3. Assessing the Resource

Collecting the following information will aid the assessment of the historic environment resource:

• the character and significance of the known historic environment features (this includes individual surface features, their landscape setting, and buried archaeological and palaeoenvironmental remains) and the potential for previously unrecorded historic environment features
• the state of preservation of the peat, its archaeological and palaeoenvironmental remains, and
• the site hydrogeology and geochemistry

Information collection is a staged, iterative process, with more detail provided as the scheme is developed and as each stage of investigation is completed. Depending on the size and complexity of each site, its deposits, and the restoration plans, the need for – and scale of – each of the following stages, will vary.

The first stage is a desk-based study (see section 3.1 for further details), drawing together existing information, supplemented with a site walkover, to determine the presence, potential, and significance of the archaeological, palaeoenvironmental and historic environment resource. Aspects of this work will also feed into work on understanding the site hydrogeology.

In some instances, the results of the desk-based work may be sufficient to indicate that the restoration proposals will not damage the significance of any heritage assets. In these cases, the information collected at the desk-based stage can be used to inform the HIA.

Usually, more detailed field-based investigation will be required (see section 3.2), for which adequate funds will be required. These might include walkover survey (locating and recording features across the site), ground-level survey (e.g. measured survey), aerial (e.g. lidar) and/or ground-based remote sensing (geophysical survey), or below-ground survey and sediment recording (sediment coring) (Figure 6). The latter (in particular) is used to produce deposit models (see section 3.3), which may help inform the location of the site works and the understanding of the likely archaeological potential within and under the peat. The need will depend on existing knowledge and the complexity of the site type and its formation processes.

Samples can be recovered at this point for scientific dating and other analyses e.g. peat preservation. Discussions with the local authority archaeologist (curator) will help to determine which are the most appropriate investigation techniques to use in any given situation.

A further phase of investigation may be needed to investigate archaeological features, through a programme of evaluation trenching or test pitting. The size and scale of these interventions should be sufficient to evaluate the significance, extent and character of known and suspected archaeological features (for example timber trackways), and to take samples of relevant materials such as waterlogged wood or other macro-remains (such as plant and insects) for assessment.

Another vital stage of assessment is to look at the state of preservation (see section 3.4) of the peat itself (i.e. its palaeoenvironmental constituents) and any archaeological materials that are encountered during the field-based investigation. This is also crucial in identifying and demonstrating the benefits of the works from an archaeological perspective, as well as helping determine the success of the restoration scheme once in place.

The high potential for preservation within peatlands is closely related to the presence of water, hence understanding the site hydrogeology (see section 3.5) (alongside peat preservation) is important to determine how waterlogged archaeological elements of the site might be impacted by the restoration proposals.

For some sites, it may be sufficient to demonstrate that the proposed changes to raise water tables and keep them high will ensure that waterlogged archaeological elements are kept permanently wet, year-round.

For more complex sites, the following need to be understood:

• the hydrological status of the site and any hydrological inputs, which contribute to waterlogged conditions at the site
• the main (natural and human) influences on these hydrological levels (including any seasonal changes), and
• how the proposed restoration work will impact and alter these current conditions

3.1 Desk-based study

Prior to any intrusive works, information on the presence or likelihood (‘potential’) of deposits and features to be encountered can be gained from a desk-based study and assessment^[footnote1] of the site, any access routes, and any areas where material for restoration (e.g. stone for dams, heather brash for revegetation) is being sourced.

Applicants should consult their local Historic Environment Record (HER)^[footnote2]. Other sources of valuable information and data include maps (historical as well as current), geological information (e.g. deposit maps and geotechnical cores), aerial sources (e.g. photographs, lidar and satellite imagery), and literature (published and ‘grey’, e.g. local county archaeological society transactions, monographs, academic publications, QRA field guides). See section 8 for a list of potential resources and contacts.

It is often quicker and easier for archaeologists to provide relevant data where they are supplied with digital mapping e.g. shapefiles of site boundaries.

