Nene Centre for Research, University College Northampton,. Northampton NN2 7AH, UK; 4Institute of Geography and Earth. Sciences, University of Wales ...
Allochthonous and autochthonous mire deposits, slope instability and palaeoenvironmental investigations in the Borve Valley, Barra, Outer Hebrides, Scotland P. Ashmore,1 B.A. Brayshay,2 K.J. Edwards,2 D.D. Gilbertson,3* J.P. Grattan,4 M. Kent,5 K.E. Pratt6 and R.E. Weaver5 (1Historic Scotland, Longmore House, Salisbury Place, Edinburgh EH9 1SH,UK; 2Department of Archaeology and Prehistory, University of Sheffield, Northgate House, West Street, Sheffield S1 4ET, UK; 3The Nene Centre for Research, University College Northampton, Northampton NN2 7AH, UK; 4Institute of Geography and Earth Sciences, University of Wales Aberystwyth, Ceredigion SY23 3DB, UK; 5Department of Geographical Sciences, University of Plymouth, Plymouth, Devon PL4 8AA, UK; 6Durham Wildlife Trust, Rainton Meadows, Chilton Moor, Houghton-le-Spring, Tyne and Wear DH4 6PU, UK) Abstract: This paper suggests that sediment depth-age anomalies, and the lithological and palaeoecological properties of a peat core from Borve mire on the Outer Hebridean island of Barra, reflect the episodic impacts of rapid mass-movement of superficial peats and mineral soils from the adjacent hillslopes in the period 3000 to 1750 14C years BP. Alternative explanations such as mismeasurement of radiocarbon or contamination by floods, are thought less likely. The research implies that there is a general need for caution in the interpretation of mire deposits from sites which are adjacent to steep peat-covered hillslopes and which have not been investigated with the support of substantial radiocarbon and lithological studies programmes. The environmental and vegetational history of this exposed and isolated Atlantic island is shown to have not been one of treeless homogeneity. A variety of deciduous and coniferous tree species colonized early in the Holocene, with distinctive birch-hazel woodland developing at one point in time. The landscape became increasingly treeless in the Bronze Age, with most but not all trees having been lost by the Medieval period. Valley side peats provide palynological evidence of pastoral and arable farming on poor soils in the Dark Age-Early Medieval period, at sites beyond the present limits of cultivation. Key words: Mass-movement, palaeoenvironments, pollen analysis, slope instability and radiocarbon dating, taphonomy, Outer Hebrides, Scotland, Holocene.
Figure 1 The location of the Upper Borve Valley, Barra, in the Outer Hebrides of Scotland: study sites, topography, and rapid peat-flows mapped in 1992–93.
Introduction This paper investigates the possible impacts of the rapid massmovement of hillslope peats and mineral soils upon the lithological and palaeoecological properties of a valley-floor mire deposit. The deposits studied are located in the exposed upper Borve Valley, a large glacial trough, in rugged terrain on the western side of the Atlantic island of Barra in the Outer Hebrides (also known as the Western Isles) of Scotland (National Grid Reference NF 674 009; Figure 1). Until recently, relatively few studies had focused upon the vegetational and environmental changes in the Outer Hebrides since the Lateglacial, compared to the relatively well-documented vegetational history of the Inner Hebrides. In many of these studies, consideration of the taphonomic factors that influence the pollen composition was often restricted to inferences concerning airfall-taphonomy as opposed to those involving the post-depositional transport and reworking of pollen which are
proposed in this study (e.g., Flenley and Pearson, 1967; Birks and Madsen, 1979; Birks and Williams, 1983; Birks et al., 1987; Walker et al., 1988; Edwards and Berridge, 1994). In the past few years, however, a number of more detailed explorations of the Holocene vegetation, environmental history and pollen taphonomy in Outer Hebrides have been published (Wilkins, 1984; Randall et al., 1986; Bohncke, 1988; Bennett et al., 1990; Kent et al., 1994; Pyatt et al., 1995; Brayshay and Edwards, 1996; Dinnin, 1996; Fossitt, 1996; Gilbertson et al., 1995; 1996; Gearey and Gilbertson, 1997; Whittington and Edwards, 1997). Initially, there was a consensus that the Outer Hebrides were largely treeless throughout the Holocene, as they are today (McVean and Ratcliffe, 1962; Birks and Madsen, 1979). As further studies were published, these assumptions of vegetational uniformity and treelessness across the islands were challenged. In their place, a picture of considerable diversity in vegetational history has emerged, with evidence from studies of beetles, plant macrofossils, pollen and pollen taphonomy suggesting that a cover of shrub and/or woodland formerly existed in a number of different parts of the archipelago. Of the few pollen diagrams published for the Holocene from the island of Barra – Port Caol, Glen Bretadale, Lochan nan Faoilean and Lochan na Cartach (Figure 1; Blackburn, 1946; Gilbertson et al., 1995; Brayshay and Edwards, 1996) – the most
complete evidence of vegetation change throughout the Holocene comes from Lochan na Cartach. This site occupies a small peatfilled basin in a sheltered, glacial trough to the northeast of the mountain of Grianan, which forms the eastern watershed with the Borve Valley (Figure 1; Brayshay and Edwards, 1996). That study indicated the former presence of a mixture of vegetation types with significant woodland occurring in the early and mid-Holocene. Subsequently, these woodland communities declined and were replaced by acid grass and heathland communities. Interestingly, in the context of this paper, the core from Lochan na Cartach yielded an apparent radiocarbon age-depth anomaly. At 440 cm organic sediments in this core gave a radiocarbon date of 6800 ± 65 years BP. This age estimate is younger than two other dates which were 40 and 110 cm higher in the core (7780 ± 75 and 7715 ± 120), posing a problem of interpretation. The new palynological evidence, radiocarbon dates and sedimentological studies from the nearby upper Borve Valley presented in this paper provide further information on the history of the vegetation and land use on the valley floor and hillside in this valley, including mention of poorly known Dark Age/Early Medieval land use. They imply that parts of the peat sequence in Borve mire near the head of the valley may reflect the repeated introduction to the mire of peats, micro- and macrofossils and mineral soils by rapid mass-movement from the adjacent steep hillslopes. The possible frequencies, taphonomic and palaeoecological impacts of past rapid peat slides and flows are indicated in the accounts by Wilson and Hegarty (1993) and Wilson et al. (1996) of recent rapid peat-flows in the mountains of Northern Ireland. These papers also summarize knowledge of the many peat-flows known from the north and west of Britain in historical times (see also Lamb, 1995: 242). Such peat-flows were shown to be more than simply a reflection of land-use intensification in recent years: particular combinations of topography, antecedent weather, precipitation, internal and surface drainage appear to have been important.
