Dark Earth in the geoarchaeological approach to urban contexts

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1. Introduction

In urban contexts, which are essentially anthro-pogenic, the processes of accumulation, transfor-mation and erosion are particularly intense and diversified (Cammas et al., 2011). These processes proceed from the urban fabric, by through dynamic interactions between societies and the materiality of spaces (Noizet, 2009). The strata resulting from the combination of these socio-environmental dynamics are more or less easily interpretable in terms of the human use of space. Any difficulties of interpretation

are both linked with the interpretive frameworks and the pedo-sedimentary history of these strata/hori-zons/contexts. Among these strata, Dark Earth are thick dark layers, which appear to be homogeneous and constitute the main archaeological documen-tation of towns from the 4th to the 11th c. Although they have been known and observed since the 19th c., specific protocols of excavation, stratigraphic analysis, archaeology and geoarchaeology have only been applied since the 1980s (Fig. 1).

Chapitre XVII

Dark Earth in the geoarchaeological approach to urban contexts

Fig. 1. Dark-Earth stratifications. A: Bayeux, rue Franche (photo: G. Schütz, 2010, Service Départemental d’Archéologie du Calvados). B: Beauvais, la Chapelle (photo: Q. Borderie, 2007). C: Metz, Sainte-Chrétienne (photo: S. Augry, 2008, Institut National de Recherches

Archéologiques Préventives). D: Evreux (photo: B. Guillot, 2001, Institut National de Recherches Archéologiques Préventives).

214 | La géoarchéologie française au xxie siècle

1.1. The geoarchaeological approach of urban contexts

The frameworks for the geoarchaeological study of urban landscapes was established in the early 1980s (Hall and Kenward, 1982) and strongly influenced by geography and geomorphology. These early work suggested that towns and cities should be perceived as artefacts whose taphonomy becomes the focus of the study (Rosen, 1986). These studies also established the foundations for the analysis of relations between the development of cities and their surrounding environ-ment (Butzer et al., 1983). Actually, the urban land-scape is the result of the deposition of heterogeneous materials, provided for construction, craft production or consumption and subject to decay and post-depo-sitional transformation, and autogenic or allogenic additions (Borderie, 2011a), irrespective of whether they proceed from purely urban activities, or from interactions with the environment (catchment, river, coastline).

Until the late 1990s, studies mainly dealt with large scales of hydrosystems, due to the collabora-tion between archaeologists and geographers during major urban rescue excavations. These enabled the development of the analysis of relationships between hydrosystems or coasts and towns, notably the study of inputs originating from outside the town, and external hydrological influences (CNAU, 1988; Bravard et al., 1989), which are still studied today (Chaussée et al., 2009). For example London Dark Earth was first thought to be a River Thames ‘flood loam’ (Macphail, 1981). On a much smaller scale, approaches limited to intra-site archaeological operations, including the analysis of Dark Earth, then enabled the establishment or refinement of the understanding of functional interpretations of spaces, both in the Middle East (Matthews, 1992; Matthews et al., 1995, 1997) and in France (Lattes; Cammas, 1994). However, these geoarchaeological approaches remained underused. For instance, only 9 articles dealing with urban contexts have been published by the international journal Geoarchaeology from 1987 to 2010.

In more recent works, the gap between these very fine, or very broad scale studies has been reduced, by combining the study of the stratification processes and the configuration of the urban site (environ-mental conditions, local sedimentary resources, etc.) with its evolution (Schwien et al., 1998; Arlaud, 2000; Deschodt and Sauvage, 2008). The nature of urban soils and deposits and their transformation, forced by activities directly or indirectly perceptible (e.g., pollution), is perceived as a co-construction of socio-environmental dynamics (Heimdhal, 2005; Davidson

et al., 2006; Golding, 2008; Noizet et al., 2011). By the 1980s, the study of Dark Earth was already part of this multi-scale perception of socio-environmental dynamics (Macphail, 1981), because their occurrence in the urban context is, the main evidence for human activities for the 4th to 11th c.

1.2. Dark Earth and the urban stratification

Dark Earth strata are located between those of Antiquity (before the 4th c.) and of the Middle Ages (after the 11th c.). The latter are more easily inter-pretable because they constitute numerous strati-graphic units (SU), resulting from distinct actions such as excavation, construction and spreading... thus forming archaeological features (such as walls, mosaics, cesspits, etc.).

