Previous Dig It articles, including #8 (Stratigraphy), #20 (The Importance of Context), and #24 (Relative Dating), discussed how important the study of stratigraphy (the layering of sediments) is in interpreting archaeological sites. In this article I will look a bit deeper into the subject to show how we do it.
An amateur British geologist, William “Strata” Smith, revolutionized the study of stratigraphy in the late 18th-early 19th Century. While working as a surveyor in coal mines and canals, he noticed that there appeared to be a predictable relationship between the various layers of rock, even when he moved from place to place across the country. Using this information, he later produced the first known geological map.
The principles that he developed can be summarized as follows:
1) In an undisturbed series of layers of sediment, the oldest layers are at the bottom, and the youngest are at the top.
2) Anything that is in an undisturbed layer cannot be older than the material that the layer is made from.
3) Identifiable layers that are interrupted by a cut are assumed to be identical.
4) Layers that intrude into other layers are younger.
Archaeologists use these simple principles to interpret the sequence of events in archaeological sites.
In the most common commonest expression of this concept, when we are digging in an archaeological site, we generally can assume that as we dig deeper, we encounter ever older layers. This is an idea that intuitively makes sense.
Figure 1 shows an example of what we encounter in many archaeological sites. At the top is modern debris. Successively older artifacts are found as we dig deeper until we get to layers that predate human occupation.
Figure 2 shows a common circumstance where we find layers that are apparently out of time sequence. When this occurs we have to explain the anomaly. If we can confirm that this is an undisturbed layer, then we would need to revisit our assumptions about the sequence of dates. In most circumstances this would suggest a more likely explanation is that the site has been disturbed, with older sediments removed from somewhere else and piled on top of younger ones.
Figure 3 shows another common occurrence, where we find a younger layer intruding into older layers, such as a hole for a storage or cache pit being dug into undisturbed ground. In this case, we would assume that the intruding layer is younger than the layers it interrupts, and we can infer that the layers on each side of this intrusion are the same. Note that in such cases it is not uncommon to find younger artifacts buried deeper than older ones, even though they were deposited later.
Careful excavation and diligent documentation of the relationship between recovered artifacts and the layers that they come from help us interpret complex archaeological sites using these simple rules of stratigraphy. Destroying this relationship through activities such as artifact hunting and construction disturbance eliminates the opportunity for us to tell an archaeological site’s story.