Reconstructing Archaeological Sites

Understanding the Geoarchaeological Matrix

Panagiotis Karkanas, Paul Goldberg

• Publisher: Wiley-Blackwell
• Year: 2018
• ISBN:
  9781119016410
• Edition: 1
• Topic: Ciencias físicas
• Subtopic: Geología y ciencias de la Tierra

A guide to the systematic understanding of the geoarchaeological matrix

Reconstructing Archaeological Sites offers an important text that puts the focus on basic theoretical and practical aspects of depositional processes in an archaeological site. It contains an in-depth discussion on the role of stratigraphy that helps determine how deposits are organised in time and space. The authors — two experts in the field — include the information needed to help recognise depositional systems, processes and stratigraphic units that aid in the interpreting the stratigraphy and deposits of a site in the field.

The book is filled with practical tools, numerous illustrative examples, drawings and photos as well as compelling descriptions that help visualise depositional processes and clarify how these build the stratigraphy of a site. Based on the authors' years of experience, the book offers a holistic approach to the study of archaeological deposits that spans the broad fundamental aspects to the smallest details. This important guide:

• Offers information and principles for interpreting natural and anthropogenic sediments and physical processes in sites
• Provides a framework for reconstructing the history of a deposit and the site
• Outlines the fundamental principles of site formation processes
• Explores common misconceptions about what constitutes a deposit
• Presents a different approach for investigating archaeological stratigraphy based on sedimentary principles

Written for archaeologists and geoarchaeologists at all levels of expertise as well as senior level researchers, Reconstructing Archaeological Sites offers a guide to the theory and practice of how stratigraphy is produced and how deposits can be organised in time and space.

1
Principles of Site‐formation or Depositional Processes

1.1 The Concept of the Deposit

In its broadest definition, an archaeological deposit is what encloses the archaeological finds and, as a result, the finds constitute an inseparable part of the deposit (cf. Stein, 1987). In other words, the archaeological deposit is the material that is excavated in order to ‘reveal’ the archaeology of a site. Deposit is a broader term than sediment: it constitutes more of a general and generic use. For example, ‘glacial deposits’ refers to a general class of sediments deposited in a glacial environment. However, archaeological sites contain only sedimentary deposits with very few exceptions like volcanic deposits. In the vast majority of cases, all archaeological objects were deposited penecontemporaneously (essentially at the same time as far as we can resolve) with the enclosing material; of course the degree of penecontemporaneity depends on temporal resolution, which is obviously greater for younger archaeological sites and less so for prehistoric ones dating to, say, one million years ago. Indeed, the separation of an archaeological object from its excavated matrix is artificial (and virtually impracticable to do). Understanding the deposits as part of the archaeological record has changed throughout time, disciplines, and regions. In many sites, excavated deposits are water sieved for recovering charcoal and macrobotanical remains, but this has not been always the case, and at many sites they do not even sieve the deposits to collect microarchaeological debris (e.g. lithic debitage, microfauna, beads) let alone botanical remains. To accept that the materials of archaeological interest are mostly deposited penecontemporaneously with their enclosing deposits is an essential tenet in interpreting formation processes.

This is not to deny that archaeological objects could not be deposited first and then covered with sediments, but we must realize that it is fruitless to treat objects and deposits separately from the excavation point of view and later in their interpretation phase. However, it should be noted that erected constructions (e.g. walls, bridges, dams, etc.) do not follow the above rule, and enclosing deposits are not always contemporaneous. Indeed, buildings are always filled with deposits after their construction, although some of them accumulated penecontemporaneously in the sense defined above (e.g. the first floor or occupational debris within the building). As we have already discussed in the Introduction, architectural features have an internal stratigraphy defined by the typology of masonry and other architectural features, and they have to be included within the depositional stratigraphy in order to be correlated with the surrounding archaeological objects.

To illustrate the above statements we will describe a rather rare example, but one that is clear and hopefully gets the point across. During a volcanic eruption the pottery left on the surface of a floor in a house was subsequently covered by volcanic tephra (the so‐called Pompeii premise) (Schiffer, 1985; Binford, 1981). In order to expose the pottery we dig through the naturally deposited tephra. Obviously, the pottery was left undisturbed intact and covered by the tephra. The study of the pottery confirms that this is an assemblage left in place. Indeed, starting from a sedimentological (depositional) point of view, we might recognize that the material that covers the pottery is a natural free‐fall volcanic tephra that covered the pottery left in the house without disturbing it. Free‐fall tephra is volcanic material ejected into the air during an eruption, which then settles by gravity on the ground.

Although the above description appears obvious for most archaeological sites buried by volcanic tephra, we should also consider other possibilities in which, for example, the tephra is not a free‐fall deposit. As Sigurdsson et al. (1982) describe very impressively, the AD 79 Vesuvius eruption produced individual sequences of different volcanic deposits in each of the cities of Pompeii, Herculaneum, and Oplontis. Each sequence points to different taphonomic histories of the archaeological material, which is far from the simplistic idea we have for this type of phenomenon; note that taphonomy is concerned with all processes happening to an organism after its death. In Pompeii, for example, all excellently preserved human bodies are found inside a second volcanic deposit in the form of a pyroclastic surge, which stratigraphically lies above an initial free‐fall tephra deposit. Pyroclastic surges and flows are fast‐moving currents of superheated tephra, the former being more dilute than the latter and therefore they move faster and generate surges. The first thick free‐fall tephra deposit was not responsible for the deaths in Pompeii, and in fact it forced the major part of the population to flee the city before the main disaster struck the following day. It produced only roof collapse in most houses. The second phase of the Vesuvius eruption first produced a very fast‐moving hot and thin pyroclastic surge that probably killed by asphyxiation those who remained in the city; then a second stage, a hot, dense, and fine‐grained pyroclastic flow, buried the human bodies. It is this last powdery material that preserved the fine details exhibited in the many human plaster casts that were obtained during the excavation of Pompeii (Sigurdsson et al., 1982). The details of the taphonomy of the archaeological materials could not have been revealed without the study of the burying volcanic deposits.

