Marine Geoarchaeology and Investigative Methodologies

Justin Dix and Fraser Sturt

Introduction

Geoarchaeology and investigative methodologies could be thought of as two of the less glamorous subjects to be addressed within a research agenda. As archaeologists we are most interested in people: past stories of interaction and change, of other ways of being ‘human’, or the readily apparent drama of events such as shipwrecks. However, as Muckelroy (1978) and Evans (2003) have argued, we can only attempt to engage with these subjects if we take the time to consider the contexts we study, and the ways in which we go about investigating them. Offshore, and away from the terrestrial heartland of archaeological research, this is not as simple or as well established as one might think.

Geoarchaeology can be broadly defined as ‘the combined study of archaeological and geomorphological records’ (French 2003, 3). As Rapp and Hill (1998, 2) note, this involves  the  integration of earth science concepts and techniques, in order to understand the context of the human past. The principal goals of geoarchaeological work thus lie  in landscape reconstruction and understanding site formation processes, these two closely connected fields forming a crucial part of what might be more broadly described as environmental archaeology.

Within the many terrestrial research frameworks already written for England, little time is spent on in-depth, separate discussions of either method or site formation processes. Where environmental and geoarchaeological concerns do surface such as in the Regional Framework documents, they exist as well- developed ‘background’ pieces, providing crucial information on the changing nature of the environment, and the current limits of our knowledge base. The presence of this chapter thus reflects the differences that exist between our understanding of terrestrial and maritime archaeological contexts in 2012.

As we move further seaward from the shoreline our knowledge of the processes of change, their impact on the archaeological record, and a consensus as to the best ways to investigate landscapes and sites begin to dwindle. Thus while accounts of best practice for terrestrial methods sit easily within guidance documents and not research frameworks (eg English Heritage’s Geoarchaeology (2007) and Environmental (2002) booklets), there is need for further discussion as to what we can/could do in the inter-tidal and offshore zones. The result of this is that questions relating to geoarchaeology, and the best methodologies for the identification and investigation of submerged archaeological material, represent real and pressing concerns within the discipline. As would be expected, the individual period chapters within this document all integrate environmental and geoarchaeological questions to some extent. However, this chapter seeks to expand the scope of these questions through linking them together, and highlighting more general areas of concern and potential.

Marine geoarchaeology is multi-scalar (full-ocean scale to wreck-site specific) and time transgressive (changes during an individual tidal cycle to hundred thousand year sea-level cycles) by nature. Further, the onshore and offshore records of environmental change are self-evidently intrinsically linked to human activity. Consequently, information gained on Roman waterfront development can help to answer questions as to variable rates of sea-level change over the Holocene. Similarly, data gathered on changing Mesolithic and Neolithic river systems and shoreline configuration both benefit from and help to refine our understanding of broader Quaternary sequences. Yet integrating the data from these two realms remains a stumbling block in archaeological research (Parfitt et al 2010).

Based on the above, the scope of this discussion is broad indeed. It needs to consider how we engage with a variety of deposits, landscapes, and sites, from those now found tens of kilometres inland due to drainage and shoreline progradation (the Fens of Eastern England), through to modern shores and inter-tidal zones, to the deepest parts of the continental shelf. These locations incorporate everything from the extreme high-energy tidal regimes of the Severn Estuary, to the low-energy estuarine back- waters and saltmarshes along the Norfolk coast.

The identification, investigation, and interpretation of archaeological landscapes and archaeological sites

As discussed in more detail in Westley et al (2004) the nature and scale of palaeogeographic and palaeoenvironmental change of our continental margins is of particular importance to the process of palaeogeographic reconstruction, as it can alter radically over not only prehistoric but also historic timescales. For a full appreciation of this topic we need therefore to understand the nature of our continental margins and the short- and long-term processes that affect them. In this respect this approach parallels current thinking in palaeoenvironmental research, specifically the use of a nested hierarchy of scales (eg Shennan et al 2000; Barron et al 2003). In an ideal world research into the archaeology of submerged landscapes would proceed at a very small, ‘local’, spatial scales (studies of the order of tens of metres through to a few kilometres), thus allowing very fine details to be observed. These smaller-scale studies could then be mosaiced into larger ‘regional’ overviews (tens to hundreds of kilometres). In practice, the realities of underwater work render such a bottom-up approach impossible to undertake. Instead, we have to accept that the majority of research on continental shelf archaeology will be undertaken on the regional scale, with only occasional, more detailed analyses of local- scale studies being possible. However, the positive adoption of a more top-down approach should be used to maximise the regional data and, through appropriate analysis, utilise it to target effectively the more labour-intensive and inevitably cost- limited local surveys. We will therefore consider these issues on three different scales with each intrinsically entailing components of identification, investigation, and interpretation of the resource.