Until the 1980s, the apparent homogeneity of massive Dark Earth SU has led to interpretations of its origin, with suggestions including its formation from urban abandonment, thick embankments or the cultivation of urban areas. These interpretations thus maintained the pervading models of cities portrayed as without active urban life, weakened by the attacks of the barbarians and abandoned by the elites (Galinié, 2010). Since the beginning of the 1990s, the geoarchaeological approach has enable the reconside-ration of these interpretations, and shown how Dark Earth can be derived from a complex combination of different processes, whose results are similar only at a macroscopic scale (Macphail, 1994; Cammas et al., 1998; Devos et al., 2009). Recent work (Cammas, 2004; Nicosia, 2012; Borderie, forthcoming). At a finer scale than the SU, studies show that this Dark Earth SUs can even contain an internal structure, such as gradients or distributions of coarse consti-tuents colours, and sometimes even include smaller structures (such as post holes, fireplaces or interior floors). This internal structure invites us to consider the importance of pedogenic processes in the trans-formation of deposits, which can but may not lead to the homogenisation of the sediment, in a context – the town – where everything is constrained by human activity. Information on the SU-formation processes (filing and transformation), as well as the characteris-tics of the SU, therefore enables us to understand the activities that are operating at the time of their origin, and therefore the functions of urban places in Late Antiquity and the early Middle Ages (Fig. 2).

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2. Methods for the study of Dark Earth

2.1. The topographic approach

Layers of Dark Earth from the 4th to the 11th c. are found in a very large number of towns in the Northwest of Europe (Macphail et al., 2003; Verslype and Brulet, 2004). However, within each town, or from one town to another, this occurrence has not yet been quantified. The establishment of a syste-matic survey is useful in order to register macroscopic characters of urban sediments such as maximum and minimum extension and thickness, colour, apparent stratification, etc. The location of Dark Earth in relation to structural elements of context and space is significant archaeological evidence of the use of space and phasing as these elements maybe contem-porary, younger or older. For example, the proximity of strata to town walls, religious buildings, and urban thoroughfares, alleyways or routeways, assists with the understanding of the use of the spaces, the rela-tionships between society and the urban ground and the organisa-tion of urban spaces , for a p e r i o d where t he use-history of the urban plot is almost unknown.

2.2. The stratigraphic approach

Firstly, in order to gain an overall vision of the organisation of spaces and activities, thick layers of Dark Earth should not be separated from the thinner SUs nor from the archaeological structures they may contain, and in which they are located (pit-houses, trenches, ditches...), or which are adjacent. Indeed, the different dynamics of accumulation and deposi-tion can be understood by comparing contexts (i.e., indoor and outdoor spaces).

Then, excavation by SU and horizontally when the SUs are very thick and difficult to understand, is the only reliable method in order to identify and to record vertical and lateral variations of the sedi-mentary characteristics and the concentrations of macro-components of a diameter greater than a few centimetres (David, 2004). The organisation of the layers can then be observed over several metres distance, including the densities and types of macro-constituents observed (fragments of pottery, tile, mortar, charcoal, slag, stones, etc.), as well as those of the structures themselves such as aggregates and

Fig. 2. Space functionalities, activities and formation processes of Dark Earth. Dark Earth are the result of complex formation processes, linked with environmental conditions (dry, wet, indoor, outdoor...) and, most of all, to activities and land use, as to their action to the soil/sediment. Thus, the functionalities of the spaces in which Dark Earth can be formed are often multiple and they can vary through time.

Tab. 1. Methods used to analyse

Dark Earth.


alignments of components such as postholes and pits (Gébus and Gama, 2004). The arrangements thus identified can sometimes be correlated with topographic features e.g. thoroughfares, buildings, etc. Finer variations can also be perceived through the observation of macro-constituents organisa-tion in sediments and by analysing latent structure (i.e., their spatial distribution in three dimensions). Different phases of input and reworking may well be highlighted (Borderie and Petronille, 2009). Finally, at the finer scale, micro-stratification can be identified and can assist in the identification of the processes of accumulation.

