The Archaeological Potential of Secondary Contexts

Module 1 - Implications for the Archaeological Record

Three questions were posed at the beginning of this interim report and are reviewed here in terms of the archaeological record and the potential understanding of the Palaeolithic material record:

1. Duration of fluvial depositional events
It is proposed that the deposition (through the redistribution of the existing channel sediments and the supply of new materials) of coarse and fine-grained sediments occurs relatively rapidly, over hundreds or a few thousand years. The duration of these depositional events is estimated from overarching fluvial models of specific systems (Cleveringa, De Gans et al. 1988; Schirmer 1988; Schirmer 1995) and general principles (Vandenberghe 1993; Vandenberghe 1995; Bridgland 2000; Maddy, Bridgland et al. 2001; Vandenberghe 2002), rather than on the hypothetical duration of individual sedimentary deposits (e.g. sand, gravels, clays and silts). It is acknowledged that certain types of deposits (e.g. matrix-supported fine gravels) can be suggestive of extremely rapid events occurring over ecological timescales of hours and days. However, it is clear that the impact of local conditions upon fluvial events is so varied (and important) that any attempt to attach timescales to deposit types would be futile.

Critical to the understanding of any archaeological material incorporated within fluvial deposits is the observation that artefacts occurring within a single homogenous deposit (as geoarchaeologically classified) may be separated by hundreds of years and that behavioural homogeneity cannot therefore be assumed. Of potentially equally important significance to the understanding of the archaeological material resource and hominid behaviour is the probability that the deposition of the sediments occurred at and shortly after (depending upon system lag effects) significant climatic shifts and transitions. This issue is further complicated by the potential of the archaeological material to be considerably older than the geological deposits - a problem that will be explored in modules 4-6 (project phase 2).

2. Dormancy in the fluvial record
It is emphasised that there is an important distinction between genuine fluvial dormancy (which may rarely if ever occur) and fluvial activity of such a low magnitude that no long-term physical traces are preserved in the Middle Pleistocene sedimentary record (e.g. the impacts of annual interglacial flooding). Intervals of c. 5-10,000 years are proposed to occur between periods of major fluvial activity (associated with glacial/interglacial and stadial/interstadial transition events). There are a series of key deposits and horizons which may be indicative of these ‘dormant’ intervals, including buried soils and palaeo-landsurfaces, ice wedges and cracks, cryoturbation, solifluction deposits, and weathered deposit surfaces. It is however effectively impossible to estimate the duration of a dormant interval from any of these features. The presence of archaeological materials above and below such features would be indicative of major temporal separation between the respective depositional ages of the artefacts (although they could of course have been contemporary in their ages of manufacture and/or used and/or discarded. Nonetheless, it is possible that all of these deposits, horizons and erosion surfaces may be associated with the short-term oscillations in fluvial regimes that occur within the major climatic fluctuations/ episodes of fluvial activity. In the absence of dated deposits above and below (a common occurrence), the horizontal extent of the features may be indicative of the duration of the ‘dormant’ interval and assist in distinguishing between the two alternative interpretations outlined above.

3. The glacial-interglacial cycle
An episodic model is proposed, with periods of major fluvial activity during periods of climatic transition, separated by longer periods of relative system dormancy. The proportion of glacial-interglacial cycles represented within river terrace sequences is difficult to estimate, partially due to the relatively low resolution geochronological record of sub-glacial/interglacial cycle climate fluctuations during the Middle Pleistocene. Nonetheless, it is suggested that no more than 25% of the cycle is represented within a terrace sedimentary sequence. This value is estimated from the presence of 21 interstadial events over the last 80,000 years, indicating 42 climatic transitions of estimated duration 500 years each, following Van Huissteden’s (1990) age estimates for the Hasselo Stadial (c. 41-38.5 kyr BP) and Hengelo Interstadial (c. 38.5-37 kyr BP). 25% is probably an over-estimation, since it is unlikely that every climatic transition is reflected in the sedimentary sequence, due to the impact of local river threshold conditions and the subsequent erosion of small-scale and/or fragile deposits. Van Huissteden, Gibbard et al. (2001: 86) have argued that the frequent climatic variations recorded in the ice-core record for stage 3 do not tend to produce a response in the river systems of north-western Europe. Moreover, the full sedimentary sequence is extremely unlikely to be represented in a single location on a terrace, as a result of localised erosion caused by fluvial migration and channel cutting. Indeed, deposits of different ages will occur at the same altitudes within terrace sequences because of the complexities of the cutting and filling episodes associated with fluvial migration and flooding events. Overall, the sedimentary and archaeological materials occurring in river terrace sequences was probably deposited at a series of episodic intervals during the glacial-interglacial cycle associated with the formation of the terrace. Current dating techniques will enable this working hypothesis to be tested (module 2). The second key to understanding the archaeological material however, stems from the recognition that the age of the material’s manufacture, use and discard may be considerably older than the age of the deposits from which it is ultimately recovered. Addressing these issues forms the core of the project’s second phase.

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