The Borve Valley The Borve Valley opens westwards to the Atlantic coast from its head at the col lying between the steep-sided hills of Grianan (294 m) and Hartaval (353 m) (Figure 1). The equable, but windy and wet climate of the area was most recently described by Angus (1991); annual precipitation is likely to exceed 1000 mm with a potential water surplus over much of the year. Although only 4.2 km long, the valley encompasses the full range of landscape types found on Barra. The pattern is one of sharp contrasts. The coast is characterized by dunes and machair grasslands with welldrained, calcareous soils. The underlying bedrock of the area is Lewisian Gneiss with a patchy and thin veneer of weathered, finegrained till or tills from former icesheets and glaciers (Peacock, 1984). Soils on till(s) and bedrock are often podzolized, gleyed and peaty (Gilbertson et al., 1995; Hudson et al., 1982). Further up the valley, a blanket cover of peat is widespread. In the winter of 1992–93, major rapid peat flows took place and were mapped on the southern and eastern margins of the valley (Figure 1). Peat appears to have been transported as flowing liquefied peaty detritus, as transported blocks or rafts, and as debris on watery runout. On the steep, north face of the mountain Hartaval, these mass-movements eroded and incorporated both the c. 1 m thickness of blanket peat and the underlying mineral soils. They travelled a substantial distance across the relatively flat valley floor,
almost reaching the core site. Mass-movements of much thicker peats are also common on both the steep and wet, western and eastern sides of Grianan (Figure 1). Today these poor, peaty upper reaches of the valley are used for rough grazing, mostly by sheep. Cultivation does not take place in the upper valley, although extensive traces of former spade cultivation exist. Settlement is concentrated further down the valley at the transition between the peat and calcareous machair. These transitional ‘blacklands’, where peat and shell sands naturally overlap, form the best agricultural land in the area, as seen in the Uists (Caird, 1977). Evidence of the scale and intensity of former human activity in the Borve Valley, as in the region generally, is emerging as a result of recent archaeological surveys (Branigan and Foster, 1995; Branigan, 1988–97). Their published and unpublished evidence suggests that the lower Borve Valley has been one of the most densely settled areas of Barra, and archaeological survey has indicated its comparative richness in surviving structures and wall systems from the historic and archaeological past. More than 160 archaeological sites have been discovered in the valley. Chambered cairns, standing stones and stone circles (probably dating from mid- to late Neolithic times, c. 5000 BP onwards), are the earliest monuments (Figure 1), but no major structures have been securely assigned to the period from Iron Age through to the Medieval Period. Rectangular huts, clearance cairns and field boundaries are suspected to be Medieval or Post-medieval in age (cf. Branigan, 1988–97).
Study sites, field and laboratory Methods Palaeoenvironmental studies were carried out at two locations (Figure 1): the upper Borve Valley mire and a hillslope cairn site west of Grianan and 0.75 km from the mire site. The mire is in the upper Borve Valley (200 m across; altitude |65 m OD; Figure 1). It lies in a rock basin, floored by thin clay-rich till resting on the underlying gneissose bedrocks. The surface is typically very wet with developing peat which supports Eriophorum, Carex, Juncus and Sphagnum spp., with Molinia caerulea and Calluna vulgaris on drier adjacent hummock areas. Pratt (1992) and Weaver et al. (1996) described in detail the present-day vegetation of the valley and the surroundings of the coring site. Six major plant community types were identified and related to local environmental gradients of nutrient status and drainage character. The mire (NF677009) is sometimes flooded by overland flows from the adjacent hillslopes. The second study site in the valley was of a small cairn located wholly within blanket peat and resting upon a thin buried soil Ao horizon on the hillslope peatface (Figures 1 and 5; NF 677 015). The studies at the mire in the upper Borve Valley involved two adjacent 5 cm diameter Russian cores. One core was used for EDMA (energy-dispersive x-ray micro-analysis) investigations described by Grattan et al. (1996). The adjacent core was investigated using x-radiography, mineral magnetism, and LOI (methods after Ball, 1974; Butler, 1992; Thompson and Oldfield, 1986) (Figure 2), AMS radiocarbon dating (Figure 3; and Table 1), and pollen analysis (Figure 4). The samples for pollen analysis and radiocarbon dating at the cairn site were obtained using a monolith tray inserted into an excavated peat. Pollen preparation of samples was carried out using standard KOH, HF and acetolysis pretreatments (Moore et al., 1991). Pollen and spore concentrations were sometimes low and a minimum
of 300 total land pollen (TLP) grains was counted (500 TLP minimum for the cairn site) (Figures 4–6). Pollen counts of selected taxa are represented variously as percentages of total land pollen or as % TLP excluding Ericaceae, Poaceae and Cyperaceae. Taxon nomenclature follows Stace (1991) and Bennett (1994). Pollen diagrams were drawn in TILIA.GRAPH (Grimm, 1991). The radiocarbon dating of 10 mire samples was carried out at the University of Arizona AMS Facility following preparation at the Glasgow University SURRC laboratory, East Kilbride. Full details of the dates and their calibration are given in Table 1. The basal peat sample from beside the sub-peat hillslope cairn was dated by radiometric methods at the Glasgow laboratory. The phases of erosion and deposition deduced from the sedimentological data are indicated by shading on the pollen diagram from the mire (Figures 4 and 6).
