Case Study 3: Using Geoarchaeological Deposit Modelling to Aid in the Identification, Evaluation and Targeting of Lithic Scatters and Sites
Carl Champness (Oxford Archaeology South)
One of the key challenges in lithic studies is developing approaches and methods by which to better predict and identify lithic scatters and sites. The need for the development of suitably sensitive archaeological methodologies, geared to identifying and characterising lithic scatters, has recently been identified in a number of publications (eg Bond 2011; Blinkhorn and Milner 2013). Recently, new approaches using geoarchaeological deposit modelling and field investigation techniques have been instrumental in helping to identify a series of large lithic sites.
Effective management of archaeological projects often starts with initial desk-based studies either as part of a desk-based assessment (DBA) or Environmental Impact Assessment (EIA). All too often DBAs and EIAs can become very formulaic and fail to provide adequate consideration of buried potential, geological context and landscape setting, leading to an undervaluing of early prehistoric potential.
Often when this potential is not recognised during the initial stages, opportunities can be missed, and less effective strategies have to be developed in the later stages of projects. This is all too familiar for Palaeolithic sites or Holocene sites in alluvial, colluvial or estuarine contexts, which are often buried beyond the reaches of more conventionally deployed approaches like aerial surveys, gradiometer surveys and HER searches. Using deposit modelling as a desk-based technique early in the planning process can help reduce risks, as well as inform mitigation by design (Historic England 2020; Carey et al. 2018).
A number of recent examples, including large infrastructure schemes like the Carlisle Northern Development Road (CNDR) and the Crossrail Project, highlight the potential for unexpected discoveries within buried environments sealed by alluvium, colluvium or drift deposits which were not highlighted in the desk-based work or targeted during evaluation stages, but discovered late during the main construction phase of work.
Last-minute discoveries can often lead to an unquantified loss of important archaeological evidence, or the under-estimation of the magnitude of a site’s scale and importance, leading to missed research opportunities or, in a planning/development context, potentially avoidable expense, delay and inconvenience.
Geoarchaeological investigation on the Carlisle Northern Development Road
A programme of archaeological mitigation works was undertaken as part of the CNDR, which crossed over the River Eden river terraces in 2010 (Fig 3.1). The geoarchaeological investigations were added later during the construction phase of the scheme, as the previous desk-based assessments and evaluations had failed to adequately highlight the early prehistoric potential of the raised river terraces
A geoarchaeological assessment of the terrace sequences and mapping of the palaeochannels through lidar (Fig 3.2) became necessary following the unexpected discovery of a potentially major lithic site during the topsoil stripping for the road scheme. Geoarchaeological test pitting and trenching was then undertaken across the mapped channel sequences to help establish the sedimentary and landscape contexts of the lithic material and its association with the wetland sequence (Fig 3.3).
The work helped to develop a suitable excavation strategy for the lithic site and associated palaeochannel sequences. The site produced nationally important Mesolithic/Neolithic remains, and one of the largest in situ lithic assemblages recorded (Brown et al. in prep; Case study 6). Unfortunately, this led to considerable unexpected costs and delay to the construction project, which may have been avoided if the potential of the terraces had been highlighted and investigated earlier in the archaeological programme.
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Evaluation sampling strategies
In the last 30 years there has been a significant shift in fieldwork techniques away from strategies that were more suited to the recovery of lithic material in the form of fieldwalking and hand-dug test pitting (Blinkhorn 2010).
Since the implementation of PPG16 there has been a push towards the greater use of geophysics and trial trench evaluation based on sampling percentages, which generally favour the identification of late prehistoric/Roman and later remains that have negative features, compared to more discrete forms of archaeology characteristic of early prehistory. This has been exacerbated in recent years as pressures to facilitate development have increased, and this has led on average to 5-2 per cent sample coverage being proposed for most sites, well below the levels advocated by Hey and Lacey (2001) for early prehistoric sites.
Current sampling levels and evaluation techniques are not well-suited to identification of lithic sites or scatters and a more targeted strategy is widely advocated where lithic potential has been identified.
The significant growth of geoarchaeological approaches and the development of deposit modelling software over the last 10 years, have helped to redress this balance and place the focus back on identifying early prehistoric remains.
