3.3: Archaeological layers

In talking about archaeological context we tend to think of it in terms of being composed of individual layers. These are discrete units of sediment, debris, rock, and other materials that form or accumulate as the result of natural processes, human activity, or both. In order to describe them and indicate their position on a site and in its sequence, these layers are generally numbered sequentially (with the lowest number being that removed first). An individual layer or deposit may be termed (following geological nomenclature) a stratum, and multiple layers are called strata. Therefore the study of their sequence is called stratigraphy (description of what we call the stratification).
One of the simplest ways to depict layers is to depict them as a vertical sequence and this is frequently done in the form of a section (or 'profile') through them.

Here is an explanation from Michigan State University 'Campus Archaeology' page "Archaeology 101: Reading stratigraphy"

This photo is a close up of some stratigraphic layers which have been labeled. Each layer is sequential: the lower B came before the lower A which proceeded the higher B, which came before the top A. When we excavated, we started with the Top A and moved downwards, back in time.

 Now let’s try one a little more complex, using a drawing of a fictional wall profile. This profile has some intrusions in it, so that makes the sequencing a little bit more difficult. See if you can place the layers in order from oldest to most recent.

The answer: C is the oldest. Then F, A, D, G, E, and B. F comes after C because it is an intrusion into that level, meaning that C must exist in order for F to be dug into it. the relationship between A, E, B, and G is also complicated. G happened first, then E, then B. It is as if E is the toothpick that is plunged into a club sandwich, and G and A are the layers of bread: E must happen after G and A. B is entirely within E, so it must have happened after E. Most likely, B is a pipe that was placed in a ditch, which is intrusion E.

In order to graphically represent the relationships between layers we might use a ('Harris') Matrix. Here is a simple example:

but they can gt much more complex, such as this one  (Point Riche, Newfoundland)

The problem is that archaeological sites are not two dimensional but complex three dimensional entities (that exist in the fourth, which is time):

Different types of archaeological deposits in an urban context (Carver 1987, drawn by Elizabeth Hooper).

Another term needs to be introduced, archaeologists refer to 'features'. Basically this rather vague term means 'something that is not a layer'. It could be a negative feature (like a pit, posthole, ditch, gully, drainage pipe trench) dug into the ground. Or it can be an upstanding feature, such as a wall, the remains of a kiln or oven, a tombstone. Within the negative features is what is called a 'fill' and this fill may have within it layers. In a matrix, the 'cut' (edges) of a negative feature is sometimes given a number and description - and in some excavations, the 'interfaces' between individual layers are also numbered separately (especially if, for example, they are worn pebble or clay floors or similar surfaces).

Tamara Kroftova comments:
"In order to interpret how sites were used and formed, the various assemblages of material (including finds) in individual layers and features need to be differentiated studied separately and in groups. In order to do that, when removing artefacts from stratified sites, those individual archaeological contexts need to be observed, described and examined seperately. Of course that cannot be done in a narrow, dark and dirty hole, one spade width across."