Box 122 The Origin And Significance Of Streamlined Glacial Landforms

Some of the most commonly used landforms in ice-sheet reconstructions are a family of ice-moulded and streamlined landforms comprising glacial lineations, drumlins, megaflutes and megascale glacial lineations (MSGL). These landforms are visible on satellite images as large-scale patterns of topographic streamlining, sometimes with distinctive cross-cutting relationships. The image below shows a Landsat subscene (left panel) and interpretation (right panel) of the glacial geomorphology of the area around Laguna Cabeza del Mar (near Seno Otway in southern Patagonia). At least two sets of streamlined glacial lineations can be traced to former ice-marginal positions in the northeast of the image. The former ice margins are marked by moraines, sandar and meltwater channels. Understanding the genesis and significance of these landforms is clearly crucial to all palaeoglaciological reconstructions. Clark (1993, 1994) provided a glaciological explanation for the origin of these streamlined land-forms based on the premise that streamlined landforms are created subglacially. Glacial lineations form parallel to ice-flow direction, providing a 'marker' that records deformation during ice-flow events. Clark argues that the palaeoglaciological record provides the surface or plan expression of subglacial deformation. This work suggests that MSGL (up to 20 km in length and with length : width ratios of up to 50 : 1) were formed under conditions of fast ice flow and their presence may thus record the locations of former ice streams or surge events (Stokes and Clark, 2002). Extensive sets of lineations must have been formed approximately synchronously, thereby indicating that lineation generation occurs over a wide range of glaciodynamic conditions, from submarginal positions to the interior portions of ice sheets. These streamlined glacial land-forms are now commonly used in palaeoglaciological reconstructions. For example, using satellite imagery, Stokes and Clark (2002) mapped over 8000 glacial lineations associated with an ice stream of the Laurentide Ice Sheet near Dubawnt Lake, District of Keewatin, Canada and used these to reconstruct areas of former high ice