1 Stratigraphy and Depositional Sequences

What stratigraphy reveals

Archaeological stratigraphy reconstructs the relative sequence of events: which events happened earlier or later. It does so by recording physical relationships among layers, surfaces, and cuts. Artifacts and other dating evidence can help assign calendar dates afterward, but they do not establish stratigraphic order.

Deposits and how they form

A deposit is accumulated material, including soil, ash, construction debris, or sediment. It may form through deliberate human activity, such as dumping refuse, laying a floor, or filling a ditch, or through natural processes such as flooding, wind-blown sediment, slope wash, or soil formation.

Deposits may be primary, accumulating where the material was originally used or produced, or reworked after being moved from an earlier location by people or natural forces. Reworked artifacts may therefore be older than the deposit that contains them.

Texture, color, inclusions, thickness, and shape can help suggest how a deposit formed. For example, a thin ash layer on a floor may represent burning in place, while mixed rubble and soil filling a cut may reflect deliberate backfilling. These interpretations should be tested against the deposit’s boundaries and relationships with surrounding units.

Units, interfaces, and cuts

Excavators identify distinct stratigraphic units, commonly called contexts. These include deposits, structural remains, and interfaces. Each unit should be defined by its observable characteristics and assigned a unique identifier according to the project’s recording system.

An interface marks a boundary or change in the sequence. Some interfaces are surfaces of deposits, such as a floor or the top of a layer. Others are cuts created when material is removed, for example by digging a pit, trench, or grave. A cut is an event rather than a deposit; material later placed in the opening is recorded separately as its fill or fills.

A pit cut into earlier soil is later than the soil it truncates. Its fill is later than the cutting event, although the cut and fill are closely related parts of the same sequence. Recording them separately prevents the act of digging from being confused with the later act of filling.

Reading relationships between units

In an undisturbed sequence, superposition means that an overlying deposit is later than the deposit beneath it. Archaeological sequences can be disturbed: later cuts may remove parts of earlier deposits, while erosion or construction may truncate layers. A lower unit is therefore not automatically older than every unit above it when a sequence has been cut or rearranged.

Interpretation depends on direct relationships: whether one unit overlies, underlies, abuts, or cuts another. Loose material commonly tends toward horizontal layering, but deposits can slope, mound, or follow the shape of a surface. If a deposit ends abruptly in a section, it may continue beyond the excavated area, or it may have been removed by a later cut or by erosion. These principles help generate questions; the observed evidence determines the interpretation.

Building and using a Harris matrix

A Harris matrix represents direct stratigraphic relationships as a diagram, placing earlier units below later ones. Units without a direct relationship should not be connected merely because they occur at similar depths. The matrix records stratigraphic order rather than every physical contact.

For example, consider an old soil, a pit cut into that soil, the pit fill, and a floor laid across the filled pit. The sequence is: the old soil formed; the pit was cut into it; the pit was filled; and the floor was laid over the fill. Thus, the floor is later than the fill, the fill is later than the cut, and the cut is later than the soil it truncates. A separate nearby layer that touches none of these units cannot be placed in this sequence from stratigraphy alone.

Building a running matrix during excavation can expose inconsistencies while evidence is still visible. A matrix is an analytical summary, not a replacement for the field observations on which it is based.

Recording the evidence

Record each unit’s description and boundaries, its location and extent, and its direct relationships to other units. Plans, sections, photographs, and written records capture different aspects of the evidence: a plan shows horizontal extent, while a section shows vertical relationships.

Record uncertainty rather than forcing a relationship that the exposed evidence cannot establish. Keep observations distinct from interpretations, and use the matrix to summarize the sequence rather than substitute for the records that document it.

• Stratigraphic unit