Area Altitude Relationship for Plateau Icefields

Many reconstructions of glaciation in mountain regions have ignored the potential impact of plateaux on glacier mass balance, probably due to the fact that early research focused largely on alpine regions. However, the general controls on plateau icefield existence were established by Manley (1955, 1959). He suggested that the larger the breadth of a glacier summit, perpendicular to the prevailing accumulation season wind, the closer to the regional snow/firn line an ice cover can be sustained. Fig. 16.1 shows data taken from Manley's original publication (Manley, 1955)

Figure 16.1 Graphic plot showing Manley's original data (with a power law fitted), and additional data for summits in North Norway. (From Rea et al. 1998).

Summit breadth (m)

Figure 16.1 Graphic plot showing Manley's original data (with a power law fitted), and additional data for summits in North Norway. (From Rea et al. 1998).

with additional data from north Norway added by Rea et al. (1998). Manley's curve on Fig. 16.1 can be best approximated by the power law:

Where sh is the summit altitude above the firn line, a and c are empirical constants and sb is the summit breadth perpendicular to the predominant accumulation wind direction. This equation can be used to evaluate icefield existence if firn line altitude can be approximated from other sources (e.g. lateral moraine elevations), where ELA and firn line are assumed to be one and the same (Porter, 2001). For decreasing plateau size, plateau altitude must increase and so correspondingly a reduction in accumulation temperature will occur, so the ice that forms is more likely to be cold-based and non-erosive, thereby reducing the likelihood of major geomorphic impact. In such situations it is important to use the evidence found in the valleys radiating from plateaux to constrain glacier geometries and to establish the local firnline/ELA using other techniques (e.g. maximum elevation of lateral moraines, accumulation area ratio (AAR) from corrie and alpine style valley glaciers). The potential presence of an icefield can be then determined by employing Fig. 16.1 and the equation above.

16.3 CONTEMPORARY PLATEAU ICEFIELDS

16.3.1 Ellesmere Island, Arctic Canada

Plateau icefields drained by narrow outlet glaciers, terminating either as piedmont lobes or calving snouts in troughs and fjords, respectively, characterize glaciation of the Clements Markham Fold Belt of northwest Ellesmere Island, (Evans 1990a, b; Rea et al., 1998; Fig. 16.2). Typical landform assemblages produced by the plateau icefields and associated outlet glaciers are associated with three depositional settings:

1. plateau surfaces

2. fjord/trough ice marginal depo-centres, and

3. undulating bedrock lowlands.

The plateau summits of northwest Ellesmere Island are characterized by a thin, often patchy residuum or weathered bedrock surface. In some areas the lithological properties of the residuum mimic exactly the underlying bedrock, indicating an autochthonous weathering origin. A few highly weathered erratics are scattered across the plateaux and document regional ice flow of unknown age over the area. Preservation of the residuum indicates that glacial coverage was cold-based and protective. Other features diagnostic of cold-based ice coverage are evident on and around the plateaux (cf. O Cofaigh et al., 2003). The summits are devoid of obvious glacial erosional features and are often characterized by well-developed patterned ground features and tors. Retreat of the plateau ice margins is often recorded by meltwater incision into residuum and/or bedrock, indicating marginal rather than subglacial drainage (Fig. 16.3).

In fjord/trough ice marginal settings, there may be considerable thicknesses of pre-existing sediments that are accessible for glacigenic erosion, transport and re-deposition. Valley systems surrounding individual plateaux receive sediment directly from lateral/proglacial streams resulting in the accumulation of thick valley-bottom sequences and alluvial fans. On Ellesmere Island, sequences of aggradation and incision are predominantly controlled by glacio-isostatically

4I0 GLACIAL LANDSYSTEMS

Figure 16.2 Part of oblique aerial photograph (1950) of plateau icefields on northwest Ellesmere Island, Canadian high arctic. (T408R-222, Energy, Mines and Resources, Canada.)

