B

Figure 2.12 Landforms produced in association with ice-margin parallel outwash tracts. A) High-elevation kame terraces and lower ice-margin parallel outwash tracts at the northern margin of Sandfellsjokull, Iceland. B) Push moraines composed of gravels derived from ice-contact outwash on the foreland of Heinabergsjokull, Iceland.

evident in the proglacial area of BreiSamerkurjokull. This indicates that they are relatively stable forms that become adourned with subglacial features (drumlins and flutings) during glacier overriding and push moraines during glacier recession but remain dominant as topographic features in the deglaciated terrain.

Figure 2.13 Portions of aerial photographs (Landm^lingar Islands and University of Glasgow, 1945 (left), 1965 (right)) of the BreiSamerkurjokull foreland, Iceland, showing the development of eskers and pitted sandur fan during glacier recession. Ice marginal ponds are visible as dark patches to the top of the 1945 image. Note how the ice-contact face of the sandur fan and the esker which lies parallel to that face become clearer between 1945 and 1965 as buried glacier ice melts-out.

Figure 2.13 Portions of aerial photographs (Landm^lingar Islands and University of Glasgow, 1945 (left), 1965 (right)) of the BreiSamerkurjokull foreland, Iceland, showing the development of eskers and pitted sandur fan during glacier recession. Ice marginal ponds are visible as dark patches to the top of the 1945 image. Note how the ice-contact face of the sandur fan and the esker which lies parallel to that face become clearer between 1945 and 1965 as buried glacier ice melts-out.

The progradation of outwash will not produce sandur fans in certain topographic settings. Rather the outwash will be deposited between the glacier margin and proglacial topographic high points to produce ice margin-parallel outwash tracts and kame terraces (Fig. 2.12a). Wherever localized glacier readvances impact upon such outwash tracts the resulting push moraine ridges will comprise glacifluvial sands and gravels (Fig. 2.12b).

Minor outwash fans composed of fine grained material ('hochsandur fans') are common at advancing or stationary temperate glacier margins where sufficient englacial and supraglacial debris is made available to meltwater streams (Gripp, 1975; Heim, 1983, 1992; Krüger, 1997). They are common at the margins of Icelandic glaciers where large quantities of englacial tephra bands melt-out and provide fine grained sediment for meltwater reworking in the ablation zone. Because hochsandur fans are produced during glacier advance they are unlikely to contain the pitted surfaces characteristic of outwash prograded over receding snouts.

Conversely, the predominantly flat surfaces of sandur fans and ice margin-parallel outwash tracts often contain numerous enclosed pits (Figs. 2.13 and 2.14) and are therefore termed pitted or kettled outwash (sandar). The nature of the pitting on the ice margin-parallel features is usually dominated by large individual kettle holes. Pitted sandur fans, on the other hand, are often characterized by numerous small kettle holes located at their apices (Fig. 2.13). This is a characteristic of jokulhlaup sandar reported from a number of Icelandic glacier snouts (e.g. Howarth, 1968; Churski, 1973; Galon, 1973; Klimek, 1973; Bodere, 1977; Olszewski and Weckwerth, 1997; Fay, 2002). The development of kettle holes at the fan apex documents the melt-out of individual ice blocks originally deposited by flood waters on the fan (Maizels, 1977, 1992).

Observations on the evolution of the ice margin-parallel features at BreiSamerkurjokull by Welch (1967), Howarth (1968) and Price (1969, 1971, 1973) indicated that the kettle holes were opening up above extensive exposures of buried glacier ice (Fig. 2.14). Consequently the BreiSamerkurjokull ice-marginal features were explained as the product of melt-out of the shallow glacier margin previously buried by glacifluvial sediment. Melting of buried glacier ice in these settings explains the unusually large size of some individual kettle holes. This type of pitted outwash is not extensively developed in front of actively receding temperate glaciers due to the absence of large tracts of stagnating ice. It occurs at the lateral margins of temperate glaciers where streams are forced to flow over and through the ice, often due to topographic constraints.

