Qih

Forward and upward motion

Forward and upward motion

August post-ablation surface, sharp channels and grainy surface

Forward motion and downward creep

August post-ablation surface, sharp channels and grainy surface

June pre-ablation surface, smooth ridqes snow fill, and glossy sublimation surflce'

Forward motion and downward creep

; Bubbly pure white ice

Strong motion throughout

Undercut by , strong ablation i /

Dirty "yellow" ice Calved ice blocks ¡-Summer stream Winter's drift accumulation

; Bubbly pure white ice

Strong motion throughout

Undercut by , strong ablation i /

Dirty "yellow" ice Calved ice blocks ¡-Summer stream Winter's drift accumulation

Increasing prism gf stagnant ice J Growing snowdrift J

Figure 3.2 The evolution of the marginal profile of the Greenland Ice Sheet at Nunatarssuaq. A) Formation of a cliffed margin and overriding of apron. B) An ice cliff and associated processes during equilibrium phase. C) Thinning of an ice cliff during period of negative mass balance. (After Goldthwait 1971).

Increasing prism gf stagnant ice J Growing snowdrift J

Figure 3.2 The evolution of the marginal profile of the Greenland Ice Sheet at Nunatarssuaq. A) Formation of a cliffed margin and overriding of apron. B) An ice cliff and associated processes during equilibrium phase. C) Thinning of an ice cliff during period of negative mass balance. (After Goldthwait 1971).

continuous with the glacier snout, forming a marginal supraglacial ramp up to the present ice margin. In addition, debris aprons may form where debris is exposed along shear planes and extrudes from the ice face. Such ramps are subject to fluvial incision and the development of glacier karst, resulting in the repositioning of supraglacial and englacial debris in the buried ice (Fig. 3.3). Glacier readvance involves re-incorporation of marginal ramps and their associated debris, contributing to the production of debris-rich, basal ice facies (Evans, 1989a).

The most prominent debris accumulations in sub-polar glacier snouts are those that comprise the basal ice facies. The thermal regime of a sub-polar glacier is critical to its ability to entrain and transport debris. Wet-based and sliding areas of the bed can erode and transport material towards the glacier margin in regelation ice, or perhaps in a deforming layer. Debris-rich basal ice is then produced in the snout due to net adfreezing, a process that is driven by the loss of heat through conduction at a rate that outstrips the provision of geothermal heat (Weertman, 1961; Hubbard and Sharp, 1989). Ice deceleration at the frozen margin of a sub-polar glacier induces compressive flow, which in turn thickens and elevates the debris-rich basal ice. Marginal debris, dry-calved ice blocks, buried glacier ice/marginal ramps and alluvium may also be entrained during glacier advance through a process known as apron entrainment (Goldthwait, 1960, 1961; Hooke, 1970, 1973a; Shaw, 1977a; Lorrain et al., 1981; Evans, 1989a; Evans and England, 1992). Englacial folding and thrusting can thicken debris-rich basal ice (Hooke, 1973b; Hudleston, 1976; Hambrey and Müller, 1978). Furthermore, debris-rich basal ice can often be traced up-glacier for up to 0.5 km where it pinches out, suggesting that in some cases, the origin of this debris may be due to glacier readvance and overriding of pre-existing moraines during the Neoglacial or Little Ice Age. The resulting end products are the thick and complex debris-rich basal ice sequences that are observed at the margins of most sub-polar glaciers (Fig. 3.4). Temporal variability in basal thermal conditions is likely to be

Figure 3.3 A marginal supraglacial ramp with its extensive supraglacial debris cover, inner Dobbin Bay, eastern Ellesmere Island.
Figure 3.4 The cliff margin of a sub-polar glacier in Phillips Inlet, northwest Ellesmere Island, showing debris-rich basal ice facies and apron of dry calved ice blocks and debris.

significant, particularly where outlet glaciers thicken and occupy lowland areas where basal ice reaches pressure melting point and subglacial meltwater is evident (Skidmore and Sharp, 1999). Therefore, geomorphic signatures may reflect warm-based conditions in certain depositional settings. This is demonstrated by van Tatenhove and Huybrechts (1996) through their modelling of the west Greenland ice sheet margin through the Holocene.

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