influenced sea level changes (England, 1983; Evans, 1990a). Commonly, piedmont lobes debouching from plateau icefields dam the regional drainage creating extensive ice-dammed lakes. An aerial photograph (Fig. 16.4) of a plateau icefield south of Phillips Inlet on northwest Ellesmere Island provides a clear illustration of well-developed river terraces and incised alluvial fans around the margins of the northerly draining piedmont lobes. Extensive stratigraphic sections in the middle reaches of the main river include delta foresets and other glacilacustrine sediments, documenting the damming of the drainage by advance of the northern piedmont lobes. The development of thrust-block moraine complexes, composed of old raised glacimarine, glacilacustrine or glacifluvial sediment (Evans, 1989b; Evans and England, 1991, 1992; Fig. 16.5) is common during such ice advances. Overriding, entrainment and subsequent release of this reworked valley bottom sediment during periods of glacio-isostatically higher sea level results in the deposition of subaqueous or grounding-line fans (Fig. 16.6). The ice contact deposits are used to demarcate the former marginal positions of outlets emanating from the surrounding plateaux (Evans, 1990a). The lateral margins of former outlet glaciers are often marked by rock glaciers, which originate as accumulations of talus over stagnant glacier ice or as supraglacial lateral moraines (Evans, 1993; O Cofaigh et al., 2003).

Figure 16.3 A) Part of aerial photograph (1959) of plateau icefield near Phillips Inlet, northwest Ellesmere Island, Canadian high arctic, showing lateral meltwater channels cut during the recession of the largest outlet glacier lobe (A 16760-99, Energy, Mines and Resources, Canada). B) Contemporary ice-marginal meltwater channels forming at the snout of a plateau outlet lobe on eastern Ellesmere Island, Canadian high arctic.

Figure 16.4 Part of aerial photograph (Al6760-l0l, Energy, Mines and Resources, Canada 1959) of a plateau icefield south of Phillips Inlet, northwest Ellesmere Island, Canadian high arctic. Note the incised and terraced valley floor sediments. These sediments are of glacilacustrine, glacifluvial and alluvial fan origin and document former damming of the valley by the outlet glaciers. Incision resulted from valley drainage and glacio-isostatic rebound/relative sea level fall.

Figure 16.4 Part of aerial photograph (Al6760-l0l, Energy, Mines and Resources, Canada 1959) of a plateau icefield south of Phillips Inlet, northwest Ellesmere Island, Canadian high arctic. Note the incised and terraced valley floor sediments. These sediments are of glacilacustrine, glacifluvial and alluvial fan origin and document former damming of the valley by the outlet glaciers. Incision resulted from valley drainage and glacio-isostatic rebound/relative sea level fall.

Undulating bedrock lowlands rather than fjord/trough systems border some plateaux. Here the lack of thick sedimentary sequences restricts the development of till blankets and ice-contact glacimarine deposits. A typical bedrock lowland formerly covered by an expanded plateau ice cap is found to the south of Cape Armstrong, Phillips Inlet (Fig. 16.7). This area is characterized by thin, discontinuous till veneers, extensive bedrock exposures and residuum. The presence and retreat of glaciers in such areas is mapped using abandoned lateral meltwater channels, making them similar geomorphologically to plateau summits.

The evidence presented thus far has been related to the more restricted ice coverage when plateaux act as individual accumulation centres. As glacierization proceeds, plateau icefields will

Outwash Veneered
Figure 16.5 Thrust-block moraine composed of glacifluvial outwash and glacilacustrine sediments, formed by proglacial thrusting at the margins of a plateau icefield outlet glacier lobe on northwest Ellesmere Island, Canadian high arctic.
Outwash Veneered

Figure 16.6 Subaqueous grounding-line fan at the head of a fjord on northwest Ellesmere Island, Canadian high arctic. Main picture shows the location of the section face in inset picture. The fan was constructed by meltwater emanating from a glacier outlet lobe that was nourished by plateau icefields surrounding the fjord. The glacier lobe flowed from left to right and occupied the fjord head, thereby damming, at least initially, the main valley in the foreground. Gravel mounds located upslope of the fan demarcate the glacier margin.

