S 10 Glaciated Valley Landsystems Synthesis

The wide variety of landforms and landform associations found in glaciated valleys reflects several interrelated controls. The most important identified in this Chapter are:

1. topography, itself a function of tectonic and denudational history

2. debris supply to glacier surfaces, in particular the relative supply of debris and snow/ice, which determines where glaciers will lie on the clean glacier—debris-covered glacier—rock glacier continuum, and

3. efficiency of sediment transport from the glacier to the proglacial environment by the glacifluvial system.

Changes to one or more of these variables will influence the character of glacial landsystems. The influence of factors (1) and (2) on glacier morphology and dynamics has already been discussed. Here we focus on the divergence of glacier margins into distinct landsystem associations due to variations in the degree of coupling between glacial and proglacial environments by the glacifluvial system (factor 3). Water discharges from glacier termini are a function of catchment area, climatically determined changes in water storage in the catchment, and mean precipitation. There is thus a climatic significance attached to the morphological outcomes, but one that is modulated by catchment size and other topographic characteristics. For example, in dry climates, the discharge needed to redistribute material supplied from supraglacial transport might require a large glacier, whereas in a more humid climate the same discharge will be issued from a smaller catchment. Similar morphologies may therefore occur under different climates but at glaciers of different sizes. Fig. 15.23 summarizes the landsystem associations typical of coupled and decoupled ice margins, that is, systems with efficient and inefficient glacifluvial transport between glacial and proglacial systems.

Figure 15.23 Conceptual relationships between constraints on landsystem development at valley glacier termini and developmental pathways into four landsystem associations. 'Covered' and 'uncovered glaciers' refer to debris-covered and clean glaciers, respectively.

15.10.1 Coupled Ice Margins

At glaciers where transfer of sediment between the glacier and the proglacial fluvial system is efficient, a coupled landsystem association exists. The efficiency of sediment transfer from the glacial to the fluvial zones means that moraine development is limited while large amounts of sediment pass into and through the proglacial zone. Most sediment leaves the glacier as fluvial load, and switching of meltwater streams across the glacier front ensures that if terminal moraines do form, they are usually rapidly destroyed. The dominant landsystem is the outwash head/aggrading sandur.

Coupled ice margins occur where powerful migratory outwash rivers leave the glacier, and the limiting factor on their rate of development is the competence and capacity of proglacial streams. Coupled landsystem associations are well displayed in the humid mountain ranges of Alaska and New Zealand. After deglaciation, the preservation potential of the ice-marginal landsystems is very high. Beautifully preserved examples date from the last glacial/interglacial transition in New Zealand and southern South America.

15.10.2 Decoupled Ice Margins

At smaller glaciers and in arid mountain areas, insufficient outwash discharge is available to transfer sediment away from the glacier margin. Coupling of the glacial and fluvial transport systems is then lost and debris is dumped around the glacier perimeter to form moraines. Over millennia, repeated superposition of moraines around the margins of debris-covered glaciers constructs giant bounding moraines, constraining small, steep glaciers on elevated beds. Breach-lobe moraine formation is commonly associated with the continued thickening of such glaciers.

'Decoupled' ice margins are characteristic of many high-altitude, semi-arid to arid mountain ranges such as the central and northern Andes and High Asia. High firn lines prevent the formation of large compound glaciers, favouring instead cirque and small valley glaciers with debris concentration ratios conducive for sediment-choked, decoupled margins. The limiting factor governing landsystem development is low ice supply due to aridity, while sediment supply to the glacier surface remains very high. These smaller, drier glaciers have enhanced morainic landsystems at the expense of the proglacial landsystem. During recession, conditions favour moraine-dammed lake development. Well-documented examples include Ngozumpa Glacier (Nepal), Hatunraju Glacier (Peru), and Miage Glacier (Italy) Benn et al. 2001; Lliboutry, 1977; Deline, 1999a, b). Rock glacier fronts fed by debris-covered glaciers are a variant on decoupled ice margins. Rock glaciers represent stores of sediment, which again reflect the inability of ice-marginal processes to evacuate sediment.

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