The use of deposit modelling and geoarchaeological techniques has helped to provide a better understanding of the buried archaeological potential, including landform assemblages like the distribution of palaeochannels, river terraces or floodplain islands that can offer insights into the distribution of archaeological remains (Carey et al. 2017).
We may still not fully understand the relationship of these palaeo-landforms to archaeological activity, but deposit modelling has proven a highly effective approach in helping identify and target areas and horizons of higher potential, using appropriate fieldwork techniques.
Application of targeted evaluation approaches based on geoarchaeological deposit modelling on the Bexhill to Hastings link road
As part of a proposed new link road between Bexhill and Hastings, in East Sussex, a deposit model was created in order to develop an interpretative framework for investigating the archaeological potential of a series of river valleys and associated bedrock ridges of Ashdown Sands and Wadhurst Clay.
The 5.6km scheme skirted around a series of alluvial sequences that have been deposited in a low-lying former coastal inlet of the Combe Haven. A staged approach was adopted for the project using multiple data sources from geotechnical investigations and archaeological boreholes, test pits, geophysics and trenching.
The model helped to define more closely and to characterise the palaeo-topography of the area impacted by the scheme, its Holocene sedimentary sequences and their archaeological potential. The model produced a zonation of landscape areas, identifying valley-edge environments containing densely stratified, well-preserved, early prehistoric remains. This model allowed these interface zones and sub-surface features to be targeted during evaluations, rather than adopting a standard blanket sampling approach across the valley sequences.
High-resolution mapping of the palaeo-landscape was undertaken by means of an electric conductivity survey using an EM31 ground conductivity meter to map the different sedimentary zones and interfaces (Fig. 3.4). The survey identified areas of high ground and submerged floodplain islands away from the main route with enhanced archaeological potential, which could be targeted for more effective evaluation strategies (Fig. 3.5).
The evaluation was able to confirm the lithic potential of the targeted areas and to help devise an appropriate mitigation strategy to reduce the impact on the archaeology by preserving some areas in situ where possible, whilst focussing excavation on the densest and highest potential areas. This and similar examples of a wider range of geoarchaeological techniques can be found in the deposit modelling guidance (Historic England 2020) and accompanying case studies volume (Carey et al. 2018).
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In order to develop more effective and targeted evaluation and mitigation strategies to look for lithic sites there is a need for greater specialist input in the early stages of projects to assess the potential of buried environments, especially for large or complex infra-structure projects, which can often cross a range of different landscapes and buried environments. Where the potential of any buried environments/horizons within DBAs/EIAs cannot be fully determined, further pre-evaluation work should be recommended, using either deposit modelling or other appropriate mapping techniques, like remote sensing, geophysics, and auger/borehole surveys.
Geoarchaeological deposit modelling and multi-disciplinary approaches have demonstrated their archaeological value in assessing the potential of buried environments. Through the use of deposit modelling, it has been possible to develop more effective targeted evaluation strategies that are better suited to identifying early prehistoric remains. This has resulted in the identification of new lithic sites that may have otherwise not been recognised or dealt with in the same way.
Bond, C J, 2011 ‘The value, meaning and protection of lithic scatters’. Lithics 32, 29–48
Brown, F, Clark, P, Dickson, A J, Gregory, R and Zant, J in prep 'From an Ancient Eden to a New Frontier: an archaeological journey along the Carlisle Northern Development Route.' Lancaster, Oxford Archaeology
Carey, C, Howard, A J, Jackson, R and Brown, A, 2017 ‘Using geoarchaeological deposit modelling as a framework for archaeological evaluation and mitigation in alluvial environments’. Journal of Archaeological Science Report 11, 658–73
Carey, C, Howard, A J, Knight, D, Corcoran, J and Heathcote, J (eds) 2018 Deposit Modelling and Archaeology. Brighton: University of Brighton
Hey, G and Lacey, M, 2001 Evaluation of Archaeological Decision-Making Processes and Sampling Strategies. Oxford, Oxford Archaeological Unit
Historic England 2020 Deposit Modelling and Archaeology: guidance for mapping buried deposits. Swindon, Historic England