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4.4 Palaeoenvironmental evidence

See Appendix 1, Appendix 2 and Figure 12

4.4.1 Soil micromorphology by Richard I. Macphail

At Braich y Gornel, the thin-section samples were taken from a buried soil within a slight lynchet - sample M6, and from a buried soil below field boundary tumble - sample M8. The results are presented in Table 7, Table 8, Table 9 and Figures 7.1, 7.2, 7.5, 7.6, 7.10, 7.23 and 7.24).

Contexts 4 over 9 (M6):
These are heterogeneous, biologically mixed (burrowed), stony soils with mainly moderately humic silty clay in upper part of slide, and iron- and iron and manganese-stained clayey soil in the lower part (Figures 7.1, 7.5, 7.6 and 7.23; Table 7). In fact, the subsoils are very strongly Mn (and Fe) stained (5.71–13.8% Fe and 17.5–36.1% Mn). Only trace amounts of fine charcoal are generally present. Nevertheless, occasional very fine charcoal and abundant very fine amorphous organic matter occur in the lower soil, which is compact (more recent fragmentation, and structure formation have apparently affected this massive soil). Overall, the soils are broadly burrowed with moderately broad to broad organo-mineral excrements, and with more fine pellety excrements upwards; this material also includes phosphate (Figure 7.23).

This thin section records the biologically mixed junction between more recent humic soil (Context 4), with phytoliths, and lower compact clayey subsoil (Context 9) which appears to have been humic and contains very fine charcoal.

Context 10 (M8):
This is a very heterogeneous, bioworked and fragmented stony soil, with moderately humic silty clay, especially in lower half of slide, and with clayey and clayey argillic soil fragments mainly in middle and upper part of thin section (Figures 7.2, 7.7, 7.8 and 7.24). Argillic soil fragments have abundant thin iron-stained limpid void coatings and ferruginous hypocoatings (max 65.3% Fe, Figure 7.24). Occasional very thin ferruginised root remains occur throughout, with fine pellety fabrics dominating the lower 'massive' soil (acidiphyle mesofauna). Vesicular arbuscular mycorrhizae (birefringent, 250μm) are present (Figures 7.9, 7.10). Only trace amounts of very fine charcoal occur.

The lower half of the slide possibly represents a truncated acidic, weakly podzolic mixed Ah and B(s) horizon, with upwards fragments of iron-stained argillic clayey soil becoming common. The presence of these clayey soil fragments indicates that the upper soil could be colluvial in character. It is worth noting that the occurrence of vesicular arbuscular mycorrhizae may be an implied proxy indicator of sheep grazing in the past, because a study of such fungal material in association with archaeological sites found that if they are birefringent, then they are probably >900 years old, and this small (250μm) size may link them to sheep management (Romans and Robertson 1983). In ecological terms, the presence of this fungal material generally means that plants were under stress (P. Wiltshire, pers. comm.) and that it would also have helped in the supply of nutrients to plants growing in poor soils (Brady and Weil 2008, 454). At Chysauster, Cornwall, similarly birefringent and 'ancient' (birefringent) vesicular arbuscular mycorrhizae were found on similarly acid buried soils (Smith et al. 1996).

Discussion:
The soils which occur between 270m and 290m OD are characterised by clayey subsoils, presumably of glacial drift origin, which have been affected by poor drainage and associated hydromorphic concentrations of iron and manganese; they occur within a mapped area of ferric stagnopodzols (Hafren soil association) formed on Palaeozoic slates, mudstone and siltstone (Rudeforth et al. 1983). At the site, it is possible that some of the relict, very fine organic matter that is present in these clayey and Fe/Mn-stained materials is of Pleistocene age. It is noticeable that in Context 10, clay soil fragments occur in the upper part of the sample, indicating colluviation from possibly plough-eroded soils upslope. No textural pedofeature evidence of cultivation is present (Jongerius 1970, Jongerius 1983, Macphail et al. 1990), however, possibly because of subsequent working of the fine soil by mesofauna. The suggested cultivated soil is currently showing acid soil traits, and may have been left to pasture as it became less productive. This speculation is further supported by the presence of vesicular arbuscular mycorrhizae here and in Context 4, where phosphate-rich biologically worked soil is present (Figure 7.23). These soils do not show the podzolic or peaty traits of the local ferric stagnopodzols (although Fe/Mn-staining is consistent with poorly drained subsoils), although they are currently acidifying, perhaps suggesting why these may have been selected for cultivation.

4.4.2 Soil pollen by Astrid E. Caseldine

Four samples (Table 10) were examined for pollen from a small sample tin (11) taken from a humic layer (13), interpreted as a former turf surface, which had been protected by one of the 'facing' stones of the boundary that had slipped downslope. The pollen record is dominated by grass pollen and other herbaceous taxa, notably ribwort plantain, especially in the top pollen level, indicating an open grassland environment and supporting the buried turf interpretation. However, Hordeum-type pollen, of either barley or possibly wild grasses, occurs in two lower levels, perhaps indicating an earlier cultivation phase. Arboreal pollen values, namely oak, alder and hazel, are consistently low, particularly in the top level, and greater stitchwort (Stellaria holostea), a species found in woodland and hedgerows, is also absent from this level. Indeterminable pollen values are relatively high (c.5–35% Total Pollen and c.5–60% Total Land Pollen), reducing the reliability of the evidence. Concentration values increase downwards perhaps indicating some downward movement of pollen. The micromorphological evidence from the buried soil (10) down slope from the boundary suggests that there had been some movement of soil downslope from the area of the early field, perhaps accounting for the Hordeum-type pollen. A piece of oak charcoal from the buried soil (10) gave a date of 1310–1050 cal BC (SUERC-33059) and is probably representative of an earlier clearance phase.

4.4.3 Charcoal by Astrid E. Caseldine

The only charcoal (Table 11) that was identified was from a layer (10) downslope from the boundary. Charcoal from a sample recovered from this layer (sample 4) consisted of a fragment of oak (Quercus spp.) and a fragment of hazel (Corylus avellana). The hazel was too small to be submitted for AMS dating while the oak provided a date of 1310–1050 cal BC (SUERC-33059). The charcoal probably represents activity associated with clearance of oak and hazel woodland during the Middle Bronze Age and is therefore unrelated to the establishment of the field boundary.


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