4.2 Geoarchaeology

Geoarchaeology, which uses earth-science techniques to study the archaeological record, can be applied to a wide range of scales, from the reconstruction of palaeoenvironments and how they have been affected by human activity (see section 4.7) to on-site analyses of human-made or altered soils and sediments in order to interpret human activities and site formation processes (Goldberg and Macphail 2006; Rapp and Hill 1998). The specific methods used are equally wide ranging, and are similar to the methods commonly used by physical geographers, geologists and soil scientists. Detailed field descriptions of soil or sediment profiles (Reed et al. 2000) are typically supported by follow-up laboratory analyses. These can include:

  1. particle-size analysis to confirm the energy and agent of deposition (e.g. by wind, water, ice, gravity),
  2. loss-on-ignition to estimate organic content,
  3. magnetic susceptibility to determine if the soil or sediment has been affected by heating,
  4. pH tests to determine the level of acidity/alkalinity, electrical conductivity tests to determine the soluble salt or nutrient content, mineralogical analyses to determine the origins of sediments,
  5. micromorphology to determine the microscopic components and structures of undisturbed soil or sediments in thin section, and
  6. elemental analyses such as soil carbon content, phosphate analysis or the increasingly more common multi-element analyses (Courty et al. 1989; English Heritage 2007; Wilson et al. 2005, 2008 ).

In recent years innovative applications of stable isotope analyses and organic residue (lipid biomarker) analysis to the study of agricultural soils has also enhanced the understanding of the biological origins of ancient manures (Bull et al. 1999; Simpson et al. 1998).

Geoarchaeological Studies in Scotland

The application of geoarchaeological techniques to the analysis of archaeological soils and sediments in Scotland has been diverse, encompassing all of the techniques discussed above, and much of the research in this field has been world-leading. Most research has focused on the elucidation of site formation processes, the identification and quantification of the chemical and organic residues of daily activities, the interpretation of the spatial organisation of indoor and outdoor activity areas and the living conditions in houses, and the understanding of anthropogenic soil development resulting from past land use. In addition to the direct application of geoarchaeological techniques to archaeological questions, a significant body of work based in Scotland has focused on the development and testing of emerging geoarchaeological techniques.

Site formation processes

Geoarchaeological methods applied to soils and sediments on Scottish archaeological sites have contributed to the understanding of the soil environment immediately predating the site, the differentiation of natural and anthropogenic sediments, the origins and modes of formation of archaeological deposits, and the post-depositional alteration of archaeological deposits. Particular attention has been paid to:

  1. the depositional and post-depositional processes that have affected the complex stratigraphy on settlement sites (e.g. Banks et al. 2000; Davidson et al. 1992; Guttmann et al. 2003; Limbrey 1989; Schwenninger 1999),
  2. the process of vitrification of Iron Age hillforts (Brothwell et al. 1974; Friend et al. 2007; Youngblood et al. 1978), the mode and tempo of midden formation (Simpson and Barrett 1996, Simpson et al. 2005),
  3. the formation of farm mounds in the Northern Isles (Davidson et al. 1986),
  4. the intensive cultivation of midden heaps in Scottish prehistory (Guttmann 2005),
  5. the movement of waste materials from urban to rural sites in Scotland (Davidson et al. 2006), and
  6. taphonomic processes such as bioturbation (Davidson 2002; Lancaster and Simpson 2003).

Prospection and the analysis of activity areas.