Shelf-scale reconstructions

At the largest continental shelf scales reconstructions are often of a first order approach, constructed through Glacio-Isostatic Adjustment (GIA) models of relative sea levels since the last glacial maximum (eg Brooks et al 2011 and references therein; Lambeck 1995a; Lambeck 1997; Lambeck et al 2000; Milne et al 2002; Peltier et al 2002; Shennan et al 2000; Shennan et al 2006). Alternatively, simply combining global eustatic sea-level curves (eg Bintanja and van de Wal 2008; Rohling et al 2009; Siddall et al 2003) with long-term estimates of crustal uplift/subsidence (Westaway 2008) and applying these to modern day coarse resolution bathymetry can give an indication of landscape change. Such ‘flooding’ models have significant limitations as they take no account of the isostatic component of sea-level change. However, such approaches, particularly when used in conjunction with shelf-scale Quaternary geological mapping (eg Hijma et al 2012), still have a place, as they represent the only option for large- scale landscape reconstruction prior to the LGM (see Westley et al 2004 for full discussion of issues related to this scale of reconstruction).

None of these interpretations is capable of accounting for changes in sedimentation patterns (either erosion or accumulation) in response to either ice sheet fluctuations or marine transgressive and regressive cycles, so should only be seen as broad indicators of the coastal morphology of late Quaternary landscapes. However, an attendant benefit of these models is that they have provided excellent platforms for research in to changing tidal (eg Neill et al 2010; Shennan et al 2000; Uehara et al 2006) and wave (eg van der Molen and De Swart 2001; Neill et al 2009) climates over the same period, essential components to understanding both landscape-scale site-formation processes as well as the potential for water-based transport.

In addition to issues of reconstruction there has been some recent consideration of the nature of shelf-scale landscape formation processes, investigating the impact of one or more cycles of marine transgression and regression either directly  on  the archaeological material or  more  realistically on the deposits, such as thick sedimentary successions with well-preserved organic horizons, and/or coarse-grained lithic deposits such as submerged fluvial terrace systems, which may contain them (Westley et al 2004; Hosfield 2007; Bailey and Flemming 2008; Ward and Larcombe 2008). This work has focused almost exclusively on Palaeolithic artefacts and is discussed in more detail in the relevent section, but it is a topic that would benefit from further research. In particular, shelf-scale models that look at the spatial variability of sea bed shear stresses, responsible for the grain (or lithic artefact) scale movement of material, and now more importantly temporal variability during the last marine transgression (eg Uehara et al 2006; Neill et al 2010) are ripe for application to the known record of archaeological scatters on the shelf.

The ability to create some of the simpler, landscape reconstructions is facilitated by the increased number of publicly accessible data sets. For the north-west European  continental  shelf  the most extensive  bathymetric  data  sets  are  the satellite-derived combined topography and bathymetry ‘ETOPO 1’ from the NOAA National Geophysical Data Centre. This represents a 1 arc- minute global relief model, so seamlessly includes both topographic and bathymetric data. Alternatively, a bathymetry only grid at 30 arc-second intervals can be obtained from GEBCO, although this product is essentially for the deep oceans and care has to be taken with data from continental shelves. Over the last fifteen years SeaZone Ltd has been both digitising extant Admiralty data (under license from the United Kingdom Hydrographic Office) and integrating modern digital surveys, as they are made available from both the UKHO and third-party sources, to provide deconflicted gridded xyz data down to an optimum resolution of 30m bins depending on the area. Alternatively, a Norwegian company, Olex Ltd (www.olex.no), has innovatively created a fishing community bathymetry project by which depth and navigation data from global fishing fleets (2500 users) are collated and integrated and the output (5m bins although with a quoted navigational accuracy of ± 10m) fed back to the community as ever-developing bathymetric charts. These data can be accessed by the non-fishing community, at resolutions dictated by the level of fishing activity, and have been used for Devensian glacial landscape evolution in the northern North Sea (Bradwell et al 2008). These latter two data sets are only available under licence and at cost.

The integration of these data is now standardly accomplished  through  a  range  of  GIS  packages, which enable not just the production of impressive imagery but facilitate critical assessment and discussion on vertical and horizontal accuracy and, most importantly, vertical and horizontal datum conversions. This is vital when considering the integration of global data sets which are defined to an arbitrary mean sea level.