2.3. Characterisation of stratigraphic units

The nature of the inputs contributing to Dark Earth may be informed by the characterisation of the SUs (e.g., charcoal, organic matter, phosphorus, heavy metals). In addition to their extrinsic characteristics, including their stratigraphic position, these characters can be used to identify the intrinsic process of accumulation and transformation. Furthermore, in support of the archaeological description, they allow the comparison of one SU with another by quantitative or semi-quan-titative criteria. Table 1 lists the main methods used to characterise Dark Earth SUs. A more detailed inventory

is given by Borderie (2011, pp. 81-82).

2.4. Micromorphology

Micromorphology has been used to study the Dark Earth since the ea rly 1980 s in Britain (Macphail, 1981;

Fig. 3. Examples of micro-elements in Dark Earth. Dark Earth contains many different micro-elements. Their combination and their state contribute to the identification of processes and activities taking part in the Dark Earth formation. A: Lead droplet embedded in calcitic ash (PPL, late Roman Dark Earth, Vine st. Leicester). B: As A but under oblique incident light showing black lead (centre), ‘red’ lead oxide (edge) and lead carbonate (contaminated ashes; Thilo Rehren, UCL, pers. comm.). C: Vivianite cristal (Iron phosphate) in a bone pore (XPL, early Middle Ages, A. Briand pl. Noyon). D: As C but under PPL. E: Gypsum crystal (PPL, Sainte-Chrétienne Metz). F: Grapes seed (PPL, A. Briand pl. Noyon). G: Burned bone fragment on a thin calcitic ash layer (XPL, Grospiron sq. Noyon). H: As G but in PPL.

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Macphail, 1983; Macphail and Courty, 1985; Courty et al., 1989) and the 1990s in France (Cammas et al., 1995; Gebhardt, 1997).

Micromorphology is the study of the organisation of soil components at the microscopic scale, which is indicative of the dynamics of depositional environ-ments and the evolution of soil profiles (Cammas and Wattez, 2009, p. 186). Undisturbed archaeological sediments and soils are observed from field to micros-cope (Courty and Fedorov, 2002) using the inter-pretative concepts of sedimentary petrography and soil science following soil-micromorphology hand-books (Bullock et al., 1985; Stoops, 2003). Analysis of micro-components, the fine fabric of archaeolo-gical sediments and their relative organisation, can then help us to “understand their formation and the manner in which they enter the archaeological record” (Goldberg and Macphail, 2010, p. 589), including the identification of micro-stratigraphic units (MSUs) or micro-facies types (Courty, 2001; Cammas and Wattez, 2009).

The interdisciplinary approach, combining micro-morphological methods with geophysical, geoche-mical, quantitative and qualitative studies of artefacts, can quickly generate results, which can contribute to

the interpretation of Dark Earth and a more general understanding of the formation process of urban stratification. Moreover, recent works show how the systematisation of these approaches is extremely profitable as it permits us to revisit the interpretations of ‘gardens’ or ‘abandonment’, which have not been established by other studies, and can demonstrate the multiplicity of processes that can potentially produce Dark Earth (waste, housing, animal housing, crafts...). It can therefore, ultimately, renew our perception of urban spaces in the city from the 4th to the 11th c.

3. Results

3.1. Physicochemical and stratigraphic characterisation of the Dark Earth

The identification of Dark Earth is based on the macroscopic observation of its colour, thickness and apparent homogeneity. Yet these characters, as well as physical and chemical characteristics, show strong variations, from one city to another, as well as from one SU to another within the same archaeological excavation.

The thickness of Dark Earth is often accepted as being between 0.4 and 0.8 m (Courty et al., 1989, p. 263; Macphail et al., 2003). However, it is sometimes thicker and it often shows in a systematic manner, at both field and microscopic scales, finer stratifica-tions or gradients of colour, textures, artefacts and concentration features (Verslype and Brulet, 2004). Sometimes, these strata consist of the superposition

Fig. 4. Organic carbon and calcium carbonate content in Dark Earth. The carbonated Dark Earth, on the right, is here

all located in the Paris basin. Dark earth from Great Britain and Belgium are less carbonated. Organic carbon content is higher

than 10 g/kg and can reach 25 g/kg. The strata underlaying the Dark Earth, which are Roman occupational layers

or alluvial substrate, or earthworks, show lower amount of organic carbon.