A very wide range of geoarchaeological techniques have been used to determine the composition of on-site archaeological soils and sediments in Scotland, and have thereby contributed to interpretations of the activities and conditions that created those soils and sediments. In particular, micromorphological analysis, magnetic analyses, and geochemical analyses, including both phosphate and multi-element techniques, have been used to locate and interpret activity areas associated with human and animal activities on prehistoric and medieval farmsteads and within buildings (e.g. Edwards 1983; Giles et al. 1999; Jones et al. 2010; Milek 2005; Smith et al. 2001). It is worth noting that the application of plant opal phytolith analysis to the interpretation of activity areas, which saw pioneering research on Scottish archaeological sites in the late 1980s and early 1990s (Powers 1993; Powers and Gilbertson 1986, 1987; Powers et al. 1986) is currently rarely practiced, even though ethno-historic work by Smith (1996) demonstrated the value of integrating this technique with the more common geoarchaeological methods, and even though it continues to make valuable contributions to the interpretation of archaeological sediments elsewhere in the world (e.g. Piperno 2006; Shahack-Gross 2011).

Anthropogenic soil development.

The long history of soil science in Scotland has meant that even before the emergence of geoarchaeology as a discipline, soil scientists were engaged with issues related to the soil environments of prehistoric settlements and the effects of agriculture on Scottish soils (e.g. Romans and Robertson 1975, 1983). Over the last two decades there has been a growing trend in research on anthropogenic soil development, the historic management of cultivated soils (e.g. by manuring), and the development of artificially thickened (plaggen) soils in Scotland (e.g. Bond et al. 2004, 2005; Davidson and Carter 1998; Davidson and Simpson 1984; Davidson et al. 2007a, 2007b; Dercon et al. 2005; Dockrill et al. 2007; Guttmann et al. 2003, 2005 , 2006, 2008; Shiel and Askew 1988; Simpson 1985; Simpson and Guttmann 2002; Simpson et al. 1997, 1998, 1999, 2005, 2007; Thomas et al. 2007; Turner et al. 2004, 2010). The techniques employed in these studies of Scottish anthropogenic soils and the manures that have been used to fertilise them have been at the cutting-edge of research, and diverse, including elemental analyses, lipid biomarker analysis, soil carbon content, stable isotopes, particle size analysis, and soil micromorphology.

Methodological Research.

Geoarchaeologists working on Scottish archaeological sites have been world leaders in the development of new methodologies, often making use of reference samples derived from Scottish experimental and ethno-historic sources. One of the first to realise the potential of using recently abandoned crofts as a testing ground for geoarchaeological techniques, and to use them for demonstrating the potential of geoarchaeology to elucidate site formation processes, was Quine (1986; Davidson et al. 1992). Since then, a number of researchers have made use of soils and sediments from recently abandoned crofts to test and validate different geoarchaeological techniques, including the use of multi-element analysis, stable isotopes, organic content, magnetic susceptibility, pH, and phytoliths analysis to identify activity areas on archaeological sites (Entwistle and Abrahams 1997; Entwistle et al. 1998, 2000; Smith 1996; Wilson et al. 2005, 2006a, 2006b, 2006c, 2007, 2008b, 2009). Other important methodological research that has focused on the anthropogenic soils associated with abandoned Scottish crofts includes the use the image analysis in micromorphology (Adderley et al. 2006; Bryant and Davidson 1996), and the applicability of agroecosystem models to historic and archaeological questions of land management and soil sustainability (Adderley et al. 2000). On the basis of burning experiments conducted on the Isle of Lewis, a technique was developed to identify different fuel ash sources based on different mineral magnetic signatures, and this technique has been applied to a range of sites in the Northern and Western Isles (Church et al. 2007; Peters et al. 2001, 2002, 2004). In addition to these Scotland-based studies, methodological research has been conducted at Scottish institutions on soil and sediment materials taken from outside the country (e.g. Adderley et al. 2001, 2004, 2007), and Scottish-based geoarchaeologists have been involved with the publication of important methodological reviews (e.g. Adderley et al. 2010; Bull et al. 1999; Guttmann et al. 2005; Davidson and Simpson 2001, 2005; Davidson et al. 2007).

