Ice Margins in High Relief Areas

5.5.1 Glacier Margins

The high-relief area that is described here is the McMurdo dry valleys, which are often called the McMurdo oasis. Glaciers of the McMurdo dry valleys can be divided into four groups: outlet glaciers, ice shelves, piedmont glaciers and alpine glaciers. Ice from the East Antarctic ice sheet flows through the Transantarctic Mountains to form outlet glaciers, such as Ferrar and Mackay glaciers, which reach the coast and form small floating ice tongues. Other outlet glaciers, such as Taylor Glacier, terminate on land. However, it could be argued that Taylor Glacier is not strictly an outlet glacier of the East Antarctic ice sheet because it flows from a local ice dome (Taylor Dome). North of the margin of the Ross Ice Shelf, ice streams that flow through the Transantarctic Mountains form outlet glaciers that feed small ice shelves. The largest ice shelf in the area is the Ross Ice Shelf, which is fed primarily by ice streams from the West Antarctic Ice Sheet. Although the Ross Ice Shelf does not directly impinge on the dry valleys today, during the Late Pleistocene the ice shelf grounded and flowed up the valleys. Consequently the ice shelf had a profound impact on the geomorphology of several valleys in the McMurdo oasis. In coastal areas of the McMurdo oasis, the slightly higher precipitation results in broad piedmont glaciers at the seaward margins of the Victoria and Wright valleys. Between the coastal piedmont glaciers and the inland glaciers, small alpine glaciers form a remarkable landscape in which bare rocky slopes contrast strongly with glacier ice. Most of these glaciers are no more than 15 km long.

Although ice margins in the McMurdo dry valleys range from gently sloping ice ramps to steep ice margins the most common and distinctive form is a 15-20 m high ice cliff. These

Figure 5.9 A) A series of thrust-block moraines on an island adjacent to the margin of an outlet glacier. B) Stratified glacimarine sediments exposed in the crest of a thrust-block

Figure 5.9 A) A series of thrust-block moraines on an island adjacent to the margin of an outlet glacier. B) Stratified glacimarine sediments exposed in the crest of a thrust-block

Figure 5.10 Sedimentary logs of sediments from the crests of thrust-block moraines. The contour interval of the Schmidt nets is two standard deviations. V, and P, give the azimuth and plunge of the principal eigenvector, S, gives the strength of clustering about the principal eigenvector and R shows the trend of the moraine ridge.

Figure 5.10 Sedimentary logs of sediments from the crests of thrust-block moraines. The contour interval of the Schmidt nets is two standard deviations. V, and P, give the azimuth and plunge of the principal eigenvector, S, gives the strength of clustering about the principal eigenvector and R shows the trend of the moraine ridge.

distinctive cliffs have been attributed to changes in the rheological properties of ice at around 20 m thickness (Chinn, 1991) and to a strong reduction in ablation from the foot of the cliffs to the glacier surface (Fountain et al., 1998). Supraglacial debris is absent from most of the glaciers and the only visible debris is often restricted to small outcrops at the foot of ice cliffs where the basal zone of glaciers is exposed (Fig. 5.11). Although the ice margins do not have the thick snow accumulations that characterize the low relief landscapes described above, they are often characterized by an accumulation of ice at the foot of the cliffs produced by episodic calving (Fig. 5.11).

All glacier margins in the McMurdo dry valleys are dry-based with basal temperatures between -16 and -18 °C, which is very similar to the mean annual temperature (-19.8 °C at Vanda

Figure 5.1 1 Glacier margins in the McMurdo dry valleys. A) Clean white ice and marginal cliff characteristic of glaciers in the dry valleys. Note the ice apron produced by episodic calving (Hart Glacier, Wright Valley). B) Stratified basal ice at the margin of Suess Glacier.

