Human Origins Research Group

Broom Excavations: Geology & Palaeoenvironment

Shakesby & Stephens (1984) concluded that the likely depositional regime at Broom was one of braided streams under cold climate conditions. It is also suggested that the deposits are at least partly fluvio-glacial, deriving from an ice margin somewhere to the north of the Chard Gap (Wessex Archaeology 1993: 166). It has also been suggested that the deposits were largely derived from the remnant gravel patches of pre-Pleistocene age on the interfluves, and from the chert Head deposits on the slopes. Both agencies indicate a starkly different fluviatile regime during the Middle Pleistocene.

Stephens (1974) has emphasised the role of the Chard Gap (90m OD, compared to the local interfluves at 230-290m OD) in the origin of the Axe gravels. He arguues that a pro-glacial lake may have existed in the Briston Channel-Severn Valley as a result of ice blocking the western end of the Channel, and that the lake may have discharged southwards through the Gap. This event would wash masses of rock debris into the Axe Valley, accounting for the thick gravel deposits and their absence along the Axe Valley east of Chard. This model follows work as far back as Maw in 1864 in its view of a Bristol Channel which was once blocked with ice. Green (1974) however has challenged this interpretation, pointing out the total absence of the erratics which should have been discharged by the meltwaters of a glacier to the north.

The pollen assemblage in the clays and silts suggested a regional vegetation of boreal forest dominated by pine and birch. The presence of silver birch indicated that the clays and silts might represent the end of the Hoxnian interglacial or an interstadial within the Wolstonian Stage. The sediments did not produce mammalian fauna or molluscs (Wessex Archaeology 1993: 166-167).

The Current Excavations

The 2001 excavations in the Railway Ballast Pit extended sections 3 and 5 further into the 'lower' gravels and confirmed the tripartite sequence. Most interestingly, the 'lower' gravels in both sections (Figures 5 and 6) were characterised by repeated colour variations (broadly characterised as 'grey' and 'orange' gravel units). The 'grey' gravel was consistently characterised by larger clasts, although preliminary analysis of clast orientation and dip has suggested that both the 'orange' and 'grey' gravels were deposited by a north-south flowing River Axe. Section 5 (Figure 6) also contained distinctive layers of manganese and iron staining (Figure 7).

Excavations in Pratt's New Pit (to the south of Holditch Lane) in 2001 complicated matters with respect to the source of the 'upper' gravels at the Broom locality. The orientation of a fine-grained channel feature in section 9 (Figure 8) suggested deposition by a north-south flowing system, as did the orientation of the clasts in the gravel deposit lying immediately below the channel feature.

Continued excavation in Pratt's New Pit during 2002 revealed the presence of significant sand units within the upper gravels, in sections 10 (Figure 9) and 13 (Figure 10). Excavations in Pratt's Old Pit (to the north of Holditch Lane) were hampered by the extensive backfilling of the area (after the commercial excavation of the gravels during the 1930's), and the sediments uncovered were not considered to be part of the Middle Pleistocene sequences.

The locations of tbe Broom pits and the sections are indicated on Figure 12. A summary cross-section of the exposed sediments is included as Figure 13.

Pollen Analyses

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The clay samples were extracted from palaeochannel deposits lying within the coarser river terrace gravels exposed in sections 2 and 5 in the Railway Ballast Pit. The channel deposits contained fine-grained sediments comprising silts and fine to medium sands. The fine-grained silts were grey and unoxidised, and were clearly lain down in a low energy environment, perhaps on a floodplain or in a meander cut-off.

The sediments were considered to have potential for pollen analysis based on the previous work of Dr Scourse (Shakesby & Stephens 1984). Samples were taken from the most representative profile (section 2) and have yielded pollen that appears to be comparable to Scourse's work. The pollen is restricted to three samples taken from the finest grained and unoxidised sediments, with the larger part of the sandy silts devoid of pollen. Diatoms were also sought but were absent.

Generally, the pollen data suggests a boreal environment with stands of pine woodland, and possibly occasional spruce and fir. It is noted however that the latter species are under-represented in pollen spectra, and may have been of greater importance than stated here. The small numbers of oak, hazel, alder, and ash are enigmatic and may represent the initial stages of vegetation succession at the beginning of an interglacial period. Alternatively, they may represent re-worked pollen from a preceding, older phase. The numbers are small though, and subordinate to the importance of the boreal woodland.

The importance of the Poaceae (grasses) either indicates a local or on-site dominance (e.g. floodplain grassland or the edges of the channel) or wider areas of open grassland within scattered coniferous woodland.

Scourse's study recorded a similar assemblage of tree pollen but made no mention of the grasses and the diverse range of thermophiles recorded in the current study. Scourse therefore suggested that the sequence might be attributable to sediments deposited at the end of an interglacial, or possibly during an interstadial within the Wolstonian cold stage. The greater significance of grasses in the current study may be due to the profile originating from a channel fill of different age, or from sediments of the same broad age, but spanning a different phase. The existence of the thermophiles may support the notion that the profile forms part of an interglacial sequence, although it is not clear whether this is pre-temperate or post-temperate phase.

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OSL analyses

Photograph archive

Figure 1: Upper gravels, clays and lower gravels in Section 2 (west face), Railway Ballast Pit
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Figure 2: Upper gravels, clays and lower gravels in Section 3 (west face), Railway Ballast Pit
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Figure 3: Clays and lower gravels in Section 5 (north face), Railway Ballast Pit
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Figure 4: Upper gravels in Section 1 (west face), Railway Ballast Pit
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Figure 5: Lower gravels in section 3 (north face), Railway Ballast Pit
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Figure 6: Lower gravels in section 5 (east face), Railway Ballast Pit
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Figure 7: Manganese staining in the lower gravels in Section 5 (north face), Railway Ballast Pit
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Figure 8: Fine-grained palaeochannel deposits in Section 9 (south face), Pratt's New Pit
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Figure 9: Fluvial sand and gravel sequence, section 10 (west face), Pratt's New Pit
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Figure 10: Fluvial sand and gravel sequence, section 13 (west face), Pratt's New Pit
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Figure 11: Pollen samples from section 2 (west face), Railway Ballast Pit
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Figure 12: Pit and section locations at Broom

Figure 13: Summary cross-section of the Broom sediments

References:

E-mail: r.hosfield@rdg.ac.uk
This page last updated at 10:36, 6^th September 2004.