Geoarchaeological Research Capacity in Scotland

In Scotland, there is expertise and facilities for the full range of analytical geochemistry techniques and loss-on-ignition in the School of Humanities at the University of Glasgow, the Schools of Geosciences and Biological Sciences at the University of Aberdeen, the School of Biological and Environmental Sciences at the University of Stirling, and the James Hutton Institute (formerly the Macaulay Land Use Research Institute), while expertise in soil micromorphology is concentrated at the Universities of Stirling and Aberdeen. There are also in-house geoarchaeologists in some archaeological consultancy firms such as AOC Archaeology Group, Argyll Archaeology, Headland Archaeology and Wessex Archaeology but as yet there is not a coherent network that can bring the smaller commercial archaeology units in contact with geoarchaeological expertise. It is notable, however, that Scottish geoarchaeologists have taken a lead in innovative methods of transferring geoarchaeological knowledge and skills to field archaeologists (Cowie et al. 2009; Wilson et al. 2008a, 2010), including the launching of the open-access, web-based Soil Analysis Support System for Archaeologists (www.sassa.org.uk).

Emerging Opportunities in Geoarchaeology

There is tremendous potential for geoarchaeological methods to make important contributions to the interpretation of Scottish archaeological sites, but, as in other areas of science assisted archaeology, it is essential that geoarchaeological expertise be consulted and integrated at an early stage in the research design. A network of archaeological science experts and archaeological consultancy firms could improve the flow of information about the benefits and costs of particular sampling strategies and techniques. It is also worth noting that the potential for geoarchaeology to contribute to the interpretation of archaeological soils and sediments could be improved by the integration of additional methodologies not normally used in conjunction with the more common geoarchaeological techniques discussed above. Plant opal phytolith analysis, for example, could be employed more frequently to locate sites where peat, turf, plant and animal waste (including ash) is expected to have been introduced into the soil (e.g. Powers et al. 1989) and to assist with the identification of fuel ash residues, animal fodder, bedding materials, and animal excrements (e.g. Shahack-Gross 2011). Biomolecular analyses, including lipid biomarker analysis using GC/MS and DNA analysis, should be employed more frequently in situations where it would be beneficial to identify dung and/or decomposed plant materials to the species level. Pollen analysis of archaeological sediments could be employed on a more regular basis in order to improve the interpretation of land use in the immediate vicinity of sites as well as the interpretation of deposits composed primarily of decomposed plant materials, such as sediments identified as originating from dung or hay on the basis of thin section or phytoliths analyses (e.g. Tipping et al. 2009; Whittington and Edwards 1999). Palynological analysis of grave fills can also potentially contribute to the interpretation of plants and flowers incorporated into burial practices (e.g. Clarke 1999). Finally, diatom analysis could be employed more frequently to locate sites where peat, turf or their ashes are expected to have been introduced into the soil, and to assist with the identification of fuel ash residues (e.g. Bathurst et al. 2010).

It is important to stress that in regards to prospecting for potential sites that the palaeo-landscape should be treated as a continuous entity from the coastal corridor through the intertidal to offshore, rather than splitting it at the present day coastline. This zone is also particularly suited to analysis using geophysical techniques. Also, as pointed out elsewhere in this panel, the geomorphology of upland landscapes might not contain all the necessary archive of palaeo-climate and its direct relevance to human inhabitation and use, this is especially true for the majority of sites which again would have been dominated by locations closer to the (palaeo)shoreline.

Due to its northerly location and diverse and often marginal environments, including both coasts and uplands, there are a number of emerging opportunities for geoarchaeological research in Scotland to contribute to archaeological and scientific research of world-wide significance. In a northern European context, for example, Scotland’s recently abandoned crofts are a particularly important resource for the development and validation of new geoarchaeological methods. There continues to be potential to extend the use of this rich source of information, particularly in the understanding of the organic and chemical residues of different activities. Scotland also has the potential to be at the forefront of research into the monitoring of site preservation conditions in the wake of human-induced and natural environmental changes, including coastal erosion related to changing sea levels and storminess, aeolian deflation and deposition in coastal regions, soil pollution related to urban settlements and mining activities, deforestation and forest plantation, flooding, and dewatering as a result of gravel or clay extraction.

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