Figure 16.6 Subaqueous grounding-line fan at the head of a fjord on northwest Ellesmere Island, Canadian high arctic. Main picture shows the location of the section face in inset picture. The fan was constructed by meltwater emanating from a glacier outlet lobe that was nourished by plateau icefields surrounding the fjord. The glacier lobe flowed from left to right and occupied the fjord head, thereby damming, at least initially, the main valley in the foreground. Gravel mounds located upslope of the fan demarcate the glacier margin.

Figure 16.7 Undulating bedrock terrain to the south of Cape Armstrong, Phillips Inlet, northwest Ellesmere Island. A river valley in the middle distance was cut by meltwater from an expanded plateau outlet lobe that advanced during the last glaciation into the bedrock lowland. This view shows the till veneer, residuum and bedrock exposures typical of plateau outlet advance into bedrock terrains.

Figure 16.7 Undulating bedrock terrain to the south of Cape Armstrong, Phillips Inlet, northwest Ellesmere Island. A river valley in the middle distance was cut by meltwater from an expanded plateau outlet lobe that advanced during the last glaciation into the bedrock lowland. This view shows the till veneer, residuum and bedrock exposures typical of plateau outlet advance into bedrock terrains.

begin to coalesce, eventually becoming overwhelmed by a regional ice sheet that may imprint its own geomorphological signature. Similarly, some linearly eroded landscapes containing plateau icefields may be dominated by outlet glaciers fed from mountainous terrain located further inland, as is the case on eastern Ellesmere Island (e.g. Rea et al., 1998; England et al., 2000). The response time of the local ice was shorter, resulting in an early thinning of the plateau ice during deglaciation and allowing the development of regional geomorphological imprints by the less responsive trunk glaciers fed from further inland. Fig. 16.8 shows the low profile lateral meltwater channels cut along the southern wall of Hayes Fjord, eastern Ellesmere Island. These channels document the recession of the Hayes Fjord trunk glacier, which drained the expanded Prince of Wales Icefield during the last glaciation. The more recently regenerated plateau ice cap on the Thorvald Peninsula plateau summit to the south of the fjord had obviously receded sufficiently to allow the incision of the regional ice-configured meltwater channels during deglaciation. In such physiographic/glaci-dynamic settings, the differentiation of plateau icefield and regional trunk glacier geomorphology is clearly essential for accurate reconstructions of palaeo-glaciation.

16.3.2 North Norway 16.3.2.1 Lyngen

Glaciers centred on the peak of Jiek'kevarri (1833 m) in the southern Lyngen Peninsula provide an excellent example of plateau icefields and valley outlet glaciers (Fig. 16.9). The landscape is highly fretted with ice supply from plateaux to valleys dominated by ice avalanching. Presently

Figure 16.8 The Thorvald Peninsula plateau icefield draining into Hayes Fjord, eastern Ellesmere Island, Canadian high arctic. Lateral meltwater channels occur along the southern wall of Hayes Fjord and document the recession of a trunk glacier in the fjord at the end of the last glaciation. The outlet glaciers from the plateau icefield on the Thorvald Peninsula have advanced across the meltwater channels since their construction.

Figure 16.8 The Thorvald Peninsula plateau icefield draining into Hayes Fjord, eastern Ellesmere Island, Canadian high arctic. Lateral meltwater channels occur along the southern wall of Hayes Fjord and document the recession of a trunk glacier in the fjord at the end of the last glaciation. The outlet glaciers from the plateau icefield on the Thorvald Peninsula have advanced across the meltwater channels since their construction.

glaciated plateaux tend to be reasonably small with Jiek'kevarri being the largest in the region at 3.70 km2, and lie well above the regional firn line. Around the margins of the icefields and on lower unglaciated plateaux, autochthonous blockfield cover is ubiquitous; in some places banding can be observed in the blockfield reflecting banding in the layered gabbros below. Despite the existence of easily removable blockfield material on the plateaux, the cold-based nature of the ice cover (Whalley et al., 1981; Gellatly et al., 1988; Gordon et al., 1988) and insignificant supraglacial debris sources restrict bed erosion and moraine formation, respectively. Some localized erosion may occur at the plateau edge where an outlet glacier exists, for example on the north side of Balgesvarri (Fig. 16.9). Gordon et al. (1988) and Gellatly et al. (1988) highlight the absence of meltwater channels, although meltwater was observed around the margins of the remnant ice cover on Bredalsfjellet (Fig. 16.9). It appears that the low bed slope angles produce meltwater ponding but this was insufficient to form significant channelized drainage. Thus, on plateaux where ice is cold-based, evidence of former glacier cover is very subtle.