Borve mire: results and discussion Stratigraphical studies The core stratigraphy in the Borve Valley mire is summarized in Figure 2. At its base (c. 5 m), the core contains sands, silts, clays and fine gravels, which are overlain by 4.4 m of peat. This peat is humified but variable in composition. It contains ericaceous twigs concentrated especially at depths of 4.4–3.7 m and 2.0– 1.5 m. The properties of the inorganic materials in the core are also particularly interesting. Consideration of both its organic and inorganic properties led to the recognition of eight sedimentary units: Base of core Unit 1: thickness 50 cm; poorly sorted clay-silts, sands and small pebbles, often angular, with a sharp upper boundary with the overlying organic muds; LOI as low as 4 % and high magnetic susceptibility. Unit 2: thickness 134 cm; transition from 2 cm of mud to peat
Figure 2 Summary results of the analyses of the physical characteristics from the core from the mire deposits in the upper Borve Valley.
with abundant wood and bark of Betula; significant sand-size component in the upper 20–30 cm where sands become increasingly common; high LOI reaching 96%, and low magnetic susceptibility. Unit 3: thickness 70 cm; fibrous, compact, humified peat with wood and twigs of Corylus and Alnus, interbedded sands and gravel (particles up to 3 mm diameter); high LOI (98%) and low magnetic susceptibility. Unit 4: thickness 90 cm; humified peat, soft, with twigs and woody remains of Betula, Corylus and Alnus; a significant content of mineral materials in distinct layers; 5–10 cm thick of clays, or sand to gravel-sized particles, sometimes sorted, sometimes poorly sorted; relatively low LOI, 3–18%, high magnetic susceptibility; with a sharp upper boundary. Unit 5: thickness 41 cm; weakly humified fibrous peat, with minor content of sand-size particles at base, with a relatively small content of woody remains of indeterminate origin; LOI increases upwards through the unit to very high values; magnetic susceptibility values are low; the upper boundary is sharp at a layer of fine sand in a peat matrix. Unit 6: thickness 37 cm; weakly humified fibrous peat, with occasional twigs of Calluna; relatively high LOI (around 83%); with magnetic susceptibility reaching high values; sharp upper boundary. Unit 7: thickness 5 cm; sand layer in a matrix of weakly humified fibrous peat, with particles up to 3–4 mm diameter; low LOI (25%) and relatively high magnetic susceptibility; sharp upper boundary. Unit 8: thickness 61 cm; weakly humified, fibrous peat, with distinct, thin bands of medium and fine sand in upper part; increasing LOI values (to 97%); magnetic susceptibility values high; upper boundary is the present mire surface, which has probably been cut for peat in the past.
The studies by x-ray radiography, EDMA, LOI and mineral magnetism of the core all point to the presence of surprising quantities of inorganic materials in distinct parts of the core – a feature that was not evident in the field. Figure 2 shows that these inorganic materials are abundant in three portions of the core. They indicate several phases of increased mineral input from the deposits and soils of the catchment onto the accumulating sedimentary surface. These episodes occur (i) within unit 1, (ii) in a complex series of deposits that occur within the upper part of units 3 and 4, and (iii) within unit 7 (Figures 2, 4 and 5). The size of materials (c. 3 mm), and the distinct sand horizons require a transporting medium of some energy. Mineral-rich deposits at the base of cores in such highland basins are common, typically reflecting greater rates of erosion and clastic sedimentation in the open landscapes of the Late Devensian. In this mire, two possibilities emerge to explain the mineralrich portions noted higher in the core, during the Holocene, when the accumulation of organic deposits had become dominant. From observations of the present geomorphology of the area, these are floodwater from the ancient Quarry Burn or the adjacent hillslopes, and/or rapid mass movement of peats and mineral soils from the mountain slopes, observable to the south and east of the mire. Either of these processes might also be responsible for the plant macrofossils detected in the core. Sediment depth-age anomalies The four dates (whether uncalibrated or calibrated) for the sediments between core depths 4.4 m to 2.9 m plot in a simple linear fashion (Figure 3; Table 1). This suggests that a relatively uniform rate of sedimentation took place from approximately 8700 BP to approximately 2945 BP. Above 2945 BP, the plot of the dates is notably different (Figure 3). In the most general of terms, the plot suggests a significant increase in the rate of sedimentation in the mire. This property has been detected in many palaeoenvironmental studies in upland Britain and reflects the impact of land-use and vegetation changes in the catchment from the Neolithic to the present; for example, locally on the nearby islands of South Uist (Pyatt et al., 1995) and North Uist (Entwistle et al., 1995). This phenomenon is not discussed further. The plot also indicates that there is no simple relationship between age and depth in this portion of the core. This is true of both calibrated and uncalibrated dates. The geomorphological context of the site suggests that this may have one or more of the following explanations. (1) It is possible that one date, AA-17441 (1745 ± 40 BP) may be a mismeasurement, and that the other dates are essentially
Figure 3 Age-depth relationships for radiocarbon – calibrated and uncalibrated – from the mire deposits in the upper Borve Valley, Barra (see Table 1).