1: Collège de France; 2: Metz Sainte-Chrétienne; 3: Beauvais Galerie nationale de

la tapisserie; 4: Beauvais Cloître; 5: Beauvais Chapelle; 6: Noyon Evêché; 7: Bayeux Rue

Franche; 8: Metz ZAC Amphithéâtre; 9: Noyon Square Grospiron; 10: London Ragoon st.;

11: London Southwark st.; 12: London Southwark Courage Brewery; 13: London

Southwark Park st.; 14: London Jubilee Hall; 15: Florence Biblioteca; 16: Brussels Rue de

Dinant; 17: Beauvais Galerie nationale de la tapisserie, earthwork; 18: Noyon Cloître

cathédral, earthwork; 19: Metz Sainte-Chrétienne, Roman deposits;

20: Beauvais Chapelle, Roman pit filling; 21: Metz ZAC Amphithéâtre, alluvial

substrate; 22: London Jubilee Hall, clay floor; 23: Florence Biblioteca, alluvial substrate;

24: Brussels Rue de Dinant. After Macphail, 1994; Cammas, 2004; Devos et al., 2009;

Borderie, 2011b; Nicosia et al., 2012; Augry et al., in press; Borderie, in press.


of many SUs, which can be seen with the naked eye (Borderie, 2011b, p. 241-281).

Similarly, the colour of Dark Earth is variable. The strata are most often of dark colour (7.5 YR 5/1 to 10 YR 2/1), due to the large presence of fragments of charcoal and burnt plant remains often observable by eye, and organic matter observable under the micros-cope. However, this colour varies within strata and gray-coloured Dark Earth, towards a more yellow or even green colour, depending on their content of char-coal micro-fragments, iron and phosphate (Macphail, 1994; Borderie, 2011b, p. 311-328; Fig. 3).

The physicochemical characteristics of Dark Earth result from both local pedo-geochemical conditions and inputs related to activities and their transforma-tions. Thus, although Dark Eartha are, in most cases, silty-sands, they are not well sorted and can be much more sandy in some alluvial contexts (see Macphail, 2003, p. 92-93; Heimdhal, 2005; Borderie, 2011b, p. 338-339; Nicosia, 2012, p. 115). Similarly, Dark Earths contain a generally low amount of carbonate in Britain or Belgium (Macphail, 2003, p. 92-93; Devos et al., 2009, p. 273) while they contain more in the Paris Basin (Borderie, 2011b, p. 340-345; Fig. 4). However, a significant proportion of these carbonates can come from the presence of derelict building mate-rials such as mortar or limestone (Macphail, 1994, 2010).

Associations of components in the Dark Earths are, thus, very variable and ref lect the nature of the inputs and the activities that produced them. Components larger than a few centimetres are most of the time altered building materials: stone tools, mortars, tiles, plaster (Guyard, 2003, p. 95; Fondrillon, 2007, p. 434 and 442). Fine compo-nents of sands and silts, are particularly relevant to the diversity of activities during the early formation of Dark Earths (Macphail, 1994, 2010; Borderie, 2011b). These components are partly the result of inputs from building materials (limestone, quartz, ceramic tiles), but they also come from domestic acti-vities (manure, bones) or craft activities using plants (phytoliths) and fire (ash, micro-charcoal fragments), sometimes formed at high temperatures (slag, fused silica, glass). The occurrence of weathered lead frag-ments embedded in ash in late Roman Dark Earth in Leicester, UK is an example of such artisan work (Macphail and Crowther, 2009; Fig. 3 A and B). The condition of these components – burnt, rolled, twisted, broken up – and their organisation within the fine fabric, provide evidence of the use of spaces and the frequency of their occupation.

Dark Earth has significant levels of organic matter. In addition to micro-charcoal fragments and

plant fragments, Dark Earths have high C/N ratios and typically a high phosphorus content (15.5-25.5 g/kg Borderie, 2011b, p. 342-343, with the method proposed by Mikkelsen, 1997). High contents of heavy metals, due to ancient polluting activities and contemporary with the formation of Dark Earths, have been recorded at more than 1800 mg/kg in Metz (Augry et al., in press). Chemical data, without soil micromorphological control needs to be interpreted with caution as post-Dark Earth land use has led to both phosphate (Deansway, Worcester, Anderitum/Pevensey Castle; Macphail, 2004, 2011) and heavy metal (London Guildhall; Macphail et al., 2008) contamination from Saxon and early mediaeval disposal of cess (Fig. 4).

