Evolution of a machair landscape: pollen and related studies from Benbecula, Outer Hebrides, Scotland

Abstract

ABSTRACTLow altitude sandy plains (machair) are a common feature of the Atlantic 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 examination of inter-tidal organic deposits, in so far as these could provide a fossil proxy record of the processes involved in machair formation. Study of a series of inter-tidal peats sites located on a beach platform at Borve, island of Benbecula, provided both spatial and chronological evidence for sand movement as well as furnishing new data on vegetational and environmental history. The pollen diagrams show a flora in whichCalluna vulgaris(heather) and Poaceae (grass) are frequent dominants, while such arbroeal taxa asBetula(birch) andCorylus avellana-type(cf. hazel) are notable. The existence of birch–hazel woodland for the periodc.8855–6190 BP might conceivably have had a retarding effect on sand movement. Later cycles of sand movement would not have met such obstruction with the consequence that sand mobility and machair plain evolution could have been faster. Sand arrival at the seaward site (Borve 3), is dated toc.6735 BP and it continued untilc.6045 BP, after which it slowed before increasing again from around 5990 BP. This sand incursion produced a machair plain over the Borve sites as part of a continuous, but variously paced, marine and aeolian process. The presence of charcoal from c. 6860 BP, with a major increase in charcoal to pollen ratios by 6190 BP, may suggest that natural or 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 destabilisation to a time long before the previously proposed Neolithic period. A wet slack deposit, dated to 3400 ± 70 BP, indicates a time when sand movement overwhelmed the area around the landward site (Borve 1) and when the rate of sand movement was likely to have been impeded. The data from Borve suggest that evolution of machair landforms occurs at a slow, variable rate and that there is considerable long-term stability.