In valleys below the plateaux the geomorphological signature is similar to what is traditionally expected of any temperate valley glacier in a mountain environment. After recession of the main fjord glaciers, where ice contacted the sea, ice-contact deltas delimit former glacier margins. Up-valley from these deltas significant quantities of glacifluvial sediment accumulated in lower gradient and overdeepened valley sections. Sequences of bouldery, frontal and lateral moraines document ice margins in the valleys. In areas where the till cover forms a thin veneer over bedrock, striae and roches moutonnées dominate the geomorphic signature of glaciation.

Figure 16.9 Aerial photograph (1978) of the fretted plateau and valley landscape centred on Jiek'kevarri at the head of Lyngsdalen (Fjellanger Wideroe 7802, 33-8 - 5820). For glacier names see Figure 16.17).

16.3.2.2 Troms-Finnmark

Plateaux in this region are lower and larger than those found in Lyngen (where plateaux are dissected), and as a result tend to act as accumulation centres for outlet glaciers in a similar style to the examples on northwest Ellesmere Island. Outlets exit steeply into valley heads as icefalls that remain connected to the main icefield above (Fig. 16.10). Generally the altitudinal range of a plateau is at most in the order of 100—200 m. Many of the surfaces are relatively flat, with slope angles generally less than 10°, only steepening in areas where they rise upward to meet nunataks/tors (Rea et al., 1996a). These larger, lower icefields are in places at the pressure melting point and sliding over their beds (Rea and Whalley, 1994). Some ice margins terminate behind moraines produced during the LIA (Gellatly et al., 1988; Fig. 16.11). In places around the ice

Figure 16.10 The main outlet of Langfjordj0kelen, which drops steeply from the bedrock plateau (800-1000 m), down to the snout at just above 300 m.
Plateau Margin Retreat
Figure 16.11 Boulder/ moraine at the eastern margin of 0ksfjordj0kelen. The middle ground shows ice plunging directly over the plateau edge, thereby prohibiting moraine formation. Note also the subglacial drainage.

margins, exposed by retreat since the LIA, the results of bedrock quarrying and abrasion are evident (Rea and Whalley, 1994, 1996). Measurements in subglacial cavities and observations of subglacial, channelized outflow streams indicate that ice is at the pressure melting point (PMP) around at least parts of the icefield margin (Fig. 16.11).

Non-erosive ice is also present, as indicated by the preservation of extensive blockfields containing patterned ground (Fig. 16.12), heavily weathered nunataks and bedrock, and a lack of moraines. Some bedrock areas show signs of 'older' subglacial erosion, but the extent of the subsequent weathering suggests that this is most likely to have taken place during a glaciation of at least pre-Weichselian age. In places, the blockfields are in excess of 1 m thick and are frost-sorted, although no permafrost has been found in the blockfields around Oksfjordjokelen (Rea et al., 1996a, b), unlike the higher plateaux in Lyngen (Gellatly et al., 1988). In both regions studies have suggested that the blockfields represent remnants of a weathering sequence, perhaps pre-Pleistocene in age (Rea et al, 1996a, b; Whalley et al, 1997).