A desk-based study also usually includes a site walkover, to check and review the evidence collected and record any visible features (including aspects of the historic landscape) which are present. This would include checking peat cuttings and grips or hagged areas of peat (Figure 7) for signs of historic features.

If there has been recent systematic archaeological study of the area e.g. in advance of a wind farm or other development, this walkover element may not be required if it can be established that the prior recording was thorough.

Establishing the character of the resource will help determine the presence and significance of archaeological, palaeoenvironmental, and other historic environment features above, within, and below the peat.

As part of the desk-based work, the existing knowledge of the state of preservation of the peat deposits and the archaeological and palaeoenvironmental remains they contain is reviewed. This draws on results of previous investigation, knowledge of other sites in the local area, and any general assessments based on current understanding of the site (in terms of peat presence, depth, survival etc).

The final element of desk-based work that is needed to understand how peatland restoration proposals will affect wetland archaeology is a review of the site hydrology and hydrogeology. Undertaking a hydrogeological assessment will help establish current water levels and the main natural and human influences on those water levels, by seeking to understand:

• How does water enter the peat? Is it from rainwater, surface water run-off, groundwater, or a combination?
• What are the main influences on water level and water level changes?
• What proposed changes are planned to the water regime as part of the restoration, and what methods, materials, and working practices are proposed to achieve these aims?
• What groundwater data have already been collected? What further data collection is planned?

Ideally, a basic level of information about the peat and the site hydrogeology (sufficient to understand the highest and lowest water levels) will have already been gathered when identifying potential restoration sites. This can be drawn together at this stage along with the archaeological information collected in the desk-based assessment.

The output from this work will be an initial hydrogeological conceptual model (as set out in Historic England 2016, appendix 3) (Figure 8); these can be produced at a range of scales, depending on the complexity of the site.

The amount of information available or collectable may be dependent on the type of peatland and its location. More complex assessments are likely to be needed in lowland peatlands with a wide range of competing influences on water levels, compared with upland peatland systems, which are largely rainwater-fed and influenced by historic drainage networks that the restoration project is usually trying to reverse.

3.2 Field-based techniques (survey)

Two key aspects of field-based investigation of peatland landscapes are above- and below-ground survey and remote sensing. These are used to explore in more detail the extent, character and preservation of any heritage assets, and thereby their potential value and significance.

• Walkover survey; when not included at the desk-based stage, initial walkover survey should be undertaken. As implied, this is a walkover of the site recording archaeological features by surveying their location and making a photographic and descriptive record.
• Above-ground survey; detailed (measured and recorded) mapping of the peatland’s surface features – both historic environment assets (such as peat cuttings, and archaeological features), and other natural features (for example, peat erosion gullies). Features not previously located in the desk-based work and walkover are often identified by this work. Measured survey is appropriate as a mitigation if a feature is included within the works area and is to be changed in some way by the restoration.
• Auger survey (sediment coring – See Figures 6 and 9) and recording exposed sections (e.g. ditch faces, peat cuttings). These methods are used to determine the depth, character and state of preservation of the peat and see how it varies across the site. An auger survey can also be used to map the pre-peatland land-surface which could also contain archaeological finds. Samples from cores (see Table 1) can be used for palaeoenvironmental analyses and scientific dating (to establish a chronological framework for peat deposits, to help inform on the significance, age, and intactness of the peat and any archaeological remains), to assess the significance of the deposits. Note that specific peat restoration grant schemes will have their own requirements and criteria for taking such samples.
• Remote sensing – aerial survey; increasingly, aerial recording techniques are being used to gather information on peatland deposits and surface features. This includes using already-acquired datasets (e.g. Environment Agency procured lidar) or obtaining newly-acquired data (using SUAs) (see Historic England 2017a, 2018). Aerial approaches are particularly useful for mapping larger-scale features (e.g. peat cuttings) over large and remote/inaccessible areas, and tracking changes over time. Datasets from other forms of sensors (e.g. hyper-spectral survey, or gamma-ray spectroscopy) on aerial platforms may also be used to assess peatland extent, depth and health (e.g. Gatis et al 2019)
• Remote sensing – geophysical survey; this is of more limited use in peatlands as a means of identifying buried archaeological remains, and so is mainly used to investigate the depth of the deposits to inform and/or complement further mitigation (e.g. coring and excavations). Depending on the target resolution required, Ground Penetrating Radar (GPR) can be effective to depths up to 11 metres but for deeper deposits, and/or in environments where GPR signal penetration is limited, Electrical Resistance Imaging (ERI) or Low Frequency Electromagnetic (LFEM) profiles may be considered (Schmidt et al 2015, 48).