More linear and complexly terraced sandar may evolve in the spillways that develop in association with proglacial lake growth. This is well illustrated by the forelands of

Figure 2.14 The pitted surface of an ice-margin parallel outwash tract at BreiSamerkurjokull, produced by the melt-out of extensive buried glacier ice. (Photograph by R.J. Price (1965).)

BreiSamerkurjokull and Fjallsjokull in Iceland. The recession of the two glaciers over the last 100 years has uncovered large depressions in which the lakes Fjallsarlon, BreiSarlon, Jokulsarlon and Stemmulon have evolved (Howarth and Price, 1969; Price and Howarth, 1970; Price, 1980, 1982; Evans and Twigg, 2000, 2002). Spillways from these lakes have routed substantial long-term discharges, which have constructed large, terraced outwash corridors. The most impressive example of a corridor lies between BreiSarlon and Fjallsarlon (Fig. 2.15), where the diversion of meltwater draining from west BreiSamerkurjokull after 1960 resulted in long-term progradation and incision of glacifluvial sediments around moraine and till-covered topographic high points (Evans and Twigg, 2000, 2002).

Ice-dammed and proglacial lakes are also common features around the receding margins of lowland temperate glaciers, particularly where the glacier has uncovered overdeepenings produced by long-term glacial erosion (Howarth and Price, 1969; Bjornsson, 1996; Bennett et al., 2000c; Evans and Twigg, 2002). These lakes are significant sediment sinks on the forelands of temperate glaciers and give rise to the accumulation of thick sequences of glacilacustrine sediments and shorelines and deltas (Shaw, 1977c; Teller, Chapter 14). The surfaces of these glacilacustrine features can be heavily pitted or extensively deformed by ice melt-out in glacier-contact settings. Lake sediments also constitute ideal material for push and thrust moraine development (Howarth, 1968; Evans et al., 1999b; Bennett et al., 2000c) and the production of glacitectonite and deforming bed tills (Benn and Evans, 1996; Evans et al., 1998, 1999c; Evans and Twigg, 2002).

The internal drainage networks of temperate glaciers are often documented in the landform record by eskers. The evolution of several esker systems at BreiSamerkurjokull, Iceland and Casement Glacier, Alaska has been documented by survey and mapping since the 1940s (Price, 1964, 1965, 1966; Petrie and Price, 1966; Welch, 1967; Howarth, 1968, 1971; Price, 1969, 1973, 1982; Evans and Twigg, 2002). The eskers are typically sharp-crested with steep sides and are composed predominantly of coarse gravels. They are arranged into complex forms comprising single ridges and multiple, anabranched sections often resembling fans. Aerial photographs taken since 1945 show that the largest of the eskers at BreiSamerkurjokull have developed at the same location as the medial moraines of the glacier, indicating that their location may be dictated by sediment availability.

The termination of some eskers at the apices of sandur fans clearly illustrates the importance of englacial and subglacial drainage systems to the construction of large areas of proglacial outwash. Based upon surveys of the eskers at BreiSamerkurjokull and Casement Glacier, Petrie and Price (1966, 1969), Price (1966), Welch (1967) and Howarth (1968, 1971) report considerable surface lowering of the original landforms. This indicates that large parts of the eskers have been deposited in englacial or supraglacial positions, an interpretation that is supported by observations of esker ice cores and eskers emanating from the wasting glacier surfaces. If drainage at BreiSamerkurjokull is subglacial at the snout (as suggested by Boulton et al., 2001a) then the streams must deposit debris in englacial tunnels before they reach the bed at the outer glacier margin. This implies that eskers are deposited englacially in increments and in a zone just up-ice of the glacier margin. As the margin recedes the drainage migrates from englacial tunnels to subglacial pathways, leaving englacial tunnel fills to melt-out on the glacier surface. Aerial photographs record the evolution of large tracts of 'kame and kettle topography' (Welch, 1967; Howarth, 1968; Price, 1969; Evans and Twigg, 2000, 2002) associated with the BreiSamerkurjokull eskers (Fig. 2.16). Evans and Twigg (2002), incorporating the observations of Price (1969), demonstrate that eskers and a pitted outwash surface visible on 1945 aerial

