The goals of the module concerned the reconstruction of the Pleistocene environment of the River Axe valley in the vicinity of the Broom locality, with specific reference to:

1. Depositional conditions
Due to the paucity of diagnostic bedforms and fluvial architecture in the Broom sediments, it is difficult to reconstruct the fluvial palaeo-landscape in great detail. The major sedimentary trend through time (coarse-grained gravels > fine-grained sediments > coarse-grained gravels) suggests a broad cycle of cold-climate river regime (multi-channel? braided?), changing to a warm-climate regime (single-channel? meandering?) before returning to a cold-climate pattern. The structure of the fine-grained sediments deposited within coarse-grained sequences (e.g. in section 9 and section 1,) suggests channels with high width-depth ratios, indicative of braided rather than meandering systems. However, the possible impacts of high energy erosion of fine-grained sediments must be kept in mind when interpreting those deposits. The laminated, fine-grained sediments in section 14 probably indicate sediment accumulation in still water conditions (e.g. abandoned channels). The fine-grained sediments in the Railway Ballast Pit (sections 2, 3 and 5) have been previously interpreted as floodplain overbank deposits (Scourse unpub. man.), and the current work has produced no evidence to reject this interpretation.

The sequence provided extensive evidence of short-term (as well as long-term) variation in the fluvial regimes, notably illustrated by the occurrence of fine-grained sediments within coarse-grained gravel deposits (e.g. section 1, section 9, section 10 and section 13). It is suggested that the Broom rivers were responding to relatively brief, sub-MIS climatic oscillations (as are increasingly being revealed in the global climatic record). These responses were recorded in the Broom sequence by major shifts between fine and coarse-grained sediment and minor variations in the grain size, sorting and (where present) bedding of the sediments.

A cautionary note is sound however, with respect to the small exposures investigated and the potential for apparently temporal (vertical) variation to be a result of migrating river channels and the exposure of a different sub-environment of the floodplain.

The clast fabric data suggest (in contrast to previous suggestions from Green (pers. comm.) that the major source of the fluvial sediments at Broom was the River Axe, for both the upper and lower gravels (and probably the middle beds as well). Nonetheless, the River Blackwater was a significant east bank tributary, and appears to have confluenced with the River Axe in the area of Pratt’s New Pit, during the period when the upper gravel sediments were accumulating. It is not yet clear whether the position of the Blackwater and its sedimentary contribution was the same during the accumulation of the Lower Gravel, as these are not currently exposed in Pratt’s New Pit.

2. Palaeo-environmental conditions
The work of Scaife (unpub. man.) and Scourse (unpub. man.) has provided good evidence for the Broom palaeo-environment. There was a general consensus, favouring a boreal environment with pine woodland and small amounts of spruce, fir and birch. The dominance of grasses was noted (although they were of greater significance in the Scaife samples), and patchy woodland coverage was proposed. Scourse favoured a model of open country dominated by heath on the higher ground, while Scaife argued for floodplain grassland with scattered woods.

The initial OSL dates (Section 5.3 below) do not support Scourse’s conclusion of deposition the end of a Middle Pleistocene interglacial, although his alternative model (interstadial age) and Scaife’s argument for an early interglacial age are both feasible on the current dates. The second stage dates and continuing work by Scaife should provide further resolution of both the palaeo-environmental conditions and the most probable age (interstadial or early interglacial).

3. Age of the Broom sequence
The initial OSL dates suggest a mid-late MIS-8 age for the middle beds and upper gravels in the Broom sequence. The age of the lower gravels remains uncertain, although the likeliest estimate is probably early-mid stage 8 (or perhaps late stage 9 at the earliest). There is some stratigraphical confusion in the initial dates (most notably the 273±22 kyr age from the section 9 fine-grained sediments), although this is perhaps unsurprising given the error bands on the dates and the potential for localised cut and fill of fluvial sediments (in other words, the relative altitude of sediments in different localities are not necessarily indicative of stratigraphic order). Nonetheless, it is hoped that the second series of OSL ages will clarify the existing discrepancies.

Given the error bands on the dates, only a very preliminary attempt was made to relate the individual sedimentary units to points on the global climatic record as represented by the marine isotope curve and the ice cores (which also have built in error ranges). Interestingly, there is evidence for rapid climate change at c. 270 kya and 250 kya, suggesting that there may be links between sub-MIS climatic oscillations/warm-cold transitions and terrestrial fluvial activity.

4. Duration of the fluvial events
Estimating the duration of the fluvial events (deposition, erosion and quiescence) represented in the Broom sedimentary sequence has been extremely difficult. Sedimentation rates have not been established (although the second series of OSL dates will provide a larger sample set for the fine-grained sediments in sections 2, 9 and 13). The potential correlation of fine-grained sediments with periods of climatic transition (and much of the extant literature – see module 1) suggests relatively rapid processes, but this line of argument can be taken no further at the present. The second series OSL dates from sections 9 and 13 will provide terminus ante quem and terminus post quem ages for the coarse-grained gravel sediments lying between the respective fine-grained sands, although it is probable that the time interval will incorporate considerable periods of quiescence alongside fluvial activity.

The evidence of multiple landsurface development throughout the sequence (indicated by iron/manganese horizons) suggests some significant hiatus and periods of relative stability, although the lack of weathering evidence, cryoturbation features and cold-climate indicators such as ice wedge clasts (in what was a predominantly cold-climate environment), would suggest that the breaks in fluvial activity were not of considerable length.

Implications for archaeological material

Module 2 Outline

Module 2 Results

Module 2 Interm Report

References

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