Deglaciation of the Dee Valley, N.E. Scotland
The aim of this study was to analyse the pattern of deglaciation in the Dee valley and, in particular, to deduce whether this occurred by active frontal retreat or by areal stagnation and downwasting. This has important implications for palaeo-environmental reconstructions of the Lateglacial period. The main techniques used were morphological mapping and construction of sediment-landform assemblages, facilitated by borehole records of the British Geological Survey. Several sections were available which have not previously been logged. The area of study has been sub-divided into three main sectors: Lower Deeside, Feughside and Upper Deeside. In each case, conclusive evidence was obtained that the last (Devensian) ice-sheet decayed predominantly by active retreat but with a stagnant margin. Linear assemblages of ice-marginal topography frequently indicate a lobate ice-front based in the main Dee valley. The closely-spaced alignment of ridges in these assemblages suggests that they represent stillstands which interrupted retreat of the ice front. The presence of ice in the main Dee valley during deglaciation meant that drainage of tributary valleys was blocked. This resulted in formation of glacial lakes as indicated by glaciolacustrine sediments, deltas and overflow channels. Topography was a major control on the pattern of deglaciation, notably in areas of compressive ice flow where supraglacial sedimentation resulted in hummocky, ice-marginal deposits. These have previously been mis-interpreted as evidence for glacial readvances initiated by climatic change. The sedimentary successions in ice-marginal ridges strongly resemble those recorded from present-day sub-polar glaciers (eg. Svalbard). This implies that deglaciation in NE Scotland occurred whilst the climate was still cold, which is consistent with evidence from the coastal zone indicating that deglaciation was in progress by 15 ka BP. Much of the ice-sheet over NE Scotland therefore disappeared before the global climatic amelioration that occurred at 13.5-13 ka BP.