Geomorphological features of mass wastage and periglacial phenomena, Cainozoic of north-east Scotland
|Merritt, J W, Auton, C A, Connell, E R, Hall, A M, and Peacock, J D. 2003. Cainozoic geology and landscape evolution of north-east Scotland. Memoir of the British Geological Survey, sheets 66E, 67, 76E, 77, 86E, 87W, 87E, 95, 96W, 96E and 97 (Scotland).
Contributors: J F Aitken, D F Ball, D Gould, J D Hansom, R Holmes, R M W Musson and M A Paul.
A wide range of phenomena that formed in former cold, periglacial environments has been recognised across the district (Galloway, 1958; FitzPatrick, 1956, 1963, 1969, 1987). Many elements of the present landscape are essentially relict periglacial landforms. The main effect of periglacial mass wasting has involved the downslope movement of till deposits and regolith and the subsequent accumulation of ‘head’ deposits as spreads on lower slopes and valley floors. These processes have created the ubiquitous rounded hill crests of the region with valley sides that are commonly convexo-concave in profile (Ballantyne and Harris, 1994). Solifluction terraces are very common, as is frost shattered bedrock and structures indicative of the former presence of ground ice, notably cryoturbation structures, ice-wedge casts and tundra polygons (P104116). Although mostly occurring at the surface, features such as these have been identified at several stratigraphical positions within the Pleistocene sequence. Aeolian periglacial features such as ventifacts, ‘cover sands’ and loess are likely to occur, but apparently have not been reported.
Intense periglacial activity last occurred during the Loch Lomond Stadial. Marked slope instability, with destruction of soils and remobilisation of diamictons by gelifluction, led to slope-foot accumulations of significant thickness, locally burying organic sediments of Windermere Interstadial age. It is very likely that permafrost had been established previously during ice-sheet deglaciation, because ice-wedge casts and polygon networks affect the sediments of low river terraces in the lower Ythan and Ugie valleys (Clapperton and Sugden, 1977; Gemmell and Ralston, 1984; Armstrong and Paterson, 1985). These sediments were formed after the retreat of the coastal ice that laid down the Logie-Buchan Drift Group, possibly during a subsequent intensely cold period late in the Main Late Devensian glaciation (Ugie Valley).
Earth-pillars have developed on the slopes of several valleys in the district from spurs and ribs of material containing durable clasts in a friable, easily eroded matrix. Large boulders cap these features, which have been eroded mainly by rain-wash on steep slopes. Good examples occur on the eastern side of the valley of the River Spey near Fochabers at Craigs of Cuildell (NJ 332 553) (P915371), where they have been carved out of weathered conglomerate (P002815).