2 Excavation Units and Field Methods

Units, contexts, and research questions

Archaeological excavation is a controlled investigation of buried deposits and features. Because excavation removes the evidence being studied, investigators plan where and how to dig in advance and record observations and finds in enough detail for others to assess the results. Methods should address the project’s research questions while disturbing no more of a site than necessary.

An excavation unit defines the area where investigators work. Within a unit, they may identify several contexts, each representing a distinct deposit, surface, cut, or built element. Keeping these concepts separate prevents a grid square from being mistaken for a depositional event.

Selecting and laying out units

Unit size, number, and placement depend on research goals, survey results, site conditions, and applicable project standards; there is no single size suitable for every project. Units may be small test units used to sample a site or larger areas opened to examine deposits, features, and spatial relationships.

Placement should test a clear question. For instance, if survey indicates artifacts in one area and a possible structure in another, units might be positioned to assess whether deposits are intact, establish their depth and extent, or expose the suspected feature. Sampling plans should consider the distribution of known evidence, topography, soil and sediment conditions, site boundaries, and areas that should remain undisturbed. Remote sensing and other survey results can guide placement, but subsurface anomalies may require excavation or another form of direct investigation to identify them.

Before digging, establish a site grid tied to a permanent or relocatable datum. Record each unit’s location and orientation on a site plan, and use consistent horizontal and vertical control so observations from different units can be compared and connected. Excavation should be conducted by qualified personnel under relevant permits and project protocols. Discoveries such as human remains require applicable consultation and reporting procedures.

Identifying deposits and sequence

Excavators distinguish deposits by observing properties such as soil color, texture, composition, inclusions, compaction, and boundary shape. A change may mark a depositional boundary, but not every variation in color or texture represents a separate event. Roots, burrowing animals, erosion, cultivation, construction, and other disturbances can alter soil. Record observations before assigning an interpretation, and revise interpretations when new evidence emerges.

Stratigraphic relationships help establish a relative sequence. If one deposit lies over another without evidence of disturbance, the lower deposit was generally in place first. A feature cut into an older layer must postdate the layer it cuts, and its fill must postdate the cut. These relationships do not by themselves provide calendar dates. Deeper deposits are not necessarily older: erosion, later cuts, redeposition, and other formation processes can disrupt or invert deposits.

The cut of a pit or posthole and the material that later filled it are different contexts. The cut records an act of digging, while each fill records material deposited afterward. When a feature has visibly distinct fills, record and excavate them separately where practical.

Excavating deposits and features

Where deposits and features can be distinguished, excavate according to their boundaries and relationships rather than removing a uniform thickness across unrelated layers. Work carefully toward visible interfaces, using tools suited to the material and task. Keep spoil from separate contexts apart so finds retain their provenience. Screen sediment as required by the research design and project standards; finer recovery methods may be appropriate when small remains are important.

When deposits have no reliably visible internal boundaries, excavators may use arbitrary levels, or measured increments of a chosen thickness, as a control. Record each level’s depth and reference point. These increments are a practical sampling device, not proof that each represents a separate ancient event, and they should not obscure visible stratigraphy. If a clear boundary appears in a deposit that seemed uniform, stop and revise the excavation and recording strategy.

Features are commonly documented in plan before excavation. Where appropriate, excavators may remove one portion first and leave the other in place to expose and record a cross-section. This can reveal the feature’s depth, shape, and fill sequence. The best method depends on the feature, research questions, and project protocol. Record feature fills separately from general unit deposits, and document the precise context of any specialized samples.

Recording observations and provenience

Documentation preserves information that excavation itself removes. For each unit and context, record its identifier, location, depth or elevation, observed soil characteristics, boundaries, relationships, and the methods used. Use scaled plans and profile drawings, photographs with logged location and direction, and written or digital field notes. Record finds and samples by their specific context or level, and label containers so their provenience remains clear.

Where a stratigraphic matrix is used, update it as contexts and relationships are identified. Check it against field records, drawings, and photographs.

Keep observation distinct from interpretation. For example, “dark brown, loose sediment with charcoal flecks and a rounded boundary” records observations, while “hearth fill” is an interpretation to be evaluated. This distinction preserves the original evidence and allows interpretations to change without losing the observations on which they were based.