In a thorough assessment of the geomorphic impact of plateau glaciation centred on 0ksfjordjokelen by Evans et al. (2002), it has been suggested that the largest accumulations of glacially derived materials occur as lateral and latero-frontal moraines in the valleys. The bouldery nature of these moraines and the angularity of the individual boulders suggest that rock avalanches and rock falls from the extensive bedrock cliffs are the main source of the debris. Moraine asymmetry attests to the variability of cliff exposures within some basins (eg. Matthews and Petch, 1982; Benn, 1989a; Evans, 1999). Rock glaciers have developed below some precipitous bedrock walls, reflecting the locally high rates of debris provision. The occurrence of some subglacially derived material in end moraines, in addition to roches-moutonnées and patchy till covers, attest to basal sliding, bed erosion and sediment deposition. Much of this was likely initiated by strain heating in basal ice passing through the steep icefalls that linked plateau summit ice to valley outlet glaciers. Where plateau outlet glaciers terminated in the surrounding fjord heads they sometimes deposited ice-shelf moraines, indicative of cold-based snouts. Recession of snouts is recorded in the shallow marine waters of some fjord heads by De Geer moraines and ice-contact, Gilbert-type deltas.

Figure 16.12 An area of weathered bedrock and blockfield showing patterned ground beyond the margin of 0ksfjordj0kelen.

16.3.3 Iceland

The temperate glacier snouts of Iceland are mainly wet-based for at least part of the year with a narrow frozen zone developing at most margins during the winter due to the penetration of the seasonal atmospheric cold wave (see Evans, 2003). With the exception of glaciers that may 'freeze-on' large quantities of debris due to supercooling in overdeepenings (Spedding and Evans, 2002), debris-rich basal ice sequences are typically thin or absent. The stapis or tuyas are ideal physiographic features for the accumulation of small plateau icefields in areas located at the limit of glacierization, good examples being @orisjokull, Eiriksjokull and Hrutfell, which surround the larger Langjokull icecap, Drangajokull in the northwest and Torfajokull in the south. Glaciers nourished on plateau surfaces are unlikely to accumulate large volumes of supraglacial debris due to the lack of extraglacial debris sources (Rea et al., 1998; Evans, in press). However, substantial accumulations of rockfall debris characterize the lateral margins of some outlet glaciers where they descend through precipitous cliffs at the plateau edge (Fig.

16.13). Consequently, lateral moraines are developed only at those margins lying beneath steep cliffs and are therefore discontinuous and asymmetrical on individual outlet lobes. Under some highly active rock walls, debris supply is sufficient to bury glacier ice during recession from the valley head, thereby producing ice-cored lateral moraines and talus-foot rock glaciers (Fig. 16.13). Beyond the influence of active rockwalls, plateau outlet glaciers deposit inset sequences of latero-frontal moraines during recession from valley heads. However, annual push moraines are not evident around the margins of many of the Icelandic plateau icefields. Instead, the valley floors are covered by a boulder veneer, which thickens at individual latero-frontal moraines (Fig. 16.13). This type of till cover suggests that the glaciers are transporting regolith from the plateau summit only short distances into nearby valley heads and that ice-marginal still-stands only occur in response to climate signals of lower frequency than the annual cycle.

Substantial rubbly moraines have been constructed on plateau summits, for example at the southwest margin of @orisjôkull, although the extent of the underlying ice core is unknown (Fig.

16.14). The moraines demonstrate that at least parts of the glacier bed are warm-based and capable of eroding regolith and/or bedrock. Some areas cleared of regolith indicate glacier ice descended from a plateau into surrounding valleys (Rea et al., 1998). Elsewhere, plateau geomorphology is dominated by or even restricted to meltwater channels, indicating predominantly cold-based ice and/or ineffective subglacial erosion.

16.4 DYNAMICS OF PLATEAU ICEFIELDS

In order to compile a plateau icefield landsystem it is important to understand the dynamics and style of glacierization of plateau summits. Broadly speaking, the glacierization style can be divided into two types, each representing a distinct phase of glaciation:

• large-scale ice cover (ice sheet) usually experienced under full glacial conditions where the topography is submerged and exerts less control on ice flow directions

• smaller scale, regional- to local-scale glaciation with ice sources centred on plateaux, the pattern of ice distribution being dictated by the altitude and latitude of the region (in highly dissected landscapes with small plateaux e.g. Lyngsdalen, valley glaciers may or may not exist during parts of this phase of glacierization)

Dissected Plateau

Figure 16.13 Aerial photograph stereopair (Isgraf/Loftmyndir and University of Glasgow, 1999) of the outlet glaciers on the northwest margin of @orisjokull. Visible are asymmetrically developed supraglacial lateral moraines and rubbly latero-frontal moraines whose distribution is controlled by the location of bedrock cliffs relative to the ice margins. Scale bar represents 1 km.

