Reconstructing the basal thermal regime of an ice stream in a landscape of selective linear erosion: Glen Avon, Cairngorm Mountains, Scotland ADRIAN M. HALL AND NEIL F. GLASSER Hall, A. M. & Glasser, N. F. 2003 (March): Reconstructing the basal thermal regime of an ice stream in a landscape of selective linear erosion: Glen Avon, Cairngorm Mountains, Scotland. Boreas, Vol. 32, pp. 191– 207. Oslo. ISSN 0300-9483. The Cairngorm Mountain area of Scotland is a classic example of a landscape of selective linear glacial erosion, with sharp contrasts in the intensity of glacial erosion between the deeply incised troughs and valleys and the undulating high plateau. This article examines the Quaternary development of Glen Avon, a 200 m deep glacial trough set within the high plateau of the mountains. Evidence concerning the aggregate basal thermal regimes of the topographically controlled ice streams that formerly developed in this area is reconstructed from the geomorphological record, including bedforms indicative of wet-based, sliding ice and of dry-based ice frozen to its bed. This mapping indicates that basal sliding was not confined exclusively to the troughs but extended towards valley heads and on to parts of the plateau adjacent to troughs. The extent of basal sliding appears to have been greatest beneath pre-Late Devensian ice sheets. Basal ice temperatures are modelled under steady-state conditions for the last ice sheet at c. 18 ka BP. Basal thermal regimes are predicted using a reconstruction of the preglacial relief and for the current topography of the area. Convergent flow of ice through the preglacial valley system appears to have been sufficient to induce basal melting and therefore to initiate valley deepening. This effect is enhanced when the model is run across the present topography. Comparison of results of the geomorphological mapping and the modelling reveals significant differences between the actual and predicted extent of basal sliding outside the main ice stream. The overall conclusion is that many ice streams in mountainous terrain are inherited from the locations of preglacial valleys, which serve to accelerate ice flow and promote frictional heating beneath ice sheets. Adrian M. Hall, Department of Geography, University of Edinburgh, Drummond Street, Edinburgh EH8 9XP, UK; Neil F. Glasser (e-mail: nfg@aber.ac.uk), Centre for Glaciology, Institute of Geography and Earth Sciences, University of Wales, Aberystwyth SY23 3DB, UK; received 19th November 2001, accepted 10th July 2002. Ice streams are regions in a grounded ice sheet that flow much faster than the surrounding ice. These features have traditionally been subdivided into two types: ‘topographic’ ice streams, which lie in bedrock troughs and whose existence is largely controlled by the underlying bedrock topography, and ‘pure’ ice streams that do not lie in bedrock troughs (Stokes & Clark 1999). Pure ice streams can develop over areas of deformable sediment (e.g. Marshall et al. 1996) or due to the self-organization of ice flow as the temperature field in an ice sheet evolves through time (Payne & Donglemans 1997; Payne & Baldwin 1999). Most recent research has concentrated on establishing the dynamics of pure ice streams in the modern glacial environment (Blankenship et al. 1986; Anandakrishnan et al. 1998; Bell et al. 1998), and on the reconstruction of palaeo-ice streams (Patterson 1998; Knight et al. 1999; Stokes & Clark 1999, 2001). In contrast, there have been relatively few studies of the development and glaciological significance of topographically con- trolled palaeo-ice streams (Evans 1996; Lian & Hicock 2000). Topographically controlled ice streams are especially important in mountainous terrain, where they control to a great extent basal thermal conditions and therefore the preservation or erosion of relict glacial and non-glacial landforms and landscapes (Kleman 1992; Kleman & Borgstro ¨m 1994). Glaciologically, the continuum between protection and erosion is a function of the extent of former frozen-bed (protective) and thawed- bed (erosive) conditions beneath former ice sheets. The controls on the locations of areas of frozen-bed conditions within large ice sheets are strongly scale- dependent (Kleman et al. 1999). At the ice sheet scale (10 3 km), the location of frozen-bed areas is a function of dispersal centre location. Ice divides are character- ized by low ice velocities and are commonly areas of high bed elevation and therefore limited ice thickness. At the mesoscale (10 2 km) the pattern of basal thermal regime is controlled by headward ice-stream erosion. Topographic control is limited, with the locations of frozen-bed and thawed-bed areas determined princi- pally by the flow patterns that develop within ice sheets at this scale (Payne & Baldwin 1999). Topographic control becomes important at the kilometre scale, especially within mountainous or dissected terrain, where topographically controlled ice streams may develop, creating sharp contrasts in basal thermal regime over small distances (Glasser 1995). The location of zones of frozen-bed and thawed-bed conditions beneath former mid-latitude ice sheets in North America and Scandinavia are now reasonably well known at the 10 2 –10 3 km scale (Sugden 1977; Dyke 1993; Kleman & Stroeven 1997; Kleman & DOI 10.1080/03009480310001100. # 2003 Taylor & Francis