Stratigraphy
Understanding the way in which deposits have accumulated to form the layers of an archaeological site requires an awareness of stratigraphy. Stratigraphy is the study of the build-up of soil, refuse, building debris and other material in the ground: the ‘strata’.
On most sites we dig from the uppermost (the most recent) layers down to the lowest (which tend to be the earliest) layers. On the way down, 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.
A cut is where somebody has removed soil, rock or building material in the past. This might have been to create a pit or post-hole, a grave, a foundation trench or defensive ditch, or to take away material such as stone or brick for re-use (‘robbing’). Finding a cut means looking for an edge, then following it down by carefully removing what is inside the feature, until it is ‘bottomed’. Cuts were often made into existing cuts, this is known as inter-cutting and disentangling it can be a very challenging task, especially where the fills are very similar in colour or texture. It is best always to try to excavate the latest cut first: this usually the one with the most complete edge. In Medieval cemeteries, for instance, grave cuts can be intercut many times by later graves. Cuts are known as ‘negative’ features, as they are evidence of something being taken away, not added.
Fills are the opposite of cuts: they are ‘positive’ features - meaning they are deposits, and they occupy the holes and spaces created by cuts. Some cuts can have many fills, but the fills of features are always stratigraphically above cuts because they occur later in the sequence, even if in reality this may have been only a few moments after the cut was made. The fill of a post-hole may contain the stone packing which once held the post in position, and sometimes fragments of the post itself may survive. There may be artefacts or radiocarbon samples which can help to date it. The fill of a grave will contain the inhumation (sometimes more than one), but also may contain evidence of a wooden, stone or lead coffin, or grave goods such as pottery, brooches or weapons.