Landscape evolution and holocene climate change in mountain areas of the northern Highlands, Scotland.

Abstract

Holocene landscape evolution in the tectonically quiet mountain areas of the Northern Highlands of Scotland has been attributed largely to postglacial relaxation, which has left a legacy of stable, relict landscapes, disturbed only by intrinsic local response, and modified to an uncertain extent by human activity. A review of this model was prompted by improved understanding of a) the variability of the Holocene climate in mid latitudes, b) the responsiveness of some geological and geomorphological systems to low amplitude climate fluctuations, and c) a small number of field studies from the region, reporting mid and late Holocene slope mass movement unrelated to anthropogenic impact. Sixteen catchments were explored using fieldwork and aerial photographic analysis. Slope activity since the end of the last glacial was investigated at five sites which contained evidence of long sequence, shallow slope failure, gully transport of slope debris, and debris fan formation. At two of these, stratigraphic sections, together with sediment and facies analysis, were combined with radiocarbon dating, in order to elucidate slope processes and constmct a chronostratigraphy. Results confirmed widespread Holocene lower slope re-organisation, with mid and late Holocene landscape rejuvenation occurring millennia after apparent adjustment to postglacial conditions at the two dated localities. Mass movement on slopes was found to have parallels in floodplain aggradation and incision. These transformations appear to have operated on several different time scales, and across a strong regional precipitation gradient. Since they are a function of the glacial inheritance of these landscapes, the potential for further transformations exists. Mid and late Holocene events are only poorly accounted for by paraglacial relaxation. A more robust model of landscape evolution in this setting, incorporates climate change (specifically, precipitation shifts) - interacting with progressive weathering and vegetation cover - as a critical environmental variable. Although no justification was found for the use of dated slope mass movements as palaeoclimate proxies, changes in event frequency on a time scale of 10^ years may contain a climatic signal.