Rock Glaciers

Rock glaciers are tongue-like or lobate masses of ice and coarse debris that flow downslope by internal deformation. They commonly have ridges, furrows and sometimes lobes on their surfaces, and have steep fronts down which debris collapses and is then over-ridden by the advancing mass (Washburn, 1979; Ballantyne and Harris, 1994). A wide range of models has been proposed to

Glacial Foreland
Figure 15.16 Deposition of the medial moraine of the Haut Glacier d'Arolla by slow terminus retreat, giving juxtaposition of Eyles (1979) facies 1 and 2 in the foreland. (Photograph by Ben Brock).

explain the genesis of rock glaciers. Some researchers use the term 'rock glacier' broadly, to include features with cores of glacier ice or ground ice (e.g. Humlum, 1982): others reserve the term exclusively for 'periglacial' phenomena (e.g. Haeberli, 1985; Barsch, 1987). A twofold genetic classification is used in some texts, consisting of periglacial rock glaciers, which involve the slow deformation of ground ice below talus slopes (e.g. Kirkbride and Brazier, 1995), and glacial rock glaciers, which form by the progressive burial and deformation of a core of glacier ice by a thick, bouldery debris mantle (e.g. Whalley et al., 1995b). This classification can be difficult to apply in practice, and rock glaciers probably form a genetic continuum with no clear division between periglacial and glacial rock glaciers.

In mountain environments, rock, snow and ice are delivered to the base of slopes by avalanches and other mass movement processes, in varying proportions over space and time. Where the rock component is negligible, clean glaciers will form where snow and ice can survive ablation over the balance year. Where the snow and ice component is zero, talus slopes will result. Between these end-members exists a continuum of forms. Debris-covered glaciers form where the rock component is relatively high, and debris accumulates as a lag on the ablation zone of the dirty ice mass. Where the rock component is much higher, avalanche snow and ice will occur as isolated but deformable lenses within a talus, and the resulting form will be a rock glacier. It is probable that many rock glaciers in high mountain environments such as the Khumbu Himal, the Karakoram Mountains and Lahul Himalaya form by this mechanism (Barsch and Jakob, 1998; Owen and England, 1998; Fig. 15.17). In the Khumbu region, rock glaciers commonly occur in relatively low-lying catchments (5,000—5,600 m) where snow input occurs only during the drier winter months. Debris-covered glaciers typically occupy higher catchments, where temperatures are low enough for summer monsoon precipitation to fall as snow. Other origins of rock glaciers,

Figure 15.17 View over the debris-covered Ngozumpa Glacier to an avalanche-fed rock glacier below the peak of Cholo, Khumbu Himal, Nepal.

such as the formation of interstitial ice within a talus by the freezing of groundwater, can also be interpreted within this continuum model.

The relative proportions of rock and snow/ice delivered to the base of a slope will change with climate. A decrease in precipitation or an increase in temperature (more precipitation falling as rain rather than snow) will increase the relative importance of the rock component, producing conditions less favourable for glaciers but more favourable for rock glacier formation (Brazier et al., 1998; Nicholson, 2000). During periods of glacier retreat, active rock glaciers may develop at the heads of former debris-covered glaciers, while the remnants of the ablating glacier tongue also evolve into rock glacier forms. Remnant ice-cored moraines sometimes develop into rock glaciers as the protected ice core begins to flow internally under the stresses imposed by the debris overburden and distal slope (Vere and Matthews, 1985; Owen and England, 1998). Such features have been referred to as rock-glacierized moraines in the Canadian arctic (Dyke et al., 1982; Evans, 1993).

1 5.7 LANDSYSTEMS OF PROGLACIAL DEPOSITION

15.7.1 Glacial-Proglacial Linkage

The proglacial landsystem comprises landform-sediment associations constructed by fluvial, glacilacustrine mass movement and aeolian processes, which redistribute glacigenic sediment. Landforms include outwash fans, sandar, terraces formed by fluvial incision into valley fills, and drapes of wind-blown sand and silt. Volumetrically, the proglacial deposits dominate at large glaciers, particularly in maritime ranges, where most of the coarse sediment from glaciers is redistributed as fluvial bedload in proglacial valley trains. At small glaciers and in arid ranges, where proglacial fluvial deposition may be negligible, a proglacial river may be a bedrock channel whose load is primarily fine-grained glacial sediment in suspension.