In terms of our overall shelf-scale understanding of the actual bedrock geology, sea bed sediments, and in particular Quaternary deposits, our knowledge is driven by five or more decades of work undertaken by the British Geological Survey and which are summarised in:

  • a series of publications and maps (1:250000 and 1: 625000) dating from the 1970s to the present day. Probably the most up-to-date largest spatial scale published, geological and archaeological reviews are those undertaken by the Strategic Environmental Assessment (SEA) programme funded by the Department of Energy and Climate Change, which have subdivided the entire UK shelf into eight regions, whilst Murphy (2007) gives a review directed at landscape archaeologists;
  • publicly searchable databases of both geophysical and geological data now accessible through GeoIndex (which has onshore and offshore variants);
  • and derived 3D Geology models (50m grids) which at present are restricted to the terrestrial record (both bedrock and superficial models exist).
  • Inevitably, given the time period for which data has been acquired, obtaining original unconsolidated geological material is very uncommon, whilst written logs are of variable use for the archaeologists and Quaternary scientists as the original investigations were frequently for deeper geological purposes.

A wide range of alternative sources of marine geological and bathymetric data can be accessed through the MEDIN (Marine Environmental data and Information Network) although not all material identifiable through this portal is publicly accessible. Of particular note is the UK DEAL website, a gateway to the UK Offshore Oil and Gas Industry which represents an extensive archive of 2D and 3D seismic and well data. Also worthy of note is EU- SEASED which contains both seabed samples and seismic data from European seas, although it does overlap with the BGS archives (as they are one of the major UK contributors). All of the sources described here represent extensive data sets of highly variable quality; however, in general they are sufficient to make first order statements of shelf-scale evolution in order to develop more detailed regional-scale reconstructions.

Reviews and data archives of actual archaeological material at the shelf scale are few and far between and certainly bear no comparison to the extensive collation work of fishermen’s finds off the Dutch coast (as described in Peeters et al 2009).  The SEA archaeological reviews come closest to this, whilst Westley et al (2004) dedicates one theme to a review of pre-submergence archaeological deposits of the continental shelf. The commissioning by English Heritage in the mid-2000s of the Rapid Coastal Zone Assessment Surveys collectively provide similar reviews of the UK coastal zone but have limited input further offshore, an issue that is mirrored in the National Monuments Record, the County Sites and Monuments Records, and Historic Environment Records. The synchronous commissioning of the Historic Seascape Character maps and resources aimed to plug this gap but the outputs from this project are restricted to overarching layer characterisation and do not represent sources of direct  information essential for landscape reconstruction and interpretation. Ultimately, this limited pre- extant collation of the wider shelf archaeological record will make this Research Framework one of the most definitive documents.

One of the most significant shelf-scale activities of the last decade has been the ALSF (Aggregate Levy Sustainability Fund) BMAPA (British Marine Aggregate Producers Association) protocols for reporting finds of archaeological interest (Wessex Archaeology 2006). This project provided both educational and reporting components distributed to wharves and vessels operated by BMAPA companies, so archaeological finds from any period can be recorded and centrally archived; with time (and expansion to other seabed user communities) this could represent a significant resource of shelf-scale archaeological material. This protocol resulted in the most spectacular finds in UK waters of 28 Middle Palaeolithic handaxes, and faunal remains were found in an aggregate licensing area off the coast of Great Yarmouth.

Regional-scale reconstructions

Regional-scale reconstructions of the order of tens to hundreds of kilometres can be constructed through geophysical and geological techniques which when combined are capable of giving much more detailed multi-period landscapes. The last decade has seen a significant increase in such reconstructions in response to geophysical data sets being made available to the archaeological and Quaternary community. It is important to recognise at this stage that much of what has been done at this level primarily represents late Quaternary environmental reconstruction. Efforts have been made to assess archaeological potential from these interpretations (eg Gaffney et al 2009; Ward and Larcombe 2008), but full integration of palaeoenvironmental and archaeological contexts at this scale is still lacking. Higher-resolution shelf-scale bathymetric data sets (United Kingdom Hydrographic Office and third party resourced grids integrated by SeaZone Ltd) have been used successfully in relatively sediment-starved sections of the north-west European shelf to reconstruct particularly erosive landscapes, such as the Channel river system described by Gupta et al (2007). Similar approaches are now being taken as standard components of the archaeological and geological sections of the Regional Environmental Characterisation (REC) projects, funded by the Marine ALSF. These commissioned projects cover the outer Thames, south coast (focused around the south-Wight region), east coast and the Humber. All reports and data from these projects have been made publicly accessible via the ALSF website.