Figure 5.1 1 Glacier margins in the McMurdo dry valleys. A) Clean white ice and marginal cliff characteristic of glaciers in the dry valleys. Note the ice apron produced by episodic calving (Hart Glacier, Wright Valley). B) Stratified basal ice at the margin of Suess Glacier.

in the Wright Valley). However, in the case of outlet glaciers such as Taylor Glacier, the ice is at pressure melting point within a few kilometres of the margin where the glacier is thicker (Robinson, 1984). The velocities of the glaciers are generally low. In the case of fully dry-based glaciers surface velocities are less than 1 m.a-1 and 3 m above the bed velocities of around 250 mm.a-1 have been measured (Fitzsimons et al., 1999). The outlet glaciers move considerably faster.

Exposures of the basal zone of the glaciers show that debris concentrations are generally low and highly variable. In the case of Suess Glacier in the middle part of the Taylor Valley, debris concentrations range from less than 0.1 per cent to more than 70 per cent by volume with an average of less than 5 per cent. Debris concentrations in Taylor Glacier, which is at pressure melting point upstream of the terminus (Robinson, 1984), are considerably greater.

Ice-marginal landforms are absent or very small at most ice margins suggesting that the cold-based glaciers are not particularly effective agents of erosion. However, a few glaciers exhibit well-developed end moraines and have been the subjects of recent investigations. These landforms are described below.

5.5.2 Ice-contact Landforms and Sediments

Several types of moraines are recognized at the margins of glaciers in the dry valleys although there is considerable uncertainty about their origin. Given this uncertainty, for the purposes of this chapter they are divided into constructional and structural features.

5.5.2.1 Constructional Landforms

Constructional moraines are formed at ice margins where the glacier has been sufficiently stable to concentrate debris, usually at the foot of an ice cliff, by ablation of the basal debris zone. Chinn (1991) has argued that the outcrops of basal debris at the foot of ice cliffs are the equivalent of inner moraines that are commonly seen at dome-shaped polar ice-margins.

In the McMurdo dry valleys, constructional moraines, often covered with an ice and debris apron, occur at the margins of numerous glaciers. These features are formed by debris released from the basal zone together with sparse supraglacial debris. Shaw (1977b) has argued that an advancing glacier may override the ice and debris aprons thereby incorporating debris into the basal zone of the glacier. This 'apron entrainment' mechanism is similar to processes described in sub-polar glaciers in the Canadian arctic (Evans, 1989a).

5.5.2.2 Structural Landforms

Small structural moraines occur at the margin of several glaciers in the McMurdo dry valleys. These features appear to consist of either sediment blocks eroded from the base of the glacier and/or marginal sediments that have been deformed in situ (Fig. 5.12). Fitzsimons (1996a) described moraines that formed at the margins of Suess Glacier as thrust-block moraines. This paper posed the hypothesis that the moraines were produced by accretion of ice and debris as the cold-based glacier margin advanced into a proglacial lake (see Fig. 4 in Fitzsimons, 1996a). Subsequent investigations have demonstrated that the formation of moraines in this location is more complex than initially thought. Isotopic analysis of the basal ice exposed at the foot of the ice cliff has shown convincingly that some basal ice has formed as water froze onto the base and/or margin of the glacier (Lorrain et al., 1999). However, excavation of a tunnel in the right side of the

The Margins Glaciers Form

Figure 5.12 Structural landforms at the margins of glaciers in the dry valleys. A) Moraines at the left margin of Suess Glacier form multiple ice-cored ridges up to 10 m high. Note the textural contrast between the small moraines in the foreground which are the product of deformation of proglacial fluvial sediments and the coarser, larger moraines produced by subglacial erosion. B) Ice-cored moraine at the margin of Wright Lower Glacier. Note the sedimentary stratification in the moraine. At least part of the moraine has been formed by deformation of the adjacent delta. The ice cliff is 18 m high.

Figure 5.12 Structural landforms at the margins of glaciers in the dry valleys. A) Moraines at the left margin of Suess Glacier form multiple ice-cored ridges up to 10 m high. Note the textural contrast between the small moraines in the foreground which are the product of deformation of proglacial fluvial sediments and the coarser, larger moraines produced by subglacial erosion. B) Ice-cored moraine at the margin of Wright Lower Glacier. Note the sedimentary stratification in the moraine. At least part of the moraine has been formed by deformation of the adjacent delta. The ice cliff is 18 m high.

glacier 100 m upstream of the moraine has demonstrated that the sediment blocks that feed the moraine have been entrained at least 100 m upstream of the glacier terminus (Fitzsimons et al., 1999).

