When archaeologists are excavating, it’s important to understand what context they are in. Stratigraphic layers are formed by periods of sediment build up – i.e how soils are deposited (or removed). The layers nearer the surface (or the upper layers) are the later/ younger deposits, and the layers that are lower are the earlier/ older deposits.
Think of it as if you’re baking a cake with lots of layers of sponge. You first have to place the bottom sponge (the red layer) on the tray before building it up. Therefore, this layer is the earliest layer. The icing with the sprinkles that you put at the top of the cake is the last one that you place. Therefore, this is the latest layer. Dating the layers in this way is based on the ‘principle of superposition’.
image source: Stratigraphy - soils and archaeology | C.A.R.T Archaeology https://cartarchaeology.wordpress.com/2014/06/19/stratigraphy/
The artefacts found within the layers help to date it. A layer cannot be older than an artefact found within it. If you have something that is clearly modern – lets say a mobile phone – alongside some other artefacts such as roman pottery, the layer would still have to be dated to modern. This is because we know the Romans didn’t have this technology when this layer was created – or because the layer has been disturbed later.
Archaeologists use the terms ‘Terminus Ante Quem’ and ‘Terminus Post Quem’. These translate to ‘limit before which’ and ‘limit after which’. A Terminus Post Quem is the earliest date the event could have happened, and a Terminus Ante Quem is the latest date. Often artefacts will help to provide a Terminus Ante Quem or a Terminus Post Quem for the layer. For example, coins are very useful in this instance. A layer that has a Victorian coin with the date 1887 on it. Because we know the earliest date for the production of this coin (1887), the layer cannot be earlier than 1887, hence the Terminus Post Quem.
When archaeologists date a stratigraphic layer, they have to consider the date of the layers around it.
Archaeologists may refer to stratigraphic layers as contexts. A context can be thought of as an action. Therefore, you can have a long list of ways contexts are formed – it can be a deposit, an accumulation, a removal (a cut), natural erosion etc. Most likely in an archaeological trench, you can have may different processes forming the stratigraphy. For example, on prehistoric sites, you may find lots of pits. These pits have been cut into natural, and filled with soil from a different context, then over many years, soil has accumulated above these features, creating a top soil. Depending on how complex the trench or site that you’re digging is, you may have many layers of contexts on top of one another, and cutting through earlier deposits, making a complicated stratigraphy.
These images have been created by the author as examples of simple Harris Matrices
When archaeologists consider the different the dates of the stratigraphic layers and actions, they can show this as a Harris Matrix. To make one of these, archaeologists will consider the Laws of Superpositionand the Law of Stratigraphic Succession.The latest contexts will sit at the top of a Harris Matrix, and the earliest at the bottom. Archaeologists will create a Harris Matrix while they work, and the finished piece will help them when they are interpreting the archaeological processes that appear within the trench / at the site.
An important question to always ask is: what action came first?