The review of Holocene and Late Glacial fluvial systems offers a number of potential contributions to the understanding of Middle Pleistocene fluvial sequences:

1. A chronological resolution of 100s (minimum) to 2-5,000 (maximum) years can be applied to the duration of the fluvial activity episodes (incision, erosion and depositional events). Activity is not assumed to occur throughout these periods, and these time-spans are a partial reflection of current dating accuracy and precision. Coarse-grained sedimentary deposits are argued to accumulate the most rapidly and infrequently (reflecting the need for peak discharges), while fine-grained sediments may be deposited more gradually over longer time periods.

2. A maximum chronological resolution of 5-10,000 years can be applied to the periods of dormancy (indicated by soil development, ice cracks, weathering, cryoturbation, and solifluction deposits) between episodes of visible fluvial activity. This emphasis is important since it is acknowledged that fluvial activity may be occurring during these periods (e.g. in response to annual flooding), but that the sedimentary products of this activity are not preserved (or are not currently detectable by geochronological methods) in Middle Pleistocene fluvial sequences.

3. Different types of fluvial activity occur in the same system at the same time, most commonly represented by the redistribution of coarse-grained sediment from one area (erosion) to another (aggradation).

4. Complex sequences of cutting and filling on river floodplains result in an essentially ‘random’ vertical and horizontal distribution of deposits. Consequently, two deposits exposed in laterally separated sections but lying at the same altitudes may be of two different chronological ages.

5. Discharges from glacial and ice-dammed lakes at the glacial/interglacial transitions were of potentially massive impact, providing the potential for major fluvial activity episodes of incision and aggradation.

At the same time, this review has raised a key question with respect to the applicability of Late Glacial and Holocene fluvial models for the understanding of Middle Pleistocene fluvial sequences. The fundamental issues are twofold:

1. What are the impacts of changing Holocene land-use on fluvial activity, specifically the balance between incision and aggradation, and rates of sedimentation? There is widespread evidence that agriculture has led to an increase in fine-grained sedimentation rates (due to changes in land drainage, vegetation development and coverage and soil cohesion). Nonetheless, there is also extensive evidence from the pre-agricultural Late Glacial that indicates broadly consistent patterns in the processes of erosion and aggradation, channel incision, fluvial dormancy and the relationships between fluvial activity and climatic transitions. Overall, it is argued that while anthropogenic activity has altered the detail of Holocene fluvial activity, the overall patterns of river development have remained consistent from the pre-agricultural era.

2. Is the Late Pleistocene and Holocene climatic cycle (oxygen isotope stages 5d-1) comparable to the earlier climatic cycles of the Middle Pleistocene, particularly with respect to the magnitude and frequency of stadial and interstadial events? This is currently difficult to answer, particularly in light of the respective levels of geochronological resolution that are currently achievable for the two periods. 21 interstadial events have been identified over the last 80,000 years, in contrast to the far coarser event resolution for stages 7 and 6 (Gamble 1999: Figures 5.2 & 5.4). Nonetheless, it is clear that a broadly similar pattern is evident for stages 5-2 and 7-6, with short climatic oscillations occurring (stadials and interstadials), characterised by relatively rapid climatic transitions. Indeed, Van Huissteden, Gibbard et al. (2001: 75) have argued that stage 3 can be considered to represent ‘average’ glacial conditions, despite its strong climatic variability characterised by rapid climatic oscillations. They suggest that the variability may be an intrinsic property of such average glacial conditions, with glacial and extreme glacial conditions characterised by far less variable climates. In view of the working hypothesis and model presented above (episodic fluvial activity in response to climatic transitions, separated by millennia of relative dormancy), it is argued here that the Late Pleistocene patterns and models can be applied to the Middle Pleistocene fluvial sedimentary record.

Implications for archaeological material

Module 1 Outline

Module 1 Interm Report

References

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