than the surrounding Roman foundations, in spite of being much more recent in age. Great care must be taken to identify any such 'cuts' and to ensure that finds or soil samples collected from them are not mixed with materials collected from outside the cut.

During excavation, complex changes of texture, colour and content of layers are observed. These are recorded horizontally in plans and vertically in sections. By detecting cuts and fills, superimposition and episodes of soil removal and re-deposition, we can tell the order in which the deposits were laid down - this is called the 'sequence'. The sequence helps to establish the chronology of activity on the site by allowing dating evidence such as artefacts or scientific dating samples to be related to the build-up of layers across the area being investigated