correct. This assumes that the dates effectively overlap and hence explains the apparent anomaly in the inverted age-depth relationships between dates AA-17445 and AA-17444 (Figures 2 and 3). (2) (a) ‘Old peat’ may have been eroded and transported to the site during flooding of the Quarry Burn and its tributaries, or by overland flow from the adjacent hillslopes (b) ‘Old peat’ may have been eroded and transported to the site by mass-movement, as observed in 1992–93. Explanation 1 The date provided by AA-17441 has no anomalous features, except that it is much younger than the dates from layers above and below it (Table 1). The associated 13C measurement is within the expected range. The difference of approximately 800 14C years between it and the date which might be expected for that layer, judging by the dates for the layers immediately above and below it, is of a magnitude extremely unlikely to occur by chance. There
is no reason to distrust AA-17441 more than any other of the dates reported here or elsewhere. Explanation 2 Before exploring which geomorphological processes might have been responsible for the age-depth anomaly, it is important to consider how much ‘old’ carbon would have had to be incorporated as a result of the hypothesized ancient transporting events – rapid mass-movement or fluvial activity – to produce the age estimates obtained at the Borve mire site. Below 290 cm, on average, 2.2 cm
of peat was accumulated in the mire every century. Above this level, judging by the dates of peats at depths 150 cm and 290 cm, the average rate of accumulation might have been about 9.8 cm per century. However, this estimate could be affected by the inclusion of ‘old’ peat and mineral material which may have been washed and eroded, or slumped and flowed, from old peats formerly higher up the catchment. If this were the case, then the true underlying rate of peat accumulation in the mire could have been little different from that which took place in the deeper parts of the sequence. If the measured date at 200 cm (AA-17442) were due to mixing of peat growing at approximately 2200 BP, with ‘old’ allochthonous peat which had grown at 8000 BP, then 0.3 times as much 8000-year-old peat compared to the autochthonous mire peat would have to have been introduced to the sample site to produce the measured date of 3215 BP. If the possible contaminant were allocthonous peat from 3600 BP (the approximate date of the peat at 310 cm in this sequence), then 2.3 times as much ‘old’ allochthonous peat as growing autochthonous peat would have to have been introduced (cf. Taylor, 1987: 102, Table 3). On the other hand, the date of 3215 ± 45 BP at 200 cm may be purely from peat which grew around 3215 BP further up the slope and might have flowed during one of the hypothesized catastrophic peat-flow episodes to where it was eventually to be sampled in the mire. These general considerations suggest that significant transport of ‘old’ peat by geomorphological processes may be signalled by the sediment depth-age anomaly. Useful negative evidence is suggesting that explanation 2(a) (deposition during flooding) is less likely provided by the general properties of the mire deposits present that span the period from about 9000 years to about 3000 years BP. Significant storm events which might have introduced ‘old’ carbon in storm floods of the Quarry Burn must have taken place in this 6000-year interval within this catchment, yet such fluvial activity or overland flow has not been detected in the mire sequence, except possibly at the top of the final episode of the influx of inorganic material, which is described later. On the other hand, the past role of rapid massmovement, as opposed to rapidly moving water, is supported by the following observations on core lithology. For example, neither x-ray radiography nor visual observation of the cores revealed any clear evidence of sorting, fining or imbrication, or other sedimentary property typically associated with rapidly moving water. The distribution of inorganic matter revealed by x-ray radiography, LOI and mineral magnetism matches fairly precisely the portions of the cores which contain apparently anomalous radiocarbon dates. This mineral matter must indicate the arrival of allocthonous material to the mire. Importantly, observations in the field in 1992–93 showed that the rapid peat-flows included mineral matter. This inorganic matter was derived from the tills and weathered bedrock found below the blanket peats of the adjacent hillslopes. Inspection of the plots of the geochemistry of the mire sediment core (Grattan et al., 1996) reveals influxes of elements, especially metals, which must have their origins in adjacent tills and bedrocks in the same sectors of the core that revealed inverted agedepth relationships and the notable levels of inorganic sediment. Although it is difficult to establish the precise causes of rapid peat-flows (which is not the objective of this paper), examination of the relevant literature for this type of terrain does suggest that such geomorphological activity may recur in particular situations (see Wilson and Hegarty, 1993), such as those which prevail around the Borve mire. Finally, an explanation which focuses upon the introduction of peat by rapid mass movement of peaty
Figure 4 Relative pollen frequencies of selected taxa from a core in the Upper Borve Valley, and deposits, which contain notable abundances of inorganic materials, as summarized in Figure 2.
Figure 5 Relative pollen frequencies of selected taxa from the upper Borve Valley expressed as total land pollen excluding Ericaceae, Poaceae and Cyperaceae.
Figure 6 Relative pollen diagram from a section through a Dark Age-Medieval
clearance cairn overlying peaty topsoil and buried in blanket peat on a hillslope in the upper Borve Valley – see Figure 1.
