Storglaciären is a small valley glacier in the Kebnekaise range, northern Sweden. Storglaciären has an area of 3.1km², a volume of 0.31km³ and elevation range 1135 to 1710m (Holmlund and Jansson, 1999). There is a long history of glaciological work on Storglaciären and surrounding glaciers included in the ‘Tarfala Mass Balance Programme’ (Holmlund and Janson, 1999). Mass balance work on Storglaciären provides a continuous winter, summer and net balance record from 1945 to present. Storglaciären has undergone retreat during most of the twentieth century in common with many Scandinavian and Svalbard glaciers. Retreat of around 750m has occurred since the Holocene maximum in response to climatic warming around 1910 to 1920. Mean annual temperature at the Tarfala Research Station, below Storglaciären at 1130m, is –4.0ºC and precipitation is around 1000mm year^–1 (Holmlund et al., 1996). The thermal regime of Storglaciären is complex with internal warm based ice and a 30 to 40m cold surface layer, which results in a 100 to 200m wide cold based zone in the thin glacier snout (Holmlund et al., 1996).

The supraglacial debris structures on the snout of Storglaciären consist of ridges longitudinal to glacier flow that are around 2m to 10m length with rectilinear proximal slopes and more irregular slumped distal slopes (figure 1). The ridges show relief of around 0.7 to 1.5m predominantly resulting from glacier ice with a thin debris cover. The proximal slopes consist of a thin (less than 10cm) band of sediment and the distal slopes show active slumping of the thin sediment cover (less than 5cm) (figure 2). The sediment character is texturally a clast rich intermediate diamicton (using the Hambrey, 1994 classification for poorly sorted sediments). Clasts vary in size from pebble to cobble and are predominantly subangular and subrounded with occasional faceted and striated clasts.

Figure 1 Photograph showing supraglacial debris ridges on the snout of Storglaciären (Photo: J Moore).

Figure 2 Photograph of a section excavated through a supraglacial debris ridge (Photo: J Moore).

Basic observations of sediment texture, clast roundness and clast surface features indicate evidence of active subglacial transport. The height of the ridges is generated by the restricted ablation of glacier ice under the debris band in comparison to the surrounding debris free ice. These supraglacial debris features at Storglaciären are analogous to debris rich englacial thrusts found a many Svalbard ice margins. Neoglacial moraine-mound complexes at some Svalbard ice margins have been related to development by melt out of thick debris rich thrusts (Hambrey and Huddart, 1995; Huddart and Hambrey, 1996; Bennett et al., 1996a, b; Hambrey et al., 1997; Bennett et al., 1998; Bennett et al., 1999; Hambrey et al., 1999). The Svalbard model of moraine-mound formation is also being applied to some British Younger Dryas ‘hummocky moraine’ sites (Hambrey et al., 1997; Bennett et al., 1998; Graham and Midgley, 2000). Debris rich thrusts develop through strong longitudinal compression of the glacier, which can be associated with a steep reverse bedrock slope, surge activity and a polythermal glacial regime. The supragalcial debris features at Storglaciären are interpreted as resulting from thrusting of subglacial sediment caused by strong longitudinal compression associated with the polythermal glacial regime.

The process of initial sediment incorporation and subsequent elevation can be attributed exclusively to a thrust process although many mechanisms of sediment incorporation to a basal ice layer are recognised. The process of regelation, involving localised melting upglacier of an obstacle and subsequent refreezing (Weertman, 1964), can incorporate sediment. This process is of very limited importance in terms of quantity of sediment incorporated, as regelation ice thickness is not usually more than a few centimetres (Hubbard and Sharp, 1989, 1993). A process of regelation into basal sediments has also been described (Iverson, 1993; Iverson and Semmens, 1995; Iverson and Souchez, 1996). Predicted penetration of ice into sediment range from millimetres to decimetres (Iverson, 2000). The process of net freeze-on by conductive cooling appears effective when winter penetration of a cold front reaches basal sediments through thin ice fronts. As well as temporal variation, spatial variation in thermal conditions, as found in polythermal glaciers, may effectively incorporate sediment (Weertman, 1961). Periodic migration of the thermal boundary may potentially develop multiple stacked sequences (Weertman, 1961). Such thickening processes are important for subsequent landform development. The net freeze-on porcess may however be limited by the ability of the refreezing ice to incorporate sediment due to segregation (Alley et al., 1997). Net freeze-on by glaciohydraulic supercooling has also been shown to form stratified debris-laden basal ice (Lawson et al., 1998; Alley et al., 1998). Shear planes and folds have been documented as capable of sediment incorporation (Clarke and Blake, 1991; Sharp et al., 1994; Glasser et al., 1998). The thrusting and folding process is of particular importance because of the elevation of material above the bed and the potential for thickening by folding of incorporated material.

Supraglacial debris structures, resulting from englacial thrusting of subglacial sediment, show the importance of glacial deformation in debris entrainment and elevation at Storglaciären. Whilst englacial thrusting can account for both sediment incorporation and elevation, a number of processes may contribute to initial sediment incorporation. The englacial thrusting is associated with strong longitudinal compression caused by the polythermal regime. The preservation potential of these supraglacial debris features in terms of landform development is limited by the thin nature of the incorporated sediment band. This work is important because it adds to data on glacial deformation and landform development from ice margins in Svalbard. A better understanding of modern glacial deformation processes and the landforms it can develop aid the successful interpretation of former glacial regimes. Such models of moraine-mound development, which include the role of glacial deformation, are now being applied to the British Younger Dryas.

Fieldwork at Storglaciären was funded by a European Union GlacioEuroLab6 grant. The organisation of the course, undertaken by John Moore, is gratefully acknowledged.