The new evidence shows that there is indeed support for the hypothesis that at least some of these features are thrust-block moraines formed as ice and debris was accreted and thrust at the glacier margin. However, the absence of the isotopic signature of ice accretion upstream of the glacier terminus suggests that a large proportion of the debris in the moraines has been entrained subglacially. The mechanism for erosion and detachment of the blocks of sediment are currently unknown. The evidence for subglacial entrainment casts doubt on whether the moraines are thrust-block moraines (sensu Kalin, 1971), which are features that are formed in a proglacial position as the foreland of a glacier is deformed.

5.5.2.3 Glacifluvial and Glacilacustrine Landforms

Outwash surfaces are absent from the proglacial areas of most glaciers in the McMurdo dry valleys. Their absence is a consequence of the low production of meltwater, low ephemeral discharges in streams, and low debris concentrations in and on the glaciers. The largest stream in the dry valleys and Antarctica is the Onyx River which flows from the Wright Lower Glacier and Lake Brownworth into Lake Vanda. Lake Vanda like many other lakes in the dry valleys has no outlet to the sea and water losses occur through sublimation and evaporation. Most lakes have a 4-6 m-thick perennial ice cover although some are frozen to their beds. The majority of these lakes receive little sediment from the glaciers again because of the low production of meltwater, low ephemeral discharges in streams, and low debris concentrations. Even the lakes in contact with glacier margins (Figs 5.13 and 5.14) are not strongly influenced by the presence of the glaciers. Divers and remotely operated vehicles operating beneath the ice cover of the lakes have revealed clear water and a lake bed covered with algae up to the contact with the glacier cliff.

5.5.2.4 Late Pleistocene Landforms and Sediments

During the Late Pleistocene the configuration of the glaciers in the McMurdo oasis was substantially different from the glacial systems described above. The alpine and piedmont glaciers are thought to have receded because their precipitation source was greatly diminished by the presence of a much larger Ross Ice Shelf. In the Taylor Valley, lacustrine strandlines and lacustrine deltas provide evidence of a large glacial lake that occupied much of the valley (Fig. 5.13). Glacial Lake Washburn is thought to have formed as the Ross Ice Shelf thickened and advanced into the valley. The grounded ice shelf deposited the younger Ross Sea Drift (Stuiver et al., 1981), which extends westward into the valley as far as Canada Glacier (Fig. 5.14). The drift consists of numerous eskers 1-5 m high and up to 2 km long, and numerous small moraines which drape the eskers in a washboard-like structure (Fig. 5.14). The eskers and moraines are overlain by marine sediments and lacustrine deltas.

After withdrawal of the Ross Ice Shelf and the draining of Glacial Lake Washburn, alpine glaciers such as Canada Glacier advanced and reached their maximum positions on top of and abutting the Ross Sea Drift (Fig. 5.14). The relationship between the alpine glaciers and the Ross Sea Drift suggests the alpine and piedmont glaciers fluctuate out of phase with the grounded ice in McMurdo Sound.

Figure 5.13 Aerial photograph of Taylor Glacier (TG), an outlet adjacent to the perennially frozen Lake Bonny. The adjacent Rhone Glacier (RG) is a small alpine glacier fringed by an older latero-terminal moraine (RM). Numerous strandlines (S) from Glacial Lake Washburn are evident on both sides of the valley. Several perched deltas (RD) deposited by streams that flowed from Rhone Glacier are evident.

Figure 5.13 Aerial photograph of Taylor Glacier (TG), an outlet adjacent to the perennially frozen Lake Bonny. The adjacent Rhone Glacier (RG) is a small alpine glacier fringed by an older latero-terminal moraine (RM). Numerous strandlines (S) from Glacial Lake Washburn are evident on both sides of the valley. Several perched deltas (RD) deposited by streams that flowed from Rhone Glacier are evident.

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