Sedimentation Poots

Their crenulate, lobate or saw-tooth plan-forms mimic the indentations of the snout margin produced by the melting back of longitudinal crevasses (Price, 1970; Matthews et al., 1979). At some locations push moraines may be coalescent or partially superimposed, indicating that one winter readvance was more extensive than that of the previous year. A stationary glacier snout may construct larger and more complex moraine ridges in this fashion (Krüger, 1985, 1993).

The local characteristics of push moraines indicate that most are produced either by ice-marginal squeezing of water-soaked subglacial sediment (e.g. Price, 1970) or pushing of proglacial materials (Howarth, 1968). The occurrence of flutings on the proximal slopes and crests of many push moraines (Fig. 2.3) prompted Evans and Twigg (2002) to propose that subglacial bedform production and push moraine formation are genetically linked on the Breiäamerkurjökull/Fjallsjökull foreland. Moreover, the fact that the flutings are arranged in

Figure 2.3 Part of an aerial photograph of the BreiSamerkurjokull foreland, Iceland (Landm^lingar Islands and University of Glasgow, 1998), showing flutings arranged in strips between push moraines and terminating at the moraine crests. Note that some larger spindle drumlins are draped by push moraine (p') and terminate at the older push moraine (pii); (scale bar at bottom left represents 500 m).

Figure 2.3 Part of an aerial photograph of the BreiSamerkurjokull foreland, Iceland (Landm^lingar Islands and University of Glasgow, 1998), showing flutings arranged in strips between push moraines and terminating at the moraine crests. Note that some larger spindle drumlins are draped by push moraine (p') and terminate at the older push moraine (pii); (scale bar at bottom left represents 500 m).

ICE-MARGINAL TERRESTRIAL LANDSYSTEMS: ACTIVE TEMPERATE GLACIER MARGINS i7

strips separated by push moraines and aligned at right angles to the push moraine at their down-glacier end indicate that they clearly record individual flow events associated with the ice margin responsible for moraine deposition. Flutings are therefore the product of subglacial erosion and transport processes that advect sediment to the ice margin, where squeezing and pushing combine to create the moraine ridge (e.g. Sharp, 1984; Johnson and Hansel, 1999; Fig. 2.4). At locations where glacier snouts are thin and winters are relatively cold the construction of annual push moraines may involve the penetration of the winter cold wave down through ice marginal tills. For example, Krüger (1995) suggests that the annual moraines produced by the recession of northern Myrdalsjökull are initiated when the sub-marginal till is frozen on to the glacier snout and then pushed forward as a frozen slab in the winter (Fig. 2.4).

The role of a marginal zone of frozen sediment, possibly even discontinuous permafrost (see van der Meer et al., 1999), is central to the explanation of larger moraines produced by stationary glacier snouts (Krüger, 1993, 1994a, 1996; Matthews et al., 1995; Fig. 2.4). The model of Matthews et al. (1995) involves the winter freeze-on of supraglacial waterlain sediments and subglacial till that accumulate on and under the snout behind a large moraine. The moraine volume is increased each year as the glacier advances over its proximal slope and then melts out, draping the moraine surface with a layer of subglacial till and supraglacial outwash. Krüger (1993, 1994a, 1996) proposes a mechanism of incremental stacking of frozen subglacial till slabs. If the glacier snout reaches the same point each year then it will produce a large moraine composed of imbricate slabs of till interbedded with debris flow deposits.

Common features on the forelands of active temperate glaciers are the more subtle arcuate, low-amplitude ridges aligned parallel to recessional push moraines (Fig. 2.5). These ridges have clearly been overridden and moulded by the glacier as indicated by flutings that continue uninterrupted from their up-ice to down-ice slopes. They are also draped in some locations by recessional push moraines. Based upon these characteristics the arcuate ridges are interpreted as overridden push moraines, initially deposited during an earlier phase of glacier advance (Krüger, 1987; Evans et al., 1999b; Evans and Twigg, 2000, 2002).

Due to the sparsity of supraglacial and englacial debris over large areas of temperate glacier snouts, hummocky moraine, defined as the product of melt-out of debris-mantled glaciers (Benn and Evans, 1998), is often sparsely developed over deglaciated forelands. Narrow, elongate zones of rubble or rubbly, low-amplitude hummocks can document the lowering of medial moraines onto the substrate (e.g. Dawson, 1979; Levson and Rutter, 1989a, b; Evans and Twigg, 2000, 2002; Fig. 2.6). However, such features are rarely reported, being difficult to identify among the stronger imprints of other landform-sediment accumulations. In addition, debris-charged glacier snouts, produced by marginal freeze-on and the development of debris-rich ice facies, may melt-out to form low-amplitude hummocky moraine (e.g. Kötlujökull, Krüger, 1994a; Kjœr and Krüger, 2001; see Chapter 1).

2.2.2 Subglacial Domain (Tills and Associated Landforms)

The former beds of active temperate glaciers typically comprise areas of striated and polished bedrock with roches moutonnées, indicating that abrasion and quarrying is widespread at the ice/bedrock interface, covered or at least partially covered by subglacial sediments and landforms such as flutings and drumlins (Fig. 2.7). Subglacial experiments conducted beneath Breiäamerkurjökull demonstrated that the till was emplaced by deformation and that it

Glacier Diagram With Moraine And DrumlinFacies Well TemplateHummocky Topograph Slope
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