Palaeoenvironmental sampling and what it adds

Palaeoenvironmental sampling in archaeology is the part of a mitigation programme that most often gets specified badly and priced vaguely. It is also the part that turns a set of excavated features into a dated sequence with an environment attached to it. A ditch recorded, drawn and left unsampled gives you a line on a plan. The same ditch with a bulk sample from its lower fill can give you a radiocarbon date, the crops being processed nearby and an indication of whether the surrounding land was wooded, grazed or cultivated.

For anyone reviewing a written scheme of investigation, the sampling strategy is where you can tell within a page whether the author has thought about the site or copied a template.

What the technique recovers

Different sample types answer different questions, and the strategy should pair each one with the question it serves.

Bulk samples, processed by flotation and wet sieving, recover charred plant remains, charcoal for identification and dating, small animal and fish bone, molluscs, and small artefacts missed during hand excavation. They are the workhorse of the discipline and suit most dry archaeological fills.

Waterlogged samples, taken from deposits permanently below the water table, preserve a completely different assemblage: uncharred seeds and fruits, wood, insect remains, mosses and sometimes textiles or leather. The information return per sample is high because the preservation is better and the assemblage reflects the immediate environment rather than only what was burnt.

Column and monolith samples, taken as a continuous sequence through a sediment profile, support pollen analysis, diatoms, soil micromorphology and the reconstruction of a landscape through time rather than at a moment. These are the samples that tell you how a palaeochannel silted up or how a buried soil formed.

Separate from all of these are samples taken specifically for dating, where the material selected, its stratigraphic security and the absence of residuality matter more than the volume.

Deposits worth sampling

A strategy that samples everything wastes money. One that samples by context type without regard to condition wastes more.

The deposits that repay sampling are:

• Waterlogged fills below the water table, in any feature type.
• Peat, palaeochannel sediments and alluvial or colluvial sequences.
• Buried soils sealed beneath later deposits or structures.
• Lower fills of ditches, pits and wells, where primary deposition is most likely.
• Hearths, ovens, kilns and industrial residues.
• Middens and dumps of domestic waste.

The deposits that rarely repay it are modern made ground, heavily truncated horizons, upper ditch fills open to later contamination, and plough soil. Sampling those produces assemblages that cannot be tied to a phase, and a reviewer who sees them in a strategy should ask why.

Where the site has an obvious waterlogged component, the archaeological field evaluation should have established the water table and the depth at which organic preservation begins. If it has not, the sampling strategy is being written without the one piece of information that determines it.

Sampling strategy in a written scheme

A strategy that a reviewer can approve without correspondence contains six things.

1. The research aims the sampling serves, expressed as questions rather than as a wish to recover material.
2. The sample types to be taken, matched to context types and to preservation conditions.
3. Target sample volumes, with the rule for small features where total excavation is proposed.
4. The named specialists, or at least the specialist disciplines, with the organisation responsible for commissioning them.
5. A processing, retention and discard policy, including what happens to residues and flots.
6. The archive destination and the standard the archive will meet.

Target volumes deserve a word. Bulk sample volumes are conventionally expressed in litres, and the appropriate figure depends on the expected density of material rather than on a standard number. Say what volume is sought and what will be done when a context yields less than that, because a context that produces three litres does not get the specified forty by wishing.

Processing and assessment stages

Sampling is not a single cost line. It runs through four stages and each should be priced separately.

Collection happens on site and costs little relative to everything after it. Processing, by flotation or wet sieving, is a volume-driven cost. Assessment is where a specialist examines the processed material, reports what is present and in what condition, and states whether full analysis would add anything. Analysis is the expensive stage and should only be commissioned where the assessment demonstrates potential.

That staged structure comes from the project management framework Historic England publishes, and it exists precisely so that money is not committed to analysing material that cannot answer a question. A programme that jumps from processing to analysis without an assessment stage is either badly planned or hiding the decision.

Reporting and what it changes

The assessment report is the document that changes decisions. It can show that a sequence is datable where it was thought not to be, that preservation is better than predicted, which in turn affects the case for preservation in situ, or that the deposits are reworked and the apparent sequence is not real.

The analysis report belongs in the final publication, with the data tables deposited in the archive. The useful discipline is to write the environmental results into the site narrative rather than appending them as a specialist section nobody reads. A site report where the charred plant evidence appears in the discussion of the enclosure's function is worth more than one where it appears on page 140.

Where the asset is a building rather than buried deposits, none of this applies, and the appropriate instrument is a heritage significance assessment with recording to the relevant level.

Cost and programme effect

Three programme items catch developers out.

Flotation and sorting take time proportional to volume, and a large sample set from a strip, map and record exercise can occupy a processing facility for weeks. Radiocarbon dating turnaround is measured in weeks, and a date needed for the assessment conclusion sits on the critical path to discharging a condition. Specialist availability for a narrow discipline, insect remains or soil micromorphology, can be the longest lead item of all.

So the sampling strategy should be agreed before fieldwork starts rather than after, and the condition discharge timetable should assume the assessment stage completes before occupation rather than before commencement. The distinction between mitigation techniques matters here too, and the reasons for choosing an evaluation over a watching brief also determine whether a useful sampling strategy is even possible.

When to decline sampling

A reviewer should be as willing to remove sampling from a strategy as to add it.

Decline where preservation conditions rule out the assemblage sought, for example seeking waterlogged remains in free-draining gravel above the water table. Decline where the stratigraphy cannot support a phased interpretation, so that any date obtained could not be attached to anything. Decline where the research question is already answered for that period and landscape, which is a judgement the regional advice and the research framework for the area should inform. And decline where the proposed volume is disproportionate to the development's impact.

An illustrative example

A pipeline crossing a floodplain exposes a palaeochannel sealed beneath 1.2 metres of alluvium. The strategy takes a continuous monolith sequence through the channel fill for pollen and micromorphology, three waterlogged bulk samples from the basal organic silt, and two dating samples on short-lived plant material from secure contexts. No sampling is proposed from the overlying alluvium or from the modern plough soil above it. This example is illustrative and not taken from a real project.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and note that environmental archaeology work should be carried out to the relevant Chartered Institute for Archaeologists standards by suitably experienced specialists.

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