Age and significance of mountain-top detritus

In north-west Scotland, mountain-top detritus forms blockfields or diamicts, depending on lithology. Clast angularity, absence of grussification and transition to underlying rock imply formation by frost-wedging of bedrock. Age is constrained by trimlines and exposure dating of weathering zones. Mountain-top detritus is ubiquitous on nunataks that remained above the level of the last ice sheet, but occurs only on well-jointed rocks in areas exposed to periglacial conditions since ice-sheet downwastage and is absent from areas exposed to weathering only during the Holocene. Most secondary clay minerals are equally represented both above and below a trimline cut by the last ice sheet, indicating formation since deglaciation, though haematite and gibbsite are preferentially represented on former nunataks. The age and significance of mountain-top detritus are determined by lithology and glacial history. On well-jointed rocks, such detritus has developed within a few millennia of exposure to periglacial conditions. On massive lithologies, however, it has formed over much longer timescales on nunataks above the last and possibly earlier ice sheets. In north-east Scotland ancient (possibly pre-Pleistocene) regolith also appears to have survived under a cover of cold-based ice. Use of the distribution of mountain-top detritus in palaeoglaciological reconstructions therefore requires caution. (C) 1998 John Wiley & Sons, Ltd.