An even more complicated case involves Herculaneum, where ample evidence shows that tephra‐rich mudflows (lahars) were also encountered in the final stages of the eruption. This type of deposit is often observed immediately after volcanic eruptions when eruption‐induced rainstorms remobilize tephra and other material, and deposit them downhill as thick slurries. Although not reported in this case, there is a hypothetical possibility that archaeological material was deposited by such mudflows, which, for instance, resulted in the mixing of objects from a room and an adjacent yard. Depending on the local depositional processes in some cases, whole parts of the original archaeological assemblage could have been moved en masse by dense mudflows. Analysis of the pottery alone, for instance, would not reveal such subtle mixing processes.

The reason for presenting the above examples is to stress that it would be naïve to think that we can understand the patterning of the artefacts and features without understanding the sedimentary matrix that contains them. It is that simple. These sediments have accumulated by a certain process, and it is this process that explains the patterning of the artefacts. Even in cases where fragments of artefacts can be put back together, making one think that therefore they come from a primary context, it is the surrounding matrix that will lead us to confirm or reject this interpretation. There is no doubt that a separate analysis of the archaeological finds and the sediments would be mutually beneficial. However, within the framework of excavation both the finds and the deposits have to be examined together in order to interpret correctly the archaeological assemblage. In other words, it is the deposit as a whole that contains the archaeological finds, which will ultimately define the context.

The Pompeii example obviously is the exception. Most of the time, archaeological objects are affected by complex combinations of natural processes and therefore are contemporaneously deposited with the enclosing sediment or incorporated within the sediments by syn‐depositional anthropogenic processes such as dumping, trampling, or construction of earthen structures (e.g. floors, walls, mounds). The archaeological objects, together with all other materials that are not of direct archaeological interest, are parts of the organization of the sediments. The way they have been incorporated inside the sediment is reflected in the fabric of the deposit, which is the three‐dimensional arrangement of the constituents and their size, shape, and form (Stoops, 2003).

All these attributes inform us about the dynamics of deposition of the materials by natural forces. We have to consider that there are only a limited number of basic human actions that occur, such as laying down, dropping, compacting, and throwing materials. Therefore, compacting, kneading, and applying, sweeping and raking, trampling, dumping, backfilling, and levelling are fundamental actions that together with burning and animal husbandry activities form the majority of archaeological sediments (see Chapter 3). It is the combinations of these actions that lead us to interpretations of overall human activity.

Likewise, in the realm of the excavation, the possibility that the archaeological objects are buried at a later time by sedimentary deposits is meaningless, with very few exceptions. By definition, the archaeological finds are recovered by digging the deposits. If we accept that there is no way of interpreting correctly the archaeology of a site without understanding its stratigraphy (see Chapter 4) and context, then the smallest depositional unit recognized in the site is what defines the smallest informational unit, its time resolution, and its archaeological/anthropological significance (e.g. Goldberg et al., 2009a). When objects are referred to their excavated unit in order to define their stratigraphic position they cannot be treated separately from their matrix.

Nevertheless, in the case of a dumped accumulation of only bones (see section 5.2.1) or the construction of a wall, the archaeological objects can be recovered as a discrete continuous body, devoid of any sedimentary matrix. In this instance, the objects can have an identifiable discrete stratigraphic position on their own without being referenced to their enclosing sediment. Such features are usually walls and similar architectural constructions, and this is the reason why, during excavation, they are commonly treated as part of a different stratigraphy, that of the architectural or construction phases. In all other cases, archaeological objects are parts of the sedimentary deposits and at a first stage cannot be studied separately. Of course in a later stage individual archaeological objects can be studied on their own or combined into groups and assemblages, but their original context is defined in the excavation and in relation to the depositional unit with which it was associated.

In sum, archaeological objects cannot practically be separated from the deposits in which they are included, and in the majority of the cases they are part of the fabric of the deposits that characterize the way they accumulated in the site. In cases where the archaeological objects can be treated separately from their deposits, they already constitute a discrete field of archaeological study (e.g. architectural phases) and normally they are not excavated or they are removed at a much later time after being documented. Other rare exceptions are artefact lags (Figure 1.1), which are concentrations of archaeological objects that are left behind after their sedimentary matrix has been winnowed out by wind or rainwash in so‐called ‘deflated’ deposits, for example (see section 2.7.1). In this case the assemblage is the direct product of a natural process of concentration, and therefore the objects by themselves make up the deposit. The same could be said when anthropogenic activities have led to a single concentration of objects in discrete, identifiable excavation bodies (e.g. shell middens) (Figure 1.2), and as a result constitute a single layer. Even with shells in middens, their orie...