15.7.2 Outwash Fans and Sandar

15.7.2.1 Aggrading Outwash Fans and Sandur (Valley Trains)

During periods of advance and extended stillstands of maritime glaciers, linkage between the ice-contact and proglacial zones is strong. Efficient fluvial redistribution of sediment aggrades the whole valley width by braided rivers, aided by switching of the loci of the outwash portal at wide glacier termini. Aggradation occurs when sediment is supplied to a proglacial river either directly from glaciers or during periods of paraglacial activity (Ballantyne, 2002b). The valley fill usually forms a sharp break of slope with the valley walls, except where tributaries build fans interfingering with the aggrading sandur surface (Fig. 15.18). During periods of glacier retreat, ice-contact lakes v v

Sinkhole New Zealand
Figure 15.18 Aggrading braided valley fill in the Godley Valley, New Zealand, a major sediment sink in the glaciated valley landsystem. Note the lack of valley-side fan development and great width of the active outwash plain.

influence downstream sedimentation by acting as sediment traps for the coarse sediment delivered from the glacier.

Glacifluvial valley fills attain thicknesses of several hundred metres and extend tens to hundreds of kilometres downstream from glacier termini. They form large sediment sinks on timescales of 104—105 years, even in tectonically active regions where they may be the last remaining depositional evidence for former glacial advances in some valleys. Proximal to the glacier, the valley fill takes the form of an alluvial fan with a broad apex at the ice margin. Downstream, decreases in gradient and sediment size are associated with the gradation of the proglacial fan into the braided river plain (sandur, or valley train). The detailed facies architecture of sandur in relation to fluvial processes is described elsewhere (e.g. Boothroyd and Ashley, 1975; Boothroyd and Nummedal, 1978; Maizels, 2002).

15.7.2.2 Incised Outwash Streams

During glacier retreat, sediment supply to the proglacial zone may be reduced for three reasons:

1. opening of a terminal ice-contact proglacial lake trapping coarse sediment

2. exhaustion of glacigenic slope mantles, and

3. stabilization of slopes by vegetation.

Reduced sediment supply can lead to incision of sandur surfaces, to give major paired river terraces, within which degradational (unpaired) terraces are inset. Multiple flights of paired terraces are associated with complex glacial histories (eg. Maizels, 1989). The transition is an important threshold in the sediment transfer system. Incision propagates downstream from the glacier terminus, initially forming a narrow inset floodplain (Fig. 15.19). The incised reach extends diachronously downstream, and may mark the first phase of a period of complex fluvial response triggered by glacier retreat and/or slope stability.

15.7.2.3 Ice-Contact Adverse Slopes (Outwash Heads)

The term outwash head describes the up-valley or adverse ice-contact slope bounding a proglacial sandur or fan. Outwash heads are associated with well-connected glacial and fluvial transport systems, in which little debris is incorporated into terminal moraines. If moraines form they have low preservation potential due to destruction by powerful, migratory outwash rivers. Though common landforms along the southern margins of the Laurentide Ice Sheet (e.g. Koteff, 1974), outwash heads are under-represented in research on valley glaciers in humid alpine regions. They are a major ice-marginal form in areas where debris-rich glaciers terminate in wide, gentle valleys (e.g. Alaska, New Zealand). The formation of an outwash head is not dependent on the presence of a debris-covered glacier, though such glaciers in humid regions invariably terminate in outwash heads.

The ice-marginal and outwash head environments at Tasman Glacier (Kirkbride, 2000) provide evidence of how Holocene glacier fluctuations reflect the dynamics of the debris-covered ablation zone. Proglacial fluvial aggradation during the Holocene created the outwash head, which now constrains a growing ice-contact lake (Fig. 15.20). Neoglacial terminal moraines are clustered in lateral-frontal positions and represent a tiny proportion of the debris discharge from the glacier. The vast majority has been transferred directly into the proglacial zone by glacial dumping and syndepositional redistribution in the proglacial fan.

Figure 15.19 Oblique aerial view of the terminus region of Maud Glacier in the Godley Valley, New Zealand. Ice retreat has opened up an ice-contact lake leading to incision of the outwash stream. Note the abandoned braided channels, and the late 19th century trimline and drift limit of the glacier. (Light autumn snow cover.)

Figure 15.19 Oblique aerial view of the terminus region of Maud Glacier in the Godley Valley, New Zealand. Ice retreat has opened up an ice-contact lake leading to incision of the outwash stream. Note the abandoned braided channels, and the late 19th century trimline and drift limit of the glacier. (Light autumn snow cover.)

Adverse Slope Profile

Distance (m|

Figure 15.20 Long profile through the terminus of Tasman Glacier, based on geophysical and bathymetric surveys. The outwash head (the former ice-contact adverse slope) ponds the growing proglacial lake. A block of separated dead ice decaying on the adverse slope will eventually form the irregular hummocky topography typical of such landforms. (Adapted from Hochstein et al, 1995).

Distance (m|

Figure 15.20 Long profile through the terminus of Tasman Glacier, based on geophysical and bathymetric surveys. The outwash head (the former ice-contact adverse slope) ponds the growing proglacial lake. A block of separated dead ice decaying on the adverse slope will eventually form the irregular hummocky topography typical of such landforms. (Adapted from Hochstein et al, 1995).

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