EVOLUTION OF A MACHAIR LANDSCAPE - POLLEN AND RELATED STUDIES FROM BENBECULA, OUTER-HEBRIDES, SCOTLAND

Low altitude sandy plains (machair) are a common feature of the Atlant ic coasts of the Outer Hebrides. They formed as a result of shoreward movement of material consequent upon a rise in Holocene sea levels. A number of earlier, partially inconclusive, investigations into machair evolution proposed that fuller understanding could arise from an exam ination of inter-tidal organic deposits, in so far as these could prov ide a fossil proxy record of the processes involved in machair formati on. Study of a series of inter-tidal peats sites located on a beach pl atform at Borve, island of Benbecula, provided both spatial and chrono logical evidence for sand movement as well as furnishing new data on v egetational and environmental history. The pollen diagrams show a flor a in which Calluna vulgaris is (heather) and Poaceae (grass) are frequ ent dominants, while such arbroeal taxa as Betula (birch) and Corylus avellana-type (cf. hazel) are notable. The existence of birch-hazel wo odland for the period c. 8855-6190 BP might conceivably have had a ret arding effect on sand movement. Later cycles of sand movement would no t have met such obstruction with the consequence that sand mobility an d machair plain evolution could have been faster. Sand arrival at the seaward site (Borve 3), is dated to c. 6735 BP and it continued until c. 6045 BP, after which it slowed before increasing again from around 5990 BP. This sand incursion produced a machair plain over the Borve s ites as part of a continuous, but variously paced, marine and aeolian process. The presence of charcoal from c. 6860 BP, with a major increa se in charcoal to pollen ratios by 6190 BP, may suggest that natural o r intentional burning of the vegetation cover of the machair occurred in Mesolithic times; if the burning was anthropogenic in origin, then it pushes the possible involvement of humans in machair destabilisatio n to a time long before the previously proposed Neolithic period. A we t slack deposit, dated to 3400 +/- 70 BP, indicates a time when sand m ovement overwhelmed the area around the landward site (Borve 1) and wh en the rate of sand movement was likely to have been impeded. The data from Borve suggest that evolution of machair landforms occurs at a sl ow, variable rate and that there is considerable long-term stability.