The vertical dimensions of the last ice sheet and late quaternary events in northern Ross-shire, Scotland

Abstract

Despite more than 100 years of research, surprisingly little is known about the precise dimensions of the last ice sheet in Northern Scotland, though recent work has suggested that it may have been much less laterally extensive than was previously assumed. This raises the possibility that some higher mountain summits in this area may have remained as nunataks above the level of the ice sheet at its greatest thickness. Detailed geomorphological mapping of a west-east transect across northern Ross-shire has revealed the existence of a periglacial trimline or 'trimzone' well outside and usually well above the limits of glaciers that developed during the Loch Lomond Stadial of ~11-10 ka. B.P. This trimzone usually consists of a zone of mass-moved frost-weathered detritus that separates in situ mountain-top detritus and frost-weathered bedrock upslope from relatively unweathered ice-scoured bedrock downslope. The trimzone declines in altitude to both the east and west of the present watershed and is interpreted as marking the approximate altitude of a former ice-sheet surface. The degree and depth of per1glacial weathering above the level of the trimzone strongly suggests that it relates to the last Scottish ice sheet at its maximum thickness rather than any subsequent ice-sheet readvance, though the possibility that thin cold-based ice-caps developed locally on plateaux above the level of the ice sheet cannot be excluded. Strong independent support for nunataks is provided by comparison the survival of of the clay mineral content of soils developed above the level of the ice-sheet trimzone and those developed on ice-scoured areas or thin incipient mountain-top detritus below the level of this zone. Clay minerals thought to be 'inherited' from pre-Late Devensian weathering were found to be either significantly more abundant (kaolinite and halloysite) or exclusively developed (gibbsite) above the trimzone, thus indicating that mature in situ mountain-top detritus above the trimzone escaped glaciation throughout the Late Devensian or possibly much longer. Trimzone evidence and evidence for former directions of ice-sheet movement was used to reconstruct the three dimensional form of the last ice sheet in northern Ross-shire. This reconstruction indicated that the former ice-sheet surface rose in altitude from ~600m in the west of the area to in excess of 850m in the Beinn Dearg massif and Fannich Mountains, and declined eastwards to ~700m in the vicinity of Carn Chuinneag and Ben Wyvis. The former ice divide was located close to the present watershed. Finally, re-investigation of the dimensions of Loch Lomond Readvance glaciers in northern Ross-shire has shown that these former glaciers may have been much more extensive than has previously been believed.