Although enabling excellent regional-scale morphological reconstructions they also highlight one of the biggest problems with this scale of research, namely assigning an accurate chronology to landscape evolution. The work on the English Channel is an excellent example of this, as the catastrophic mega- floods postulated to have generated the English Channel (‘Fleuve Manche’) river network by Gupta et al (op cit) could originally only be  constrained to one or more overflow events through the Dover Straits at either MIS 12, MIS 10–6 or at least by MIS 5e. More recent work, dating associated distal sediments in the Bay of Biscay, would now suggest catastrophic activity was actually initiated during MIS 12 (Toucanne et al 2009b). Chronological control for all current coastal and marine deposits is a major challenge and beyond the scope of this overview, but the reader is referred to English Heritage best practice guidelines for scientific dating as well as the National Heritage Science Strategy report.

Such regional-scale reconstructions are not limited to sediment-starved erosive regimes as the work of Gaffney et al (2007; 2009) on a 3D seismic mega-survey (made available from Petroleum Geo- Services publicly viewable Data  Library) from  the Southern North Sea – Doggerland clearly demonstrates. Here an extensive (23,000km2) primarily fluvial and estuarine-dominated emergent plain is within the top 200ms (c 160m) beneath the modern seabed. As with the relic English Channel (‘Fleuve Manche’) river system this major landscape reconstruction  is based almost exclusively on geophysical data  and initially lacked any form of absolute dating despite significant efforts by the authors to extract all extant data available for the area. This problem has partly been addressed by coring as part of the Humber REC project, where 31 cores have been acquired from eight localities and from which a total of 25 OSL and radiocarbon dates have been reported so far. Again no direct archaeological material was recovered or available with sufficient accuracy to integrate into their final model, but the authors did modify terrestrial Heritage Landscape Characterisation schemes to facilitate both interpretation and the identification of potential high archaeological preservation zones. The potential of these zones has yet to be tested.

Finally, such an approach is implicitly restricted to those areas that have undergone 3D seismic exploration, currently the North Sea, the Irish Sea (Fitch et al 2010) and restricted parts of the English Channel such as Poole and Christchurch Bay (Gaffney et  al 2007). Further, the software and data storage requirements for the analysis and visualisation of these large 3D data volumes are currently restricted to either the academic community or the original oil and gas sector from which they are derived.

The Regional Environmental Characterisation projects not only rely on the larger-scale bathymetric data sets, but have also been able to acquire new geophysical data. The RECs follow a corridor approach to survey, with high-resolution swath bathymetry, side scan and sub-bottom profiler (typically boomer) data being collected along c 300m wide corridors at c 8–10km spacing across hundreds of square kilometres. This enables the next level of landscape identification and reconstruction which can start to enhance the shelf-scale models described above.

Through positive collaboration with the marine aggregate industry (co-ordinated through the BMAPA), the data acquired as part of the REC have been supplemented by additional sub-bottom profiler data and core log material acquired during prospection, environmental impact assessment, and monitoring phases of individual aggregate deposits. In the Thames Estuary such an approach has facilitated a reinterpretation of the offshore relic landscapes, identifying river systems that may represent multi-phase lowstand incision from as early as c 700ka BP. Again the REC project formats do not support the acquisition of new core data which could be used for a range of palaeoenvironmental analyses and, most critically, dating of sediments. However, a follow up MEPF project to the Thames REC has funded the acquisition and analysis of 30 vibrocores from the outer Thames Estuary (Dix and Sturt 2011) specifically to constrain the chronology of the relic landscapes identified during the original geological and archaeological analysis.

In addition to the RECs, the ALSF has put significant resources (c £25.5 million between 2002 and 2011; Dellino-Musgrave et al 2009) into marine research projects. The heritage component of this represents the single largest investment in marine archaeological research the UK has seen. This has not just funded the regional-scale activities such as the RECs but also a series of smaller (tens of kilometre scale),  more detailed  geoarchaeological studies, of specific locations including: Humber (Wessex Archaeology 2007b); Great Yarmouth (Wessex Archaeology 2008b); Happisburgh and Pakefield (Wessex Archaeology 2008c); offshore Arun River (Gupta et al 2004; Wessex Archaeology 2008a; 2008d); Eastern English Channel south-west of Beachy Head and the Severn Estuary (MoLAS 2007b). All but the latter involved the acquisition of new geophysical data, and in some cases core and grab sample data were also acquired.

