loch lomond stadial                                             

Lateglacial on Orkney

Definition: the period between deglaciation and the start of the Holocene.

The Lateglacial Interstadial was a period of mild climate which began around 13,000 radiocarbon years ago. The Greenland ice core data indicate that the amelioration was remarkably rapid, possibly occurring within 50 years. As the oceanic polar front in the North Atlantic migrated to the north of Scotland, North Atlantic Drift waters reached the Scottish coast and both marine and atmospheric temperatures rose to close to present values (Sutherland, 1993). Marine sediments show that a weak North Atlantic Drift became established off Western Scotland and in the Norwegian and North Sea by 12,800 radiocarbon years BP. On Orkney, the oldest radiocarbon dates from lake basins lie at around this date. If these radiocarbon dates are correct and not affected by hard water errors then Orkney must have been largely free of glacier ice by 15.814.4 cal ka.

This rapid change in climate was not immediately registered in the nature of the vegetation. At Loch of Winless, Caithness, the Lateglacial vegetation was grassland and tall-herb communities, with abundant Cyperaceae and a variety of open ground herbs (Peglar, 1979; Birks, 1993). In west Mainland, a sparse vegetation of dwarf shrub heath and arctic-alpine communities developed (Bunting, 1994). Towards the end of the Interstadial (between ca. 11.5 and 11.14 14C ka) there was a decline in temperature which heralded the onset of the Loch Lomond Stadial. After 11 14C ka the climatic decline intensified and arctic conditions became established. The Loch Lomond Stadial in Scotland saw renewed glacier growth, the return of tundra plant communities, widespread  slope instability and erosion and the return of polar waters. On Orkney, the end moraines at Enegars Corrie and at Dwarfie Hamars mark the formation of corrie glaciers at this time. The associated equilibrium line altitudes are as low as 150 m OD. These  glaciers therefore formed  at some of the lowest elevations in Scotland and point to the severity of the Stadial climate. In Caithness, there was widespread solifluction of glacial deposits and frost shattering during the Loch Lomond Stadial (Hall, 1989). The contrast between the degree of development of scree slopes within and beyond the moraine in Enegars Corrie suggest that widespread  frost action and mass movement occurred on Hoy at this time (Sutherland, 1993). On West Mainland erosion of soils around small basins increased after 11 14C ka and the vegetation became sparser and more herb-dominated, with species such as Artemisia indicating disturbed surface conditions (Bunting, 1994). After 10 14C ka however grassland communities returned and rates of erosion declined at the start of the Holocene period.