After the ice: Holocene geomorphic activity in the Scottish Highlands

A rich variety of processes have modified the glacial landscape of the Scottish Highlands since the last glaciers disappeared 11,500 years ago. Many of these processes can be described as paraglacial (glacially-conditioned): retreat of glacier ice has resulted in exposure of metastable sediment sources (rock slopes, drift-mantled slopes and valley-floor glacigenic deposits) that have been reworked over a range of timescales. Much of the reworked sediment has been deposited in paraglacial sediment stores (talus accumulations, debris cones, alluvial fans, valley fills, river terraces and deltas). The trajectory of paraglacial sediment transfer can be approximated by an exhaustion model. A corollary of this model is that all sediment stores eventually undergo a change from net accumulation to net erosion, reflecting the non-renewable nature of sediment sources. This concept helps to explain why most talus accumulations, debris cones and alluvial fans in the Highlands are now relict, and why many floodplains appear to have experienced an initial period of aggradation followed by later river incision and terrace development. Since deglaciation, rock slopes have experienced paraglacial stress release due to differential deglacial unloading. Many rock slopes have remained stable or experienced gradual adjustment through intermittent rockfall. At over 600 sites, however, large-scale rock-slope failure has occurred, either in the form of catastrophic failure with complete or arrested debris runout, or as major slope deformations. Modification of drift-mantled slopes has been dominated by rainstorm-generated debris flows activity, which have stripped sediment from slopes and redeposited it in debris cones. Radiocarbon dating of buried organic horizons shows that debris flows have occurred intermittently throughout the Holocene. Snow avalanches have played only a localized role in paraglacial sediment transfer. The Holocene alluvial history of the Highlands is poorly understood. Some alluvial fans formed during a few exceptional flood events, and there is evidence that postglacial floodplain aggradation peaked in the late Holocene and was succeeded by floodplain incision and terrace development, possibly reflecting upstream reduction in paraglacial sediment supply. Other processes have operated independently of glacial conditioning. The effects of wind action are dominant in the coastal zone, forming dunefields through aeolian reworking of beach sand, and on plateaux in the form of deflation surfaces and sand sheets. Extensive erosion of plateau-top sands and aeolisols occurred in the period AD 1550-1700, a period of exceptional storminess. Small-scale periglacial processes have also been active on high ground throughout the Holocene, including granular weathering of exposed rock surfaces, the development of small-scale frost-sorted patterned ground on unvegetated terrain, and slow downslope movement of solifluction sheets, solifluction lobes and ploughing boulders.