For the majority of the regional projects described here the primary mode of landscape reconstruction and interpretation was based on GIS integration of the extant record, bathymetric data sets and spatial interpretations of sub-bottom features, where possible calibrated against new or extant core data. Alternatives to this approach are becoming more widely available with packages such as Fledermaus Viz 4D (purely for the integration of high-resolution geophysical data) and Rockworks15 (purely for the integration of geological and geotechnical data sets) already having been used, particularly in the commercial sector (for partial review see Bates et al 2009). SMT’s Kingdom Suite, GeoSoft’s Oasis Montaj, and Schlumberger’s Petrel enable the full integration  of geophysical and geological data sets in a single package, but as yet these programmes have not been widely used in the archaeological community for landscape reconstruction. The quantitative and qualitative outputs from all these packages can either be presented as final image products (static or moving) or exported for inclusion and further visual manipulation within GIS software.

As evidenced in this section, the funds and opportunities provided by the ALSF has been the principal driver behind the recent rapid advancement in our understanding of landscapes in both selected parts of the north-west European shelf  and to a lesser extent the wider shelf environment. There are currently a number of new opportunities for archaeologists to gain access to essential high-resolution geophysical and geological data. Firstly, swath bathymetry (1m binned) and back- scatter data (25cm binned) collected as part of the Civil Hydrography Programme (administered by the Maritime Coastguard Agency) in collaboration with a number of external partners (including the Strategic Regional Coastal Monitoring Programme) is being made freely available through the Channel Coastal Observatory. A series of such surveys has already been undertaken of the coastal strip (1km offshore from Mean Low Water Neeps) including: the southern tip of Cornwall (Lizard Point, Land’s End); south-east Devon and south-west Dorset coast (Torbay to Abbotsbury); and Christchurch Bay to the Isle of Sheppey including the Isle of Wight. These developments were all strongly influenced by the Joint Irish Bathymetric Surveys (JIBS), a swath bathymetry IHO Order 1 data set acquired over   an area within the 3 nautical miles coastal strip between Malin Head and Rathlin Island and also freely available online but at a sub-sampled resolution of 10m bins; this has already been analysed for both landscape and wreck-based archaeological material (Westley et al 2011; Plets et al 2011). Similar activity is likely to continue and needs to be embraced by the archaeological community.

The next phase of regional-scale data  acquisition suitable for use in landscape reconstructions are the prospection, environmental impact, and monitoring surveys required for the extensive offshore civil engineering projects currently being undertaken for the renewable energy sector. All windfarm (Rounds 1–3), wave, and tidal turbine installations and offshore components of rekindled nuclear power station sites require extensive geophysical and geological data acquisition. All of these data are naturally assessed for archaeological potential as part of planning constraints; however, post-consent, data are increasingly being made available to the wider archaeological community via the Cowrie Data Management System, with Round 2 data already available.

There is also the opportunity to enhance these large-scale, geophysical reconstructions with core derived geological data. Geoarchaeological analysis of offshore cores follows the protocols that have been long established onshore, and which are summarised in documents such as Environmental Archaeology: A Guide to the Theory and Practice of Methods, from Sampling and Recovery to Post-Excavation (English Heritage 2002); and Geoarchaeology: Using Earth Sciences to Understand the Archaeological Record (English Heritage 2004). Specific issues related to the acquisition and analysis of offshore cores for archaeological purposes has been reviewed in the Offshore Geotechnical Investigations and Historic Environment Analysis: Guidance for the Renewable Energy Sector (Gribble and Leather 2010). There has also been extensive discussion within the academic literature of the methods and role of integrated stratigraphic data sets eg Bates (1998; 2000; 2003), Bates and Bates (2000), Bates et al (2000; 2007a), and Bell and Walker (2005).

Detailed study of the sediments, their faunal, floral, and very occasionally direct archaeological content, enables the establishment of palaeoenvironmental conditions (with  particular  reference  to their location to sea level) and their variation through the stratigraphic sequence. It also facilitates the establishment of absolute chronologies and, when integrated with additional boreholes and/or geophysical data, the full regional environmental context. For a typical core, analyses can include: detailed visual lithological and stratigraphic logging; particle-size analyses; x-ray photography, CT scanning, micro-morphology; macrofossil, macrofaunal, and microfossil (diatoms, ostracods and foraminifera) content; lithic analyses of gravel content; palynology; and geochemical analyses, the latter being particularly useful in establishing hinterland industrial activity from the Roman period onwards.

