Age and significance of aeolian sediment reworking on high plateaux in the Scottish Highlands

Holocene aeolian sand sheets at the margins of high-level deflation surfaces in the Scottish Highlands commonly comprise two units: a lower weathered unit representing slow accumulation throughout much or all of the Holocene, and an upper unit of structureless sand that is inferred to represent recent erosion of aeolian deposits and aeolisols from adjacent plateaux. OSL dating of samples from above and below the upper-lower unit contact at three sites in NW Scotland places the onset of upper unit sediment accumulation within the interval AD 1550-1700. Accumulation rates calculated from OSL ages confirm rapid accumulation of upper-unit deposits. The timing of the onset of upper-unit sand accumulation excludes expansion of sheep grazing, intrinsic instability and atmospheric pollution as triggers of plateau-surface vegetation degradation and consequent erosion, but favours climatic deterioration (increased wind stress and possibly prolonged snow-lie) during the 'Little Ice Age' as the likely cause. This explanation does not, however, apply in all cases; at an OSL-dated site in the Grampian Highlands upper-unit aeolian sand accumulation commenced around AD 1900.