Allochthonous and autochthonous mire deposits, slope instability and palaeoenvironmental investigations in the Borve Valley, Barra, Outer Hebrides, Scotland

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.

Using palaeoecology to advise peatland conservation: An example from West Arkengarthdale, Yorkshire, UK

“…For example, Ashmore et al (2000) theorised that an inverted age-depth model from Borve Valley, Barra, Outer Hebrides may have been caused by 'old' carbon being incorporated as a result of hypothesized ancient transporting events such as rapid mass-movement or fluvial activity. However, the lithological evidence does not suggest that this has taken place at West Arkengarthdale.…”

Erosional Processes and Sediment Transport in Upland Mires

“…Johnson and Dunham (1963) report extensive clay laminae in the lower peats at Moor House associated with colluvial delivery of mineral material to incipient blanket peat downslope. In some locations mires continue to trap mineral sediment periodically; for example, Ashmore et al (2000) report major mineral horizons within blanket bog stratigraphies on the Hebrides caused by upslope mass movement events.…”

Surface pollen‐vegetation relationships on the Atlantic seaboard: South Uist, Scotland

“…In particular, the nature and extent of Holocene woodland has intrigued researchers since the initial work by Blackburn (1946), Heslop‐Harrison & Blackburn (1946) and Birks & Madsen (1979), which emphasized the supposedly relatively treeless nature of Outer Hebridean Holocene landscapes. More recent studies ( Ashmore et al. , 2000; Bohncke, 1988; Bennett et al.…”

“…, 1996 ). Palynological records from the peat and lake deposits on the islands of the Outer Hebrides ( Ashmore et al. , 2000; Bennett et al.…”

“…These are strongly differentiated by the acid/alkaline environmental gradients that occur between the west coast sand dune machair and the blacklands/peatlands of the island interior and east coast (Kent et al , 1994;Gilbertson et al , 1996). Palynological records from the peat and lake deposits on the islands of the Outer Hebrides (Ashmore et al , 2000;Bennett et al , 1990;Brayshay, 1992;Fossitt, 1996) demonstrate a complex history of Holocene vegetational and environmental change, culminating in the present-day pattern of plant communities. In particular, the nature and extent of Holocene woodland has intrigued researchers since the initial work by Blackburn (1946), Heslop-Harrison & Blackburn (1946) and Birks & Madsen (1979), which emphasized the supposedly relatively treeless nature of Outer Hebridean Holocene landscapes.…”