3.3 Deposit modelling

Once the desk-based and field survey elements of work are completed, information can be brought together into a deposit model. The model does not need to be complex; the aim is to show the vertical and horizontal make-up of a site’s deposits (see Historic England 2020), where possible down to and including the pre-peatland surface.

The model can be used to determine the optimum location for the proposed works – from the historic environment point of view – and inform any further investigations (see section 5.4).

3.4 Assessing the state of the preservation of the peat

An initial indication of overall peat preservation can be determined by an on-site inspection, which might show whether the peat is waterlogged or desiccated/humified for example (Figures 10a & 10b). For a more detailed insight, material from peat samples taken for palaeoenvironmental analysis can be used to assess the state of preservation of any archaeological and biological remains present (see Historic England 2016, appendix 2; Historic England forthcoming – Waterlogged Wood guidance).

Where well-preserved waterlogged archaeological and palaeoenvironmental remains are present, site restoration will ensure that the same site conditions that have enabled the survival of these remains to date are maintained within the new scheme. Conversely, the assessment of the state of preservation may indicate that the peat deposits affected by the restoration proposals are already degraded and do not contain material of archaeological or palaeoenvironmental value.

Knowing the physical condition and state of preservation of material remains at a known point in time means that it can be used as a baseline with which future preservation assessment analyses can be compared, in order to track any changes (degradation) over time.

3.5 Understanding site hydrogeology

Depending on the existing information available via the desk-based work, it may be necessary to collect additional groundwater data in order to construct an initial hydrogeological model and/or to improve and quantify a model. The level of additional data required will depend both on the robustness of the initial model and the nature of any changes proposed within the restoration programme.

The goal with any hydrogeological assessment is to collect sufficient information to inform decision-making and reduce risk (Historic England 2016, appendix 3) (see Table 1). It is important to understand how water is entering the site. In many cases, water level monitoring will already be in place as part of restoration planning and can therefore be applied to the archaeological scoping.

Upland peats/peat deposits are usually fed by rainwater (and subsequent run-off from higher, dryer areas to lower, wetter areas). Hydrogeological characterisation might identify those areas that are permanently wet, where maintaining or improving this condition (through the reversal of drainage, for example) will help to preserve any surviving archaeological or palaeoenvironmental material.

In areas of lowland peatlands/wetlands, the main historic environment objective is to maintain existing waterlogged, anoxic conditions of the peat which support the preservation of archaeological and palaeoenvironmental remains.

Where restoration works include raising or changing water levels, it is essential to ensure that: i) doing so will not harm the significance of historic environment features at the surface, and ii) that water levels are not allowed to fluctuate at the level of any known archaeological remains at depth (see Historic England 2016). The latter will be achieved by ensuring that post-restoration water levels are set high enough to take seasonal variations into account and maintain sufficiently higher levels during periods of low water availability (during summer and autumn).

Footnotes

1. This is compiling information that can be gathered without any fieldwork – it may, or may not, be equivalent to a ‘Desk-based Assessment’ (DBA) as defined by CIfA. ↩
2. Note that there may be a charge for this service. See Historic Environment Records (HERs) for more information. ↩

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