Archaeological Stratigraphy


By definition archaeological stratigraphy is the study of stratification within the archaeological record. All archaeological sites are stratified. The study, during an excavation, includes physical deposits and other stratigraphic occurrences, e.g., post-holes and in-filled pits. These depositional events revealed by strata comprise the site through time. Therefore, if the excavation of a site is possible, then it will be found to be stratified.

This essay will outline the laws of archaeological stratigraphy which are: (1) the Law of Superposition; (2) the Law of Original Horizontaiity; (3) the Law of Original Continuity; and (4) the Law of Stratigraphical Succession. These laws will then be related to the use of the Harris Matrix for excavations. The whole will be seen against the background that recognises archaeological stratigraphy as the unintended outcome of human activity. The record of these past activities can only be established by excavation, and where the Harris Matrix records the structure of a site destroyed by excavation.

Archaeological Stratigraphy

The laws of archaeological stratigraphy apply to all archaeological sites. These laws evolved from those used in geological practice. There are four basic laws of archaeological stratigraphy of which the first three were adapted from geology. The first three laws are concerned with the physical aspects of accumulated strata or depositional layers. These laws permit archaeologists to work the stratigraphic relationships existing on an excavation site. During the 1950’s it was Sir Mortimer Wheeler who stressed the importance of stratigraphy for archaeological excavations.

The stratigraphic Law of Superpositionassumes that strata andfeatures found in them are in a similar position as to when they were first deposited. Therefore, with a series of layers and their interfacial features, the upper layers are younger and the lower older. The establishment of superpositional relations is important for excavation and archaeological stratigraphy. In addition it is important when analysing sequences of deposition, to determine interfaces between strata. These interfaces, or interfacial units, are abstract units or layers and related to the strata they lie above. Therefore, when amassing data in order to establish the stratigraphic sequence, it is essential to record the superpositional relationshipsof units and layers.

The Law of Original Horizontalityassumes, during the formation of strata, that they will tend towards the horizontal. The position of a layer or deposit depends on natural forces such as gravity. Archaeological layers are unconsolidated when laid down and thus have a natural tendency towards a horizontal position, as well asaccommodating to the position of pre-existing layers or contours. This law has to take account of dry land conditions as well as limitations imposed by people, e.g., man-made ditches. For example, the progressive in-filling of ditches will gradually tend to the horizontal. For archaeologists this provides a useful guide to identify significant interfacial evidence.

The Law of Original Continuityis based on extent, topographically, of a deposit or aninterfacial feature. It will be seen that a basin of deposition will function as a boundary to an archaeological deposit. This then demarcates the original laying down or creation of the layer or an interfacial feature. Therefore this allowsstratigraphic correlations to be made when analysing separated sections of an original deposit.

Axiomatic is the fourth law which is the Law of Stratigraphic Succession. Stratigraphic sequences that are multi-linear are a feature of most excavation sites. This is caused by the extent of the strata themselves, the existence of any upstanding strata or features, and other interfacial evidence. In essence the law statesthat any stratification unit assumes its place in the site stratification sequence, depending on its position between the earliest (under-most) and the latest (upper­most) of all units considered together.

The Harris Matrix and Stratigraphic Sequences

When constructing the stratigraphic sequence the Law of Superposition is of paramount importance. It is this law that provides a body of stratification with its chronological direction. The question thus posed about strata is which came first? And consequently `…the units of stratification can be placed in sequential order in relative time, one after another.’ (Hams, in Fagan, 1996). During site excavation the Harris Matrix provides archaeology with a method, one which enables stratification sequences to be, using simple terms, displayed as a diagram.

The Harris Matrix comprises a grid, on paper, of rectangular boxes. Its format is designed to show the stratigraphic relationships found on site. The matrix is the resulting diagram and represents the excavated stratigraphic sequence. The stratigraphic sequence is therefore `…the order of deposition of layers and the creation of feature interfaces through the course of time.’ (Harris, in Fagan, 1996). This order is interpreted using the first three laws of archaeological stratigraphy. However, the translation of these relationships that are uncovered are achieved using the fourth law, that of stratigraphical succession. The matrix sequential diagram thus builds up on the paper grid as the excavation progresses. The placing of layers and other features in sequential order thus becomes the main objective of the matrix.