3.2. Formation processes

Most often, Dark Earth is composed of a succes-sion of numerous thin inputs, directly reworked by biological activity, as a cumulative soil (Cammas et al., 1998). The MSUs (micro-stratigraphical units), which can then be observed, result from the combination of the volume and the frequency of the inputs, the inten-sity of the bioturbation, trampling, and the humidity of the environment. Indeed, competition between these processes determines the creation of interfaces and their preservation. When interfaces are not visible to the eye, it is through the organisation of the micro-fabric that the succession of surface and sub -surfaces layers can be identified micrscopically. In all cases, these accumulations are characteristic of progressive inputs, more or less rhythmic, which persist over time. Dark Earth can also result from the inputs made for urban layout (floors, spreading of materials...), exclu-ding massive deposits. The latter, suggests substantial earthwork or dumps and generally constitute only a very small part of the whole Dark Earth. Interior spaces, such as rooms or use-areas are, in most cases, relatively easy to identify in the field and include; floors, occupation deposits, cleaning/maintaining, etc. However, the transformation processes of stra-tification can make them less easily identifiable and interpretable. Thus, at A. Briand in Noyon (France), a succession of inputs was characterised by thin ash deposits, weakly bioturbated, containing small and fragmented constituents. They were overlain by fine plant fragments and sometimes coarse constituents in thin layers with sub-horizontal porosity. The large number of bio-nodules and the weak aggregation suggest a resumption of earthworm activity, which had been interrupted by rapid burial and compaction (Borderie, 2011b, p. 282-295).

The apparent homogeneity of Dark Earth is, above all, due to the simultaneous combination of

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depositional processes and bioturbation, rather than a partial or total restructuring of stratification. The most homogenous units of Dark Earth can result from mechanical processes of reworking, such as tillage (Devos et al., 2009). However, even in these thick units, it is possible to perceive different processes and inputs due to the superimposition of pedo-sedi-mentary features (Borderie, in press). The part played by bioturbation processes in the formation of Dark Earth was demonstrated in the 1980s by Macphail (1981, 1994). In most cases, this bioturbation indi-cated spaces which had been weakly trampled, as the results from similar studies from Paris (Cammas, 2004, p. 49) and Metz (Gébus and Gama, 2004) have shown. However, such bioturbation may also be accompanied by substantial archaeological evidence for the use of these areas, as at Beauvais (Borderie, 2011b, p. 249-254 and 280-281), in Paris and Macon (Cammas, 2004, p. 49 and 53). It may even take place in refuse areas of interior space, as at Beauvais and Noyon (Borderie, 2011b, p. 265-274 and 316-319).

The role of percolation and stagnation of soil water in the formation of Dark Earth has been demonstrated by Macphail (1994). Both can contribute to the dissolu-tion of some constituents, including carbonate, which can also be altered by variation in the acidity of the sedi-ment. The conservation of ash lenses (Fig. 3 G and H), which are particularly prone to dissolution, has also been observed, as at Noyon (Borderie, 2011b, p. 284-295). In this situation preservation can then be explained by the speed of burial by relatively permeable materials such as clayey-sand and particularly compacted strata. Damp environments can also cause the formation of vivianite. These iron phosphate crystals (Fig. 3 C and D) may form in the sub-surface contexts, outdoors, in indoor floors (Borderie, 2011b, p. 337). It is indicative of the amount of phosphate materials in the deposits, usually derived from faeces or bone (Courty et al., 1989, p. 267) and the decomposition of plants (McGowan and Prangnell, 2006). Therefore, the formation of Dark Earth results from a complex combination of the processes of accu-mulation and transformation associated with the use of space and the nature of prevalent materials.

4. Beyond Dark Earth

4.1. From Dark Earth to urban life in the early Middle Ages

Detailed understanding of the formation of Dark Earth is very valuable for informing us about human activities in urban environments with such

information including the sources of the inputs, inout characters and input transformations.

