Introduction and Rationale

Glacier surging represents a cyclic flow instability that is triggered from within the glacier system rather than by external climate forcing. The active phase of a surge involves the transfer of ice from a reservoir area to the snout of a glacier and can produce ice flow velocities up to one thousand times the flow rate of intervening non-surge phases (Clarke et al., 1984; Raymond, 1987). This may result in the rapid advance of the glacier front and a concomitant thinning of the reservoir area. Between surges, periods of slow flow, or quiescent phases, are characterized by snout stagnation and ice build up in the reservoir area. Although individual surging glaciers display uniform return periods there are large variations between glaciers and regions (Post, 1969; Clarke et al., 1986; Dowdeswell et al., 1991; Hamilton and Dowdeswell, 1996). A climatic linkage to surging was modelled by Budd (1975), who suggested that a continuously fast-flowing glacier is capable of discharging its annual mass balance, whereas a surging glacier has a total mass throughput that is too small to sustain fast flow but too large to be discharged by slow flow alone, thereby initiating a regular surging cycle. As the reservoir zone builds up, a thermal boundary may exist at the down-glacier end, and water storage increases at the bed. Thus, it should be noted that, while there is a climatic linkage, ultimately surging is the result of oscillations in the internal dynamics of the glacier. Specifically, the large changes in glacier velocity are driven by reorganizations in the subglacial drainage system (Clarke et al., 1984; Kamb et al., 1985; Clarke, 1987; Fowler, 1987; Kamb, 1987). In Iceland, surging is likely associated with geothermal activity as this can allow cyclical build up and release of large subglacial meltwater reservoirs (Bjornsson, 1975, 1992). However, many of the surging glaciers may be predisposed to this type of behaviour. For example jokulhlaups drain regularly from Grimsvotn lake out beneath Skiedararjokull (intervals of 1—10 years) but these do not induce the glacier to surge, an event which occurs independently (Bjornsson, 1998). Regardless of the trigger mechanism, the landform-sediment assemblages produced by surging glaciers appear to be consistent and predictable. Significantly, moraines deposited by surging glacier margins cannot be modelled as the products of steady-state glaciers in equilibrium with climate. Moreover, the identification of former surging within Pleistocene ice sheets remains a problem area that glacial geologists continue to tackle through the identification of diagnostic palaeo-surge signatures in the geomorphological, sedimentological and stratigraphical record.

Contemporary surging glaciers are instantly recognizable by their surface forms. For example, prior to the surge there may be surface bulging associated with the filling of the reservoir area which may be coincident with a thermal boundary (e.g. Trapridge Glacier). Associated with the passage of the surge front is extensive crevassing, thrusting and folding of the glacier surface, and the formation of looped medial moraines (Meier and Post, 1969; Clarke et al., 1984; Raymond et al., 1987; Clarke and Blake, 1991). Once such features have melted out on the glacier surface and become more subdued, or when the entire glacier has gone, the identification of palaeo-surges becomes problematic. Addressing this problem requires systematic investigations of the marginal areas of known surging glaciers and the integration of the landforms, sediments and stratigraphy into a diagnostic landsystems model. Previous attempts to identify the landform-sediment assemblages of Pleistocene ice sheet surging have been piecemeal, employing various combinations of diagnostic geomorphological and sedimentological criteria. For example, Clayton et al. (1985) identified extensive tracts of hummocky moraine as characteristic features of surging margins in the southwestern part of the Laurentide Ice Sheet. Dredge and Cowan (1989b) proposed that inset fluting fields terminating at major moraines were the product of palaeo-surges on the Canadian shield. Eyles et al. (1994) interpreted till diapirs as ridges deformed upward below masses of dead ice in shallow water following onshore surging of an ice margin in east Yorkshire, England. Because such studies have focused on a limited selection of landforms and sediments, they are too restricted to use 'globally' for the identification of palaeo-surging. Diagnostic landforms from contemporary surging glaciers have been previously identified by Sharp (1985a, b; thrust moraines, crevasse-fill ridges, flutings and hummocky moraine), Croot (1988a, b; thrust moraines) and Knudsen (1995; concertina eskers). However, individually these models are not comprehensive and, therefore, a surging glacier landsystems model is presented here, comprising a suite of landform-sediment associations believed to be diagnostic of glacier surging (cf. Evans and Rea, 1999).

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