detritus or intact blocks or rafts from adjacent hillslopes also makes no assumptions of significant error in any of the dates and this study has not detected such error. Interestingly, radiocarbon anomalies are well-known features of those limnological profiles where sediments incorporate much allochthonous material (e.g., Edwards and Rowntree, 1980; as well as the nearby Barra site of Lochan na Cartach – Brayshay and Edwards, 1996); although they are less a feature of the peaty mire deposits described here. At present, the convergence of all these lines of lithological and geomorphological evidence lead to the conclusion that the suite of inverted age-depth relations detected in the Borve mire core is a consequence of the intrusion of peat and mineral matter introduced by mass-movement onto a rapidly developing mire surface in the period from approximately 3300 14C years to about 2200 14C years ago. More specifically, dates AA-17444, AA17442, AA-17440, AA-17439 and perhaps AA-17443 have been affected by the introduction of ‘old’ carbon, transported in prehistoric rapid peat-flows. The consequences for the interpretation of the pollen record are discussed further below. Unfortunately, similar information is not available for the third and uppermost episode of accelerated inorganic sedimentation represented in unit 7. In the absence of appropriate research, peats of early- or midHolocene age in locations from which peat might have flowed to the site in times past, have not been identified and dated on the steeper slopes of this valley. Informed field observation, however, suggests that it almost certainly exists. At present, the only pollenanalytical and radiocarbon study of a hillslope peat on the island comes from beneath and aside the peat-covered cairn immediately northwest of the site. This investigation indicated that the present blanket peat accumulated in the late Holocene (Figures 1 and 5), although it is evident from other investigations in that the area that hillslope peat soils have been forming at some locations in the region for at least 8000 years or so (e.g., Bennett et al., 1990; Brayshay and Edwards, 1996; and see below). Palynological evidence Palynological evidence provides environmental contexts for the inferred erosional-depositional processes and events mentioned above (Figures 4 and 6) is seen to coincide with pollen and spore spectra typical of Lateglacial and early-Holocene assemblages from the Outer Hebrides with dominant Salix, Ericaceae (mainly Empetrum nigrum), Poaceae and cryptogams (Birks and Madsen, 1979; Edwards and Whittington, 1994; Brayshay and Edwards, 1996). The high mineral content of the deposits is indicative of eroded substrates exhibiting little weathering or pedogenesis. In the period from approximately 8700 to 3000 BP, before the inferred episodes of rapid mass-movement of peat and mineral soils (Figures 2 and 4), the lithology of the core is characterized by woody peat. The pollen assemblages of these deposits are dominated by Cyperaceae, Poaceae, Lactuceae, Epilobium type and Ericaceae (mainly Calluna vulgaris) which, together with Sphagnum, probably reflect taxa growing on the mire surface and on peaty hillslopes nearby. The early expansion of Betula and the subsequent appearance of Corylus avellana-type pollen (c. 7530 BP) reflect the presence of distinctive birch-hazel woodland in the pollen rain (especially evident in Figure 6). Wood fragments of Betula, up to 3 cm long and 1 cm in diameter, were recovered
from 445–373 cm. Birch was therefore locally present, as it was earlier in time downvalley at Port Caol on Borve Headland (Figure 1; Brayshay and Edwards, 1996). Hazel was probably also growing locally – though Corylus pollen at nearby Lochan na Cartach, northeast of Grianan (Figure 1) expanded at an estimated date of 8870 BP and attained levels of more than 60% TLP. The hazel signal at the Borve mire site is overly muted as a result of local mire taxa, although this could also be attributable to low pollen productivity for trees and shrubs responding to excessive exposure to powerful and continuous, salt-laden winds from the west (see Gearey and Gilbertson, 1997). In both cases, the spectra in the Holocene before the beginning of the second erosional phase are characteristic of locally dominant herbaceous plants, conceivably with herb and light birch-hazel cover on adjacent hillslopes. The intermittently high Pinus values are taken to represent long-distance pollen transport, though Scots pine was present on Lewis during the Holocene (Fossitt, 1996). This vegetation cover does not appear to have been associated with the introduction of allocthonous materials – peats or inorganic sediments – to the mire. The peat lithology shows a change from wellhumified peat to humified fibrous peat at 372 cm (approximately 6000 BP), with a layer of heather twigs occurring at this level. This may reflect some increase in sediment accumulation rates as the mire surface became wetter, Calluna perhaps became waterlogged and unviable, whereas Cyperaceae became more dominant (Figure 4). The second, dominant and complex inferred phase characterized by the influx of allocthonous sediments is associated with more-humified deposits and higher values of the Ericaceae, and Potentilla values and a lower frequencies of Cyperaceae and Sphagnum. These palynological changes suggest drier landsurface conditions. These may have occurred on the mire surface itself. Alternatively, if the arguments concerning past rapid massmovement of peat are correct, then these changes are those that could be anticipated from the influx onto the mire of drier hillslope peats with their characteristic drier heathland vegetation (and pollen load) described by Weaver et al. (1996). The influx of spores of Huperzia selago and Pteropsida might also reflect those cryptogams growing on eroding, rocky slopes above the site, as much as taxa living on the mire. The role and significance of human activity in these changes is difficult to assess. The palynological evidence suggests that the upper Borve Valley from around 3000 BP may have been experiencing the strains of increased middle Bronze Age land use identified in archaeological surveys (Branigan, 1988–97; Branigan and Foster, 1995). Herbaceous taxa commonly found in areas under human pressure, such as Plantago lanceolata, Lactuceae, Artemisia and Pteridium aquilinum, are present and increases in Ericaceae might also reflect more widespread leaching and podzolization of soils, as well as the spread of blanket peat. Particularly notable are the summary pollen curves for trees and shrubs calculated using a TLP sum excluding mire taxa (Figure 5) where arboreal frequencies decline sharply immediately before the arrival of mineral matter. The ideas and evidence presented above imply that the loss of tree pollen may have several explanations. Alternatively, Callunarich, tree-pollen-poor, peaty sediments may have been reworked as peaty detritus or intact blocks of peat into the accumulating mire from transported heathland-blanket peats, as indicated by the increased levels of mineral matter that occur slightly higher in the core. At present, it is not possible to distinguish clearly between
these possibilities. Nevertheless, these palynological data do point to some human impact in the landscape. At the uppermost levels in this second phase of increased inorganic sedimentation, the deposits again yielded notably lower frequencies of tree pollen, with the notable exception of Quercus – see below. Whether or not the sequence in these upper levels is or is not largely in situ, it is clear that above this level tree taxa became of minimal significance in the catchment. At the conclusion of this second episode of increased inorganic sedimentation, the pollen spectra return to a simple dominance of Cyperaceae, perhaps suggesting the undisturbed wet surface conditions that characterized the actual mire for most of the early and middle Holocene. The beginning of the final, short and less intense, episode of greater influx of inorganic sediments to the mire was (phase 3) also characterized by pollen spectra indicating the existence of drier peaty soil conditions. In the field, the peat appeared relatively humified, but, on this occasion, Ericaceae grains were relatively scarce. This association might also suggest the return of drier conditions on the mire, or, once again, it might be a consequence of the introduction of detrital peat, pollen and mineral matter from adjacent hillslopes. However, as other indications of hillslope erosion do not appear to be reflected in the pollen spectra, the allochthonous inorganic material might possibly in this case include a waterborne component which had spread across the mire surface in times of extreme flood. Normal peat accumulation followed this episode. If high-intensity land use was taking place on the valley floor during this period, the pollen spectra from the mire profile are insufficiently sensitive to reflect it. The presence of oak pollen in the uppermost sediments suggests that peat has been cut from the bog surface – oak being completely absent from the area today, the high Quercus values may reflect non-local sources.