Although examples of such work are very limited offshore (effectively to a sub-set of the ALSF projects described above; the diver-based sampling and hand augering undertaken over the last decade at Bouldnor Cliff; and restricted reports from the commercial sector), there are numerous case studies from currently terrestrial coastal lowland sites around the UK coastline that demonstrate good practice in core-based palaeoenvironmental analysis.

Despite all the data now potentially  available, the archaeological community still too often fails to consider onshore-offshore sites as a single seamless landscape. One of the most obvious examples of this has been the exemplary work done for onshore East Anglia on the Palaeolithic estuarine and coastal landscapes that contain the earliest evidence of hominin occupation of the British Isles. This extensive work is currently based exclusively on deposits from terrestrial exposures and has so far failed to include any offshore component (such as that acquired as part of the Seabed in Prehistory project). Consequently, current and future projects should as a matter of course look towards bringing together data from either side of the coastal strip. This step is facilitated by the digital nature of the data types now being acquired both onshore (LiDAR and georectified aerial photographs) and offshore (surface and sub-surface geophysics), which can be seamlessly merged within the software packages previously described. However, although this contiguous approach is commonly regarded as the way forward, Bates et al (2007a) clearly articulate the considerable difficulty in extrapolating between terrestrial and maritime situations without careful investigation in the coastal transition zone. Indeed they argue that rather than assuming similarities between patterns of landscape evolution between the onshore and offshore systems, one should anticipate dissimilarity of patterns, missing sequences and different landscape formation processes.

Finally, much of what has been discussed above primarily, although not exclusively, relates to prehistoric reconstructions. The sedimentary record can and should be considered for all periods. However, once written records begin the availability of text and in particular maps can be used with caution for the interpretation of coastal change. Georectified historic maps can provide strong evidence of coastal change, yet quantification is required to avoid over reliance on documents of unknown original accuracy.

Site-scale reconstructions

Although regarded as the most common scale of investigation, the concept of site as a unit of analysis is in many ways problematic, within both traditional and geoarchaeological research. As ever, the main issue is where one draws the boundary between the site level and the regional context, such distinctions often being arbitrary in nature. It is crucial to realise that site-level processes are necessarily informed by regional regimes (Muckelroy 1978; Ward et al 1999; Quinn 2006); thus for an understanding of site formation to be developed, it is necessary to tack between fine-grained, small-scale, high-resolution data, and broader regional analysis. A considerable amount has been written on this subject with regard to terrestrial sites (see Rapp and Hill 1998; French 2003; Goldberg and Macphail 2006), but again there is limited information on this topic for offshore and inter-tidal locations.

The current suite of established offshore geophysical tools (swath bathymetry, and sub-bottom, magnetometer, and sidescan sonar) along with diver and remotely operated vehicle (ROV) survey, have proved effective in identifying both exposed and buried larger (and particularly metal) wreck sites. ALSF-funded work by Bates et  al  (2009),  Dix et al (2008a; 2008b) and Wessex Archaeology (2003; 2006; 2007b; 2008a–d; 2008f) has explicitly described and evaluated the strengths and weaknesses of these tools under different conditions and in different areas. Furthermore Plets et al (in press) provide a full review of current technology as well as detailed guidance notes on standards for the application of marine geophysics for such archaeological work. However, whilst these techniques appear to work well, much of the success in identifying wreck sites via geophysical methods lies in the skills of the operator. Determining which acoustic anomaly is likely to indicate wreck material rather than changes in seabed morphology is not an exact science. As such, there is need for a continued commitment to training and open discussion within the field as to how techniques might be improved. These vagaries in terms of identification of anomalies in geophysical data sets, and the value we place upon them as archaeologists, necessitate a continued commitment to ground truthing (where possible) by ROV or diver.

In the near-shore and inter-tidal zones the range of techniques increases to include LiDAR, aerial photographs (extensive coverage of which is freely available from the Channel Coastal Observatory), and more detailed records from a greater time depth of archaeological investigation. However, in all of these cases, and particularly in the case of buried material, identification relies largely upon the extent and obtrusiveness of the wreck. This means that there is a bias in our identification methods towards larger (and with this often more recent) wreck material. As such, there is a need for continued research into how we go about identifying and ground truthing the presence of more ephemeral wreck material. Work by Arnott et al (2005) and Plets et al (2007, 2008 and 2009) points to the potential of identifying waterlogged wood remotely, via integration of sub-bottom and borehole data. Thus although exacting and time consuming, this would appear a worthwhile area for further research.

