Stratigraphy is a term used by archaeologists, geologists, and the like to refer to the layers of the earth that have built up over time. Stratification is defined by the depositing of strata or layers, one on top of the other, creating the ground we walk on today.
Stratigraphy is a relative dating system, as there are no exact dates to be located within the ground, and areas can build up at different rates depending on climate, habitation, and weather. It is the finds within these strata that help us date these slices of human activity, what we’re really after when we want to date something anyways. This is why context and association are so important when excavating. If multiple objects are found in association with each other, it is a good indication that they were buried at the same time. As long as the layers haven’t been disturbed, and has been sealed, we can say these objects are no later or no more recent that the deposit itself.
If coins are found within strata, or pieces of organic material that can radio carbon dated, then more exact dates can be attributed. Once a collection is formed over various layers in the earth, we are then able to create a proper timeline. Analysis of stratigraphy is then used to create a matrix, sorting out the layers to create a visual timeline. Harris Matrices are the probably what you’ve heard about the most.
Stratigraphy works off four basic Laws/Principles:
The Law of Superposition: The upper strata or layers occurred more recently in history in comparison to those below it as each layer had to build up on top of the other.
The Law of Original Horizontality: Any layer deposited in an unconsolidated will form horizontally on the ground.
The Law of Original Continuity: Any deposit laid down will taper out unless confined within a feature. Without this tapered edge, it can be deduced that part of the layer has been removed from either excavation or erosion.
The Law of Stratigraphic Succession: Any given layer within the archaeological stratigraphy has its place in history after the layer directly below it, and before the layer directly above it.
It is also important to note that when something is found within a layer, it is the final use date of this artifact, and may not always give the date of the layer itself as objects can be used for many years, if not decades before being buried within the archaeological record.
Stratigraphic layers can also end up being reversed due to both natural and cultural forces. Landslides, human interaction with digging pits, graves, etc. can flip the strata on its head, meaning it would need to be interpreted upside down.
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Looking to Find Out More?
K. Kris Hirst, Stratigraphy - Earth's Geological and Archaeological Layers. What Is Stratigraphy and Why Do Archaeologists and Geologists Care?
Harris, E. C. (1989) Principles of Archaeological Stratigraphy, 2nd Edition. Academic Press: London and San Diego.
Practices in Archaeological Stratigraphy, edited by Edward C. Harris
Reverse Stratigraphy :Archaeological Method and Theory: An Encyclopedia
edited by Linda Ellis