Stratigraphy

Well-defined stratigraphic layers.

Stratigraphy is the one of the most important concepts for archaeologists to use in the interpretation of a site.  For archaeologists, stratigraphy is the order in which strata, or layers of soil, relate to each other and archaeological remains.  Stratigraphy is based upon the law of superposition, which states generally that younger layers lay above older layers.  Knowing which stratum artifacts are found in gives archaeologists and idea about how old they are, relative to the other artifacts. 

Notice the brick and concrete in the top stratum; this is a historic layer. 

In lower strata, older prehistoric pottery was found.

One strategy for excavation and artifact recovery is to excavate following stratigraphy.  Our students, like all archaeologists, are trained to notice changes in soil color or texture – both of which can suggest important archaeological features.  We excavate in 10cm levels, but follow the natural stratigraphy within the level; if mid-level the soil changes we will stop and note the depths of the change and continue excavating, but keeping the new soil type separate from that above it; by doing this, we have a more exact context for any artifacts recovered.  Even the smallest of differences are important to note.  If we ignored the changes, we may be ignoring the only evidence of archaeological features, such as wall trenches, posts, and trash pits. 

Features are not always very clear. This profile has a pit running from the 

shell in the center towards the bottom-left of the photo.

However, as we excavate down by level, changes and stains are not always apparent.  That is when we rely upon studying the stratigraphy of the wall profiles of each unit.  Profiles are the best place to see and analyze stratigraphy.  It is important for archaeologists to keep walls straight and smooth while excavating to best see soil changes.  Archaeologists look at profiles during excavation to see if there are features (such as pits or wall trench remains) from levels above continuing into lower levels.  When they are noticed, archaeologists can take extra care excavating features properly and separately from the surrounding matrix (soil type).

We also look at stratigraphy after excavation.  This helps with interpreting the unit as a whole, and to notice strata or features that may have been missed during excavation.   By looking at profiles we can understand which features are oldest and youngest. We can also understand if newer features are disturbing older ones. For example, there may have been a pit in which there were several fill episodes, and possibly later a wall trench dug into it.  Understanding the law of superposition tells archaeologists which is oldest and which is youngest.

On the left is the east wall profile of a unit, and on the right is the south.

Looking at profiles side by side lets archaeologists see the continuity of strata.

You can also see a layer of the shell midden.

However, archaeologists never gain all the answers from one profile, so looking at all profiles as well as the floor of each level together is best. For this reason, photos are taken and maps are drawn at the end of each level, and the same for all four walls of a unit.  Having this documentation also provides future archaeologists and researchers the with the ability to analyze the stratigraphy, as it will never be the same as when a unit was originally excavated.  Stratigraphy provides an invaluable resource for archaeologists to use during and after excavation to interpret a site.