GIBBSITIC SOILS ON FORMER NUNATAKS - IMPLICATIONS FOR ICE-SHEET RECONSTRUCTION

X-ray diffraction analyses of soils above and below a periglacial trimline developed across the basalts of the Trotternish Escarpment (Isle of Skye, Scotland) demonstrate that gibbsite is restricted to soils above the trimline. This suggests that the gibbsite is a relict of pre-Late Devensian weathering, and that the trimline did not develop after the last ice sheet achieved its maximum thickness. The sharpness of the boundary between frost-weathered regolith and gibbsitic soils upslope and ice-scoured bedrock associated with gibbsite-free soils downslope suggests that the trimline represents the altitude of the last ice sheet at its maximum thickness rather than a former boundary between passive cold-based ice and erosive warm-based ice. These findings illustrate how identification of high-level periglacial trimlines and associated contrasts in clay mineralogy provide a means for constraining reconstructions of the form of the last ice sheets.