Figure 16.13 Aerial photograph stereopair (Isgraf/Loftmyndir and University of Glasgow, 1999) of the outlet glaciers on the northwest margin of @orisjokull. Visible are asymmetrically developed supraglacial lateral moraines and rubbly latero-frontal moraines whose distribution is controlled by the location of bedrock cliffs relative to the ice margins. Scale bar represents 1 km.

The onset of full glacial conditions will generally induce a decrease in annual air temperatures ensuring that newly forming or expanding plateau icefields are likely to be/become cold-based. Such cold-based ice acts as a protective cover over the plateau surface. Gradually, as the larger regional and continental ice masses grow, thick valley glaciers and eventually ice streams may form, which reach the PMP (periodically or for long periods) enabling glacial erosion and overdeepening of valleys. This is classic 'selective linear erosion' (Sugden, 1968; 1974), which is believed to be responsible for the formation of through valleys and the overdeepening of favourably orientated valleys. It is during this phase of glaciation that erratics may be transported onto plateaux (see Ellesmere Island above; Sugden, 1968; Sugden and Watts, 1977; Rea et al., 1998).

Figure 16.14 Aerial photograph (Isgraf/Loftmyndir and University of Glasgow, 1999) of the southwest corner of @orisjokull. A substantial rubbly, ice-cored moraine has developed at the Little Ice Age maximum limit, indicating that the glacier margin has eroded and transported the underlying blockfield. Scale bar represents 0.5 km.

Figure 16.14 Aerial photograph (Isgraf/Loftmyndir and University of Glasgow, 1999) of the southwest corner of @orisjokull. A substantial rubbly, ice-cored moraine has developed at the Little Ice Age maximum limit, indicating that the glacier margin has eroded and transported the underlying blockfield. Scale bar represents 0.5 km.

Under conditions of local, topographically controlled glacier coverage, ice on plateaux is thinner but will thicken with increasing plateau size. It is during this phase of glaciation that the thermal regime may become more complex. In order for an icefield to form or expand on a plateau there must clearly be a positive mass balance. Manley (1955), and Fig. 16.1 demonstrates that there is an important area/altitude relationship that dictates plateau icefield accumulation. Simplistically, below a critical summit size, the smaller the plateau the higher it must be above the firn line to support an icefield, and thus, the lower will be the temperature of the ice. The warmest ice will therefore be found on the lowest plateaux, which Fig. 16.1 suggests will tend to have large icefields. Thus, ice which forms on plateaux that are well above the firn line (e.g. Lyngsdalen) is most likely to be cold-based and non-erosive. In some situations the ice may reach the PMP (e.g. 0ksfjordj0kelen). In polar regions, where accumulation temperatures are lower, even the lowest altitude, large icefields will be unlikely to reach the PMP at their base. However, the larger the icefield the greater the potential for producing a clear geomorphic signature.

Due to the shallow slope angles, basal shear stresses and thus strain rates will be low in the basal layers of ice on plateaux. In areas where the bed steepens (i.e. towards outlets), the opposite occurs. If the basal ice temperature is close to the PMP the increase in strain heating may be sufficient to induce basal melting and thus sliding (e.g. Balgesvarri mentioned earlier). At this point the glacier will begin to erode its bed. Basal ice may also reach the PMP around parts of an icefield margin. For example, at least two parts of the plateau-terminating margin of 0ksfjordj0kelen are warm-based (Gellatly et al., 1988; Rea and Whalley, 1994) and have eroded the bed to produce moraines; a similar situation has arisen at the southwest margin of @orisjokull in Iceland (see above). The ice in these locations may reach the PMP due to:

1. the percolation of meltwater towards the bed

2. the penetration of a summer warm wave through the thin ice

3. increases in strain heating as bed slope angles increase, or

4. some combination of these factors.

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