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Age and significance of mountain‐top detritus
Author(s) -
Ballantyne Colin K.
Publication year - 1998
Publication title -
permafrost and periglacial processes
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.867
H-Index - 76
eISSN - 1099-1530
pISSN - 1045-6740
DOI - 10.1002/(sici)1099-1530(199810/12)9:4<327::aid-ppp298>3.0.co;2-9
Subject(s) - geology , detritus , bedrock , deglaciation , weathering , lithology , ice sheet , residuum , glacial period , ice age , geomorphology , paleontology
Abstract 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. © 1998 John Wiley & Sons, Ltd.