Figure 2.15 Portion of aerial photograph (Landm^lingar Island and University of Glasgow, 1998) of the terraced outwash corridor located between the proglacial lakes BreiSarlon (top right) and Fjallsarlon (centre left) on the forelands of BreiSamerkurjokull and Fjallsjokull, Iceland.
Figure 2.16 Portions of aerial photographs (Landm^lingar Island and University of Glasgow, 1945 (left), 1965 (middle), 1998 (right)) showing the evolution of a fan-shaped arrangement of anabranched eskers from pitted outwash.

photographs evolve into eskers and kame and kettle topography by 1965, and then by 1998 into a complex anabranched system of esker ridges arranged in a fan shape. This indicates that a considerable expanse of glacier ice lay below a thin cover of glacifluvial outwash and that the larger volumes of esker sediment within the ice ensured their preservation after melt-out. These complex esker networks clearly develop in parts of the glacier snout that occupy topographic depressions, where a thin cover of glacifluvial sediment can accumulate, although landform evolution is difficult to reconcile with predominantly subglacial drainage. At the location of one of the BreiSamerkurjokull medial moraines in 1945, outwash was forming a fan that emanated from a prominent single esker. On 1965 aerial photographs it is clear that the fan apex was also the apex of a fan of eskers within the buried glacier ice. It appears that the outwash fan was thin and had developed where the englacial stream responsible for esker deposition became supraglacial rather than subglacial.

Although their models are based upon examples at the Malaspina Glacier with its marginal stagnant ice, Gustavson and Boothroyd (1987) have provided explanations for the variety of glacifluvial and glacilacustrine depositional environments that exist at active temperate glacier margins. Fig. 2.17 depicts modified versions of their original diagrammatic explanations. In each example the source of debris for the meltwater streams is assumed to be the glacier bed but, based upon the BreiSamerkurjokull esker and medial moraine associations, this need not be the case at every location. Fig. 2.17a depicts a sandur fan emanating from a fountain fed by subglacial meltwater. Such fountains are common at temperate glacier margins. A sandur fan fed by subglacial meltwater draining from beneath the glacier is depicted in Fig. 2.17b. Note that englacial tunnels will have the potential to rework debris from medial moraines. Fig. 2.17c depicts a subglacial tunnel connected to an ice-contact proglacial lake. Debris-charged meltwater will deposit a subaqueous fan by the progradation of density underflow deposits. This may be complicated where meltwater issues from an englacial tunnel (Fig. 2.17d) and deposits a subaqueous fan over glacier ice. In some circumstances englacial meltwater may exit the snout and travel a short distance over the glacier surface to feed an ice-contact delta (Fig. 2.17e). Ice-cored eskers that terminate at sandur fan apices demonstrate that englacial meltwater must exit the glacier and drain supraglacially even in the absence of ice-dammed lakes. Moreover, this englacial meltwater need not penetrate to the glacier bed (Price, 1969). In examples 2.17b to 2.17e, note that englacial sediment-filled tunnels may become supraglacial during glacier recession, thereby explaining ice-cored eskers and outwash. Additional complexity arises in the landform record where meltwater drainage pathways change location. This is well illustrated at BreiSamerkurjokull where a single large esker ridge terminates at a sandur fan apex but a smaller esker runs along the base of the ice-contact slope, at right angles to the larger esker (Boulton, 1986; Evans and Twigg, 2002). The larger esker clearly originally fed sediment to the fan surface but could no longer do so once the glacier margin had ceased to press against the fan's ice-contact slope. Although meltwater continued to be delivered to this part of the glacier snout it began to flow sub-marginally along the base of the ice-contact slope of the fan, thereby producing a smaller esker running parallel to the glacier margin.

Project Management Made Easy

Project Management Made Easy

What you need to know about… Project Management Made Easy! Project management consists of more than just a large building project and can encompass small projects as well. No matter what the size of your project, you need to have some sort of project management. How you manage your project has everything to do with its outcome.

Get My Free Ebook


Post a comment