Attempts to understand the site formation processes at work on wreck sites, and their impact on the distribution and survival of archaeological material, have been undertaken for several decades. Muckelroy (1978, 169) was at pains to point out the ‘scrambling devices’ which can occur before, during and following wrecking processes. His work on the Kennermerland still stands out as a landmark attempt to engage with site-scale reconstruction. Interestingly, the bulk of the work on site formation processes in the UK since has continued to focus on site-specific investigations and almost exclusively on Designated Wreck sites. Notable examples that involve significant site formation process research include: Stirling Castle, Mary Rose, Hazardous, Grace Dieu, Yarmouth Roads, Pomone, Swash Channel, Royal Anne Galley, Resurgam, Duart Point, and of course the Kennermerland The most up-to- date sources of information on these can be most easily accessed through the English Heritage’s UK Maritime Designated webpages. It is also interesting to note that work in site formation processes is driven by heritage management rather than a mode of enhancing our understanding of the taphonomy of individual and collective archaeological artefacts and so our understanding of the archaeology itself.

What is more lacking has been extensive generic research on the processes (physical, biological, and chemical) that operate on underwater sites. Ward et al (1999) provide the most recent overarching summary, although this is still primarily extrapolated from a single wreck, the Pandora, offshore New South Wales. The role of sediment dynamics on submerged archaeology has been explored, through a combination of in situ investigation, laboratory based experiments, and more recently in the numerical domain by a number of authors, both on a site scale (eg Dix et al 2007 and 2009a; Quinn et al 1997; Quinn 2006) and on the artefact scale (eg Dix et al 2009b; Rangecroft et al 2008; Tomalin et al 2000). There has also been recent work on modelling physical processes on a regional scale in an attempt to characterise accumulation and erosion potential at a resolution capable of identifying which of the c 30,000 known wrecks in UK waters could potentially be at risk, prior to more detailed site-scale investigations. This has been approached in two ways: geostatistical analysis of the extant archaeological and environmental data (Merritt 2008) and nested numerical models of sediment transport of the southern North Sea and English Channel (Dix et al 2008b; 2009a). These two projects are currently coming together under a Marine ALSF-funded project (AMAP1 and AMAP2, 2009–12).

In terms of fundamental biological processes, work has been undertaken by UK researchers, on both macro- (Wessex Archaeology 2008e) and meso- faunal (shipworm, eg Jones 2003; Palma 2005, 2008; Palma and Gregory 2004) activity, and as contributors to major EU heritage conservation projects such as MOSS and MACHU. Finally, there has been no known active research into the role of chemical processes on submerged archaeological sites in the UK since the work on the use of sacrificial anodes for the preservation of large iron artefacts on the Duart Point wreck (Gregory 1999). Consequently, there is significant scope for fundamental research in all of these areas.

In comparison to the depth of research carried out into individual wreck sites, relatively little has been done on detailed, site-level analysis of submerged former terrestrial sites. Excluding cases of large- scale land movement, such as at Dunwich (Sear et al 2009), which have led to submergence of more substantial modern material, the history of sea-level change and coastline reconfiguration means that submerged terrestrial sites in English waters are most likely to date to the Palaeolithic, Mesolithic, and to a lesser extent the Neolithic. The large-scale commercial surveys carried out for wind farms, aggregate extraction, telecommunications, and oil and gas activities, rarely operate at the resolution or line spacing required to pick out ephemeral prehistoric sites. Here, the terrestrial record suggests we should be looking for lithic scatters, and pit and hut sites. Within land-based commercial activity these features are most readily detected via evaluation trenches and open area excavation, practices which have not been extended into underwater commercial contexts in English waters. Work by Plets et al (2009) has shown that it is theoretically possible for site-level identification to be achieved through 3D geophysical survey, but that it needs to be tightly focused (ie cannot be used for prospection surveys) and have plenty of time to be achieved.

It is significant that at present there is only one continually submerged prehistoric area of activity being excavated within English waters: Bouldnor Cliff. There have been a great many more inter-tidal and coastal sites discovered, and these are discussed within the period-specific chapters of this research framework. This stands in marked contrast to large number    of prehistoric submerged sites investigated in the relatively benign waters of the Baltic. The difference in histories of sea-level change and inundation between the two areas is significant, but so too are the ways in which the researchers engage with the offshore archaeological record.