Excavated strata cannot be dated realistically without examining the remains found in the deposits. The dating of strata and artefacts can be inferred once the stratigraphic sequence has been determined. From this cart be calculated the date of origin of an artefact, its main period of use, and then the date or time of its deposition. Three questions can thus be posed for the time an artefact has lain in a stratum. Is it indigenous?Has it been infiltrated?Or is it residual?The aim of analysing found artefacts is to date found layers and interfaces, based on the stratigraphic law of superposition of strata. By way of this method the aim is to relate stratigraphy to chronology.

The Harris Matrix is therefore a method of straigraphic recording, of an excavation site, where each deposit is allocated a number. Each deposit can be seen as a time capsuleor unique indicator of evidence encompassing cultural, environmental, and chronological data. The stratigraphic analysis gives a relative scale, a stratigraphic time that is established during excavation and recording. The study of found artefacts gives an absolute date or calendar time. For the Harris Matrix stratification presents as a three-dimensional body of deposits and features `…from which a fourth dimension of relative time can be inferred…’ (Harris, in Fagan, 1996), thus the stratigraphic sequence, the order in relative time, of thedeposition of layers and the creation of interfacial features. (Harris, in Fagan, 1996). This, via the Harris Matrix, is translated into an abstracted diagram.In this way the Harris Matrix system can readily cope with deeply stratified excavations, an example would be York, and other rescue operations within urban environments. The Harris Matrix did not deny the validity of the stratigraphic ideas of Sir Mortimer Wheeler. The Harris Matrix systematised these ideas by referring to units of stratification rather than layers or strata, especially with stress upon interfacial features.

Summary and Conclusions

Archaeological excavation sites vary according to soil features and material culture content, depth and extent of stratified deposits. Stratigraphic sequences also show varying examples on non-historical objects. Firstly, indigenous remains are those from the time of layer formation, which implies objects and the layer are contemporaneous. Secondly, residual remains are objects made during a time a time prior to the formation of the layer. This suggests such objects were present in earlier layers but were dug up or disturbed subsequently. Thirdly, infiltrated remains arethose created at a later date than the layer formation. Archaeological stratification is therefore a layering of deposits due to human activity, whereas archaeological stratigraphy is the chronological and sequential relationships between the deposits, strata, and associated interfaces.

Using the Harris Matrix helps to determine the cultural history of a site by recording the sequences in a diagram. The essence of the Harris Matrix system is the placing of each unit (on the diagram) in its stratigraphic place in relation to features above and below. A Harris Matrix diagram therefore illustrates and encapsulates the archaeological site record within one diagram.

Finally, artefactual analysis has to contextualised during and after an excavation. It is not simply placing an artefact or object in a stratigraphic sequence. The describing, drawing, and handling an artefact from a stratum may avoid the making of interpretations. This process may miss important details and overlook that beyond individual artefacts there is a context. So, stratigraphic sequences and theHarris Matrix system enable questions to be asked — was the artefact found in a well, a grave, a ditch? In addition — how did it end up there and why? What other artefacts were associated with it? Were associations ritual, mundane and everyday, or with,‘ richer finds? Unless artefacts are placed in the archaeological record in this way the artefact will become decontextualised. Taking an artefact out of its topographical, geographical, social and historical context leaves it orphaned. If it is lifted out of time it is disembodied with its physical circumstances denied. In this sense the Harris Matrix is a means whereby artefacts and sites remain dynamic rather than become passive objects and open to all manner of inaccurate personal interpretations.

Essay contribution to University of Oxford Undergraduate Certificate in Archaeology (2003).

References and Sources

Green, K. (2002). Archaeology, An Introduction. UCL Press, London.

Harris, C. H. (1996). Stratigraphy. In Fagan, B. M. (ed). The Oxford Companion to Archaeology. OUP, New York.

Renfrew, C. & Balm, P. (2001). Archaeology. Theories, methods, practices. Thames & Hudson, London.

Roskams, S. (2001). Excavation. CUP, Cambridge.


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