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. © 1998 John Wiley & Sons, Ltd. Dans l'Ecosse du Nord-Ouest, les sommets des montagnes sont recouverts de champs de blocs et de “diamictons” qui varient selon la lithologie. L'angularite des debris, l'absence de debris fins et la transition de ces depots avec le bedrock sous-jacent indiquent qu'ils sont dus a l'action du gel. Leur âge doit etre considere en relation avec les limites de la glaciation derniere et de la datation cosmogenetique. Les couvertures de blocs existent partout sur les nunataks de la derniere calotte glaciaire; elles se trouvent seulement sur des roches presentant des joints marques dans des regions exposees aux conditions periglaciaires depuis la disparition de la calotte glaciaire. Elles sont absentes dans les regions exposees a l'alteration depuis seulement l'Holocene. La majorite des mineraux argileux secondaires sont egalement representes a la fois au-dessus et au-dessous de la limite qui a ete recoupee par la derniere calotte glaciaire, ce qui indique qu'ils ont ete formes apres la deglaciation, bien que l'hematite et la gibbsite soient preferentiellement presentes sur les anciens nunataks. L'âge et la signification des couvertures de blocs des sommets des montagnes sont determines par la lithologie et l'histoire glaciaire. Sur les roches bien fissurees, de tels champs de blocs se sont developpes en quelques millenaires d'exposition a des conditions periglaciaires. Toutefois sur des lithologies massives, ils ont demande pour apparaotre des periodes beaucoup plus longues, et se sont formes sur des nunataks qui ont existe pendant la derniere periode glaciaire mais aussi peut-etre pendant des glaciations plus anciennes. Dans l'Ecosse du Nord-Est, une ancienne regolithe (peut-etre pre-Pleistocene) a survecu sous une couverture de glace basale froide. L'utilisation pour des reconstructions paleoclimatiques de la distribution des couvertures de blocs observees au sommet des montagnes demande de ce fait de la prudence. Copyright © 1998 John Wiley & Sons, Ltd.

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