In Dark Earth, information from the nature and transformation of properties can assist us in the inter-pretation of site function because the nature of the strata and functional transformation are very closely intertwined. However, SUs have, moreover, broad rates of formation and, thus, it is rarely possible to give the precise location and intensity of activities which contributed to their formation. Functional interpretative grids that are usually used for the interpretation of archaeological structures and urban spaces (CNAU, 2007) are difficult to adapt for Dark Earths. Indeed, if progressive accumulations of waste contain evidence of domestic and craft activities, but at a microscopic scale, it requires an important interpretative leap to be made as if the data is to be regarded as evidence of spaces dedicated to “craft” or “housing”. The vast majority of these indices are situated in deposits, which are interpreted as being due to repeated dumping. Although the spaces, where this dumping is located, are sometimes interpreted as dumping areas, the studies made on waste disposal in ancient and medieval urban contexts suggest that these spaces can have other functions in addition to the regular reception of waste (Keene, 1982; Bridges, 1991; Leguay, 2003; Bourgeois, 2003; Golding 2008; Macphail 2010).

The superimposition of the accumulation processes and the richness and rhythmic characte-ristic of the refuse origin of Dark Earth can suggest, moreover, important functional changes over time and multi-functional spaces, in waste accumulation in activity areas with diffuse boundaries. The very high content of lead recorded in Metz (Augry et al., in press) shows how intense and polluting artisanal work (but probably requiring light infrastructure), can be detected by the sedimentary composition of the associated Dark Earth.

5. To be continued…

The study of Dark Earth is of major importance in the history of societies from the 4th to the 11th c. in Northwest Europe. Benefiting from recent advances in geoarchaeology applied to urban contexts, the study of Dark Earths has been advanced through the implementation of interdisciplinary approaches. Dark Earth strata are the result of a combination of processes related to environmental conditions and, especially, socio-spatial practices. Even if a compo-nent of some processes comes from outside the city, such as the case of alluvial deposits (Heimdhal, 2005;


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Nicosia et al., 2012), “natural” process are only discer-nible when considered in the context of their social-environmental co-construction.

Dark Earth is not synonymous with abandon-ment, and it is seldom the result of an urban practice of horticulture. Dark Earth strata are mainly the result of gradual accretion, by deposition and trans-formation in situ, of the wastes of urban activities that produce and release various composite materials, especially organic, in spaces which can be dedicated to various functions, and which can be both indoor or outdoor. This particular type of waste disposal and, finally, this particular relationship to the ground, raises the question of the relationship between the use and the status of some spaces, – such as the presence of Dark Earth near the cathedrals at Beauvais, Noyon, Paris, Rouen and Reims, or their absence inside the internal bank of the walls of late Antiquity sites such as at Noyon and Evreux.

Dark Earth is a very rich source of ‘documenta-tion’, which has only recently begun to be extensively

exploited. Extensive spatial and temporal data can be collected if Dark earths are studied systematically. The protocols are now well defined for the analysis and interpretation of formational and diagnostic processes, which can provide valuable information on human activities in their urban environment. A better understanding of the urban contexts from the 4th to the 11th c. can therefore now be gained, through multi-plying the number of observation points, by different form of comparisons, and by the establishment of a repository or archive at the town scale. Greater accu-racy in chronology will lead, moreover, to a better spatio-temporal perception of urban dynamics. Finally, the detailed analysis of certain components and characteristics of Dark Earth, such as organic matter, concentrations of heavy metals and micro-charcoal fragments, is proving particularly promising. This information, stored in Dark Earth, is the imprint of the life-ways and life-conditions of early mediaeval urban societies.

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Quentin Borderie

University of Panthéon-Sorbonne (Paris 1), ARSCAN Laboratory – UMR 7041 CNRS/Universities of Paris 1 and Paris 10/Ministry of Culture and Communication,

Nanterre, France

Yannick Devos

Université Libre de Bruxelles, Centre for Archaeological Research, Brussels, Belgium.

Cristiano Nicosia

Geoarchaeology and Soil Micromorphological

Consultant, Vicenza, Italy.

Cécilia Cammas

National Institute for Preventive Archaeological

Research (INRAP Centre and Île-de-France),

Laboratory Archaeology of Mediterranean Societies –

UMR 5140 CNRS/University of Montpellier 3/Ministry of

Culture and Communication/INRAP, Lattes, France.

Richard I. Macphail

University College London, Institute of Archaeology,

London, UK.