Pollen analysis of the sub-peat hillslope Cairn The cairn site provides the only palynological information presently available on the ecological and agricultural history of hillslope peats in these Southern Isles of the Outer Hebrides (Figures 1 and 6). A small cobble- and boulder-built cairn, .0.6 m wide and .30 cm high, perhaps a field-clearance or marker cairn, rests on a thin layer of in-situ peat which is part of the peaty topsoil developed upon the underlying acidic, gleyed mineral soil. The construction date of the cairn is suggested by the radiocarbon date of 1120 ± 50 BP (cal. ad 883–890; GU-4125) from the peaty topsoil upon which the cairn was constructed. This date reveals the blanket peat at this site to be relatively young in comparison to those inferred for the transported hillslope peat materials which are suggested above to have reached the nearby mire by rapid mass-movement. The presence of sandy inclusions in peats by the cairn demonstrate the erosion of the nearby mineral soils sometime after its construction. Previous studies of peat-buried walls and archaeological features in the Outer Hebrides have shown them to be notably older that at this Borve hillside – e.g., approximately 3200 radiocarbon years BP at Callanish (Bohncke, 1988), about 3000 years BP elsewhere on Lewis (Newell, 1989), and close to 2600 BP at Loch Portain, North Uist (Mills et al., 1994). Evidence of vegetation, and especially land use, in this Dark Age or early Medieval period are therefore relatively unusual. The pollen assemblages present at this hillside site convey a
different impression of land use to those recovered from the nearby Borve mire. Both the peaty topsoil and the overlying blanket peats which have buried the cairn, are dominated by Calluna vulgaris and Poaceae pointing to the presence of acid moorland vegetation and peaty, drier soils, as opposed to the very wet mire in the valley floor. Arable and probable pastoral activity are indicated with the pollen of Avena/Triticum-type (cf. oats), Hordeumtype (barley), and perhaps Chenopodiaceae, Spergula, Plantago lanceolata, Plantago major/media, Rumex acetosa, Lactuceae, Caryophyllaceae and Ranunculaceae. Broadly similar pollen assemblages, associated with buried walls, have been suggested to indicate local arable cultivation in otherwise pastoral landscape on peatlands in Ireland (O’Connell, 1990). The evidence from the cairn site reveals the presence of significant agricultural activity on hillslopes in the Borve Valley in the past, well beyond the present limits of cultivation in the valley (Figure 1). These data indicate that arable and pastoral practices have taken place in situations that might have contributed to the rapid flow of hillslope peats and mineral matter in the past, variously discussed above.
Conclusions This paper suggests that the lithological, radiocarbon and palaeoecological properties of cores from Borve mire on Barra reflect the episodic impacts of rapid mass-movement of superficial deposits from the adjacent hillslopes in the period 3000 to 1750 14C years BP. Alternative explanations which assume mismeasurement or contamination by floods are thought less likely. This research implies the need for caution in the interpretation of mire deposits which have accumulated at sites adjacent to steep peat-covered hillslopes. Few other studies of Holocene mire deposits from glacial troughs in the Scottish Highlands and Islands have been supported by such a detailed dating framework, provided here by 10 radiocarbon dates, and many similar studies are based on fewer radiocarbon dates. Had this been the case here it is unlikely that the inclusion of older allocthonous organic material would have been inferred. This study has re-emphasized the need for fuller taphonomic information on the origins and significance of older carbon and pollen. The frequencies of coring, sampling and dating have probably missed other episodes of possible peat intrusion and reworking. The lithological and pollen data presented here are insufficient to identify, with any confidence, the cause(s) of the inferred rapid mass-movements of peat and mineral soils on the adjacent steep hillslopes in the past. Although there have been significant climatic fluctuations in the Holocene, it seems that probable combinations of large antecedent rainfall, saturated soils, followed by heavy precipitation, are likely to have occurred at many times. Such rapid flows have not been detected in the lower half of the core, or at least they did not reach the mire. As a result, it is tempting to speculate that the notable human exploitation of the valley in the Bronze Age (Branigan, 1988–97) may have been a trigger or a further key predisposing factor, but there is little hard evidence for these views. Nor is there clear evidence to suggest that overgrazing was the cause of the recent peat slides observed on Barra. Casual observations suggests that rapid mass-movement involving peats in the Outer Hebrides are not uncommon, especially given the bedrock of impermeable Lewisian Gneiss,
steep slopes and abundant water as trigger and lubricant. The environmental and vegetational history evidenced for the Holocene reveals a picture of early colonization by a variety of deciduous and coniferous tree species, with the distinctive presence of birch-hazel woodland. The landscape of the region became increasingly treeless in the Bronze Age, with most trees having been lost from the landscape by Dark Age-Medieval times. Unusually, the valleyside blanket peats provide palynological evidence of pastoral and arable farming on relatively poor peat soils in the Dark Age-Medieval period well beyond the present limits of cultivation.
Acknowledgements The authors are grateful to the following people and organizations for their help, advice and support: Historic Scotland for the AMS and conventional radiocarbon dates; the Department of Archaeology and Prehistory at the University of Sheffield and its SEARCH project, directed by Professor Keith Branigan, of which this contribution forms part; the people of the Borve Valley in particular and Barra in general for their help, advice and support; Dr Mary Elliott and Dr Stewart Angus of Scottish Natural History; Brian Rogers, Tim Absolom and Matthew Chambers of the Cartographic Unit of the Department of Geographical Sciences of University of Plymouth for the illustrations; the SERC for postgraduate studentships held by Drs Brayshay and Pratt; and the University of Plymouth Research Support Fund. The useful comments of referees are gratefully acknowledged.
References Angus, I.S. 1991: Climate and vegetation of the Outer Hebrides. In Pankhurst, R.J. and Mullin, J.M., editors, Flora of the Outer Hebrides, London: Natural History Museum Publications, 28–31. Ball, D.F. 1974: Loss-on-ignition as an estimate of organic matter and organic carbon in non-calcareous soils. Journal of Soil Science 15, 84–92. Bennett, K. 1994: Annotated catalogue of pollen and pteridophyte spore types of the British Isles. Cambridge Department of Plant Sciences, University of Cambridge. Bennett, K.D., Fossitt, J.A., Sharp, M.J. and Switsur, V.R. 1990: Holocene vegetational and environmental history at Loch Lang, South Uist, Western Isles, Scotland. New Phytologist 114, 281–98. Birks H.J.B. and Madsen, B.J. 1979. Flandrian vegetational history of Little Loch Roag, Isle of Lewis, Scotland. Journal of Ecology 67, 825–42. Birks, H.J.B. and Williams, W. 1983: Late Quaternary vegetational history of the Inner Hebrides. Proceedings of the Royal Society of Edinburgh 83B, 269–92. Birks, H.J.B., Andrews, M.V., Beck, R.B. and Gilbertson, D.D. 1987: The past and present vegetation of Oronsay and Colonsay. In Mellars, P.A. (ed). Excavations on Oronsay: prehistoric human Ecology of a small island. Edinburgh: Edinburgh University Press, 52–77. Blackburn, K.B. 1946: On a peat from the island of Barra, Outer Hebrides. Data for the study of post glacial history. New Phytologist 45, 44–49. Bohncke, S.J.P. 1988: Vegetation and habitation history of the Callanish area, Isle of Lewis, Scotland. In Birks, H.H., Birks, H.J.B., Kaland, P.E. and Moe D., editors, The cultural landscape: past, present and future, Cambridge: Cambridge University Press, 445–61. Branigan, K. 1988–97: SEARCH Interim Reports on the Western Isles Project. Sheffield: Department of Archaeology and Prehistory, University of Sheffield. Branigan, K. and Foster, P., editors 1995: Barra: archaeological research on Ben Tangaval. Sheffield: Sheffield Academic Press. Brayshay, B.A. and Edwards, K.J. 1996: Late-glacial and Holocene vegetational history of South Uist and Barra. In Gilbertson, D.D, Kent, M. and Grattan, J.P., editors, The Outer Hebrides: the last 14,000 years, Sheffield: Sheffield Academic Press, 13–26. Butler, S. 1992: X-radiography of archaeological soil and sediment pro-
files. Journal of Archaeological Science 19, 151–61. Caird, J.B. 1977: Land use in the Uists since 1800. Proceedings of the Royal Society of Edinburgh 77B, 505–26. Dinnin, M. 1996: The development of the Outer Hebridean entomofauna: a fossil perspective. In Gilbertson, D.D, Kent, M. and Grattan, J.P., editors, The Outer Hebrides: the last 14,000 years. Sheffield: Sheffield Academic Press, 163–84. Edwards, K.J. and Berridge, J.M.A. 1994: The Late Quaternary vegetational history of Loch a’Bhogaidh, Rinns of Islay S.S.S.I. Scotland. New Phytologist 128, 749–69. Edwards, K.J. and Rowntree, K.M. 1980: Radiocarbon and palaeoenvironmental evidence for changing rates of erosion at a Flandrian stage site in Scotland. In Cullingford, R.A., Davidson, D.A. and Lewin, J., editors, Timescales in geomorphology, Chichester: John Wiley, 207–23. Edwards, K.J. and Whittington, G. 1994: Lateglacial pollen sites in the Western Isles of Scotland. Scottish Geographical Magazine 110, 33–39. Entwistle, J.A., Lamb, H.F., Abrahams, P.W. and Dodgshon, R.A. 1995: A lake-sediment record of changing land-use intensity on the Isle of Lewis, Scotland. In Butlin, R.A. and Roberts, N., editors, Ecological relations in historical times, Oxford: Blackwell, 45–67. Flenley, J.R. and Pearson, M.C. 1967: Pollen analysis of a peat from the island of Canna (Inner Hebrides). New Phytologist 66, 229–306. Fossitt, J.A. 1996: Late Quaternary vegetation history of the Western Isles of Scotland. New Phytologist 132, 171–96. Gearey, B. and Gilbertson, D.D. 1997: Pollen taphonomy of trees in a windy climate: Northbay Plantation, Barra, Outer Hebrides. Scottish Geographical Magazine 113, 113–20. Gilbertson, D.D., Kent, M. and Grattan, J.P., editors, 1996: The Outer Hebrides: the last 14,000 years. Sheffield: Sheffield Academic Press. Gilbertson, D.D., Kent, M., Weaver, R.E., Wathern, P., Mulder, Y.L., Brayshay, B.A. and Gill, W.J. 1995: The vegetation of the Tangaval Peninsula: present and past. In Branigan, K. and Foster, P., editors, Barra: archaeological research on Ben Tangaval. Sheffield: Sheffield Academic Press, 15–30. Grattan, J.P., Gilbertson, D.D. and Pyatt, F.B. 1996: Geochemical investigations of environmental change in the Outer Hebrides. In Gilbertson, D.D., Kent, M. and Grattan, J.P., editors, The Outer Hebrides: the last 14,000 years, Sheffield: Sheffield Academic Press, 27–43. Grimm, E. 1991: TILIA and TILIA.GRAPH. Springfield: Illinois State Museum. Harkness, D.D. 1983. The extent of 14C deficiency in the coastal environement of the United Kingdom. 14C and Archaeology, Groningen, 351–64. Hudson, G.W. 1991: Geomorphology and soils of the Outer Hebrides. In Pankhurst, R.J. and Mullin, J.M., editors, Flora of the Outer Hebrides, London: Natural History Museum Publications, 19–27. Hudson, G.W, Towers, G., Bibby, J.S. and Henderson, D.J. 1982: Soil and land capability for agriculture: the Outer Hebrides. Aberdeen: Macaulay Land Research Institute. Kent, M., Brayshay, B.A., Gilbertson, D.D., Wathern, P. and Weaver, R.E. 1994: A biogeographical study of plant communities and environmental gradients on South Uist, Outer Hebrides, Scotland. Scottish Geographical Magazine 110, 85–99. Lamb, H.H. 1995: Climate, history and the modern world (second edition). London, Routledge. McVean, D.N. and Ratcliffe, D.A. 1962. Plant communities of the Scottish Highlands. London: HMSO, Nature Conservancy Monograph No. 1. Mills, C.M., Crone, A., Edwards, K.J. and Whittington, G. 1994: The excavation and environmental significance of a sub-peat stone bank near
108 The Holocene 10 (2000) Loch Portain, North Uist, Outer Hebrides. Proceedings of the Society of Antiquaries of Scotland 124, 155–71. Moore, P.D., Webb, J.A. and Collinson, M.E. 1991: Pollen analysis (second edition). Oxford: Blackwell. Newell, P.J. 1989: A buried wall in peatland by Sheshader, Isle of Lewis. Proceedings of the Society of Antiquaries of Scotland 118, 79–93. O’Connell, M. 1990: Early land use in north-east County Mayo: the palaeoecological evidence. Proceedings of the Royal Irish Academy C90, 259–79. Peacock, J.D. 1984: Quaternary Geology of the Outer Hebrides. Report
of the British Geological Survey 16, 1–26. Pratt, K.E. 1992: The vegetational history of the Upper Borve Valley, Barra, Outer Hebrides. Unpublished MSc dissertation, University of Sheffield. Pyatt, F.B., Grattan, J.P., Lacy, D., Gilbertson, D.D., Brayshay, B.A. and Wadsworth, W.A. 1995: Exploratory studies of loch-sediment chemistry and soil history from a 10,000+ year-old core from Loch Hellisdale, South Uist, Outer Hebrides. Scottish Geographical Magazine 111, 106–12. Randall, R.E., Andrew, R. and West, R.G. 1986: Pollen catchment in relation to local vegetation: Ceann Ear, Monach Isles N.N.R., Outer Hebrides. New Phytologist 104, 271–310. Stace, C. 1991: New flora of the British Isles. Cambridge: Cambridge University Press. Taylor, R.E. 1987: Radiocarbon dating: an archaeological perspective. London: Academic Press. Thompson, R. and Oldfield, F. 1986: Environmental magnetism. London: Allen and Unwin. Walker, M.J.C., Ballantyne, C.K., Lowe, J.J. and Sutherland, D.G. 1988: A reinterpretation of the Lateglacial environmental history of the Isle of Skye, Inner Hebrides, Scotland. Journal of Quaternary Science 3, 135–46. Weaver, R.E., Kent, M., Gilbertson, D., Wathern, P. and Brayshay, B.A. 1996: The acidic and upland vegetation of the southern Outer Hebrides. In Gilbertson, D.D., Kent, M. and Grattan, J.P., editors, The Outer Hebrides: the last 14,000 years, Sheffield: Sheffield Academic Press, 147–62. Whittington, G. and Edwards, K.J. 1997: Evolution of a machair landscape: pollen and related studies from Benbecula, Outer Hebrides, Scotland. Transactions of the Royal Society of Edinburgh: Earth Sciences 87, 515–31. Wilkins, D.A. 1984: The Flandrian woods of Lewis (Scotland). Journal of Ecology 72, 251–58. Wilson, P. and Hegarty, C. 1993: Morphology and causes of recent peat slides on Skerry Hill, Co. Antrim, Northern Ireland. Earth Surface Processes and Landforms 18, 593–601. Wilson, P., Griffiths, D. and Carter, C. 1996: Characteristics, impacts and causes of the Carntogher bog-flow, Sperrin Mountains, Northern Ireland. Scottish Geographical Magazine 112, 39–46.