In English waters most prehistoric areas of interest are identified via chance finds, or research- led investigation of areas designated to be of high potential (submerged forests, sub-tidal peat beds, and areas in close proximity to known sites on land). Given the generally shallow waters in and around Denmark, a different approach has been adopted. Here site evaluations ahead of construction projects have been carried out, with underwater test pits producing significant quantities of archaeological material (Dencker and Dokkedal 2004). Often this evaluation work is carried out in conjunction with predictive modelling exercises. These models rely on close integration of onshore and offshore regional data with extensive records of past archaeological work. Furthermore, they are iterative models which are adapted as new data come to light. Significantly, regional-level survey is only used to construct landform and deposit models (regional-level analysis), not for the most part to identify sites. This has significant implications for prehistoric site-level research within English waters:

  • Current surveys do not operate at the scale or resolution required to be able to identify ephemeral prehistoric sites.
  • We do not have a system in place for further evaluation of high potential submerged prehistoric surfaces.
  • We cannot generate accurate predictive models for submerged regions beyond basic landscape/ geomorphological characterisation due to the resolution of surveys and lack of offshore excavation.

Until these issues are addressed, it will be difficult for site-level underwater prehistoric archaeological reconstruction to move forward. A case study of one site (Bouldnor Cliff) is not enough to develop a national strategy. As such, how we engage with submerged prehistoric sites remains largely hypothetical in an English context. Thus, it would appear that a crucial avenue for further research lies in carrying out pilot projects in English waters.

Despite the lack of excavation work being carried out on  submerged prehistoric  sites, there is a wealth of knowledge on how to survey and excavate underwater. This has principally been derived from overseas exemplars, work on Bouldnor Cliff, and from wreck projects within English waters  (most  notably the Mary Rose). ALSF- funded projects have addressed both the methods available to archaeologists and the additional knowledge we can draw from time series data (Merritt 2008; Wessex Archaeology 2006; 2007b; 2008a–d, 2008f; 2009b). In addition, the IFA have published an account of best practice for under- water and maritime work (Oxley and O’Regan 2001). However, the discipline has progressed significantly since the 2001 publication date of this report. Similarly, whilst the Nautical Archaeology Society’s Underwater Archaeology (Bowens 2009) provides comprehensive coverage of a number of different methods, it does not address ways in which methodologies might move forward.

Of  critical  concern  in  all  work  underwater (as on land) is the ability to position the location of recovered material and excavated contexts accurately  in  three  dimensions.  Traditionally  this has been resolved through use of arbitrary grids/ positions underwater which are then linked back  to a known point, whose positional absolute error is known. These techniques have been shown to work well on sites when multiple direct measurements can be taken to develop survey redundancy over relatively short distances. However, particularly  for work on submerged landscapes away from the inter-tidal zone, where elevation data is crucial for linking into histories of sea-level change, the error margins involved often reduce the value of the data obtained. As such, there is a need for commitment to increased use, and continued research into, under- water survey and positioning systems. At present combined use of echo locators and GPS (Geographical Positioning System) buoy arrays (such as those produced by Sonardyne and ACSA, and utilised on the  protected  wreck  surveys)  represent  the  best method through which to locate underwater work in real world coordinates. Such tools are not cheap, but in order for underwater excavation to be worth- while, it needs to be exacting.

Key research areas for marine geoarchaeology and investigative methodologies

It is clear from the discussion above that the discipline is in good health, rapid advances in offshore research having been made over the last decade. Much of this progress is due to the increasing volume of data released by the commercial sector, and the software needed to integrate it. However, there are also clear avenues by which research may move forward at all three of the scales discussed above.

Landscape and regional levels

There is an urgent need for increased dating of offshore deposits to improve landscape reconstructions and our understanding of offshore archaeological potential.

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This strategic objective applies to all levels of site. Of particular importance would be the identification and dating of additional sea-level index points on the shelf to offset the present skew towards current coastal and wetland sites.

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Active
Categories:
Chronology

More work needs to be carried out on integrating offshore and onshore records to create unified reconstructions

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Active

Site level

Evaluation of submerged prehistoric land surfaces already noted to be of high potential needs to be carried out.

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It is only through  doing  this  that we will be able to determine the value that such work may truly have. It is entirely possible that landscape and regional-level reconstruction may be the most appropriate unit of analysis for prehistoric contexts.

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Active
Categories:
Submerged landscape, Evaluation

Although considerable work has been done on identifying, investigating, and monitoring wreck sites, additional research is still required in many areas.

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Including: how reliably we can objectively identify and map wooden and ephemeral wreck debris from standard geophysical data types; and generic studies on all site formation processes (physical, biological, and chemical) operating on submerged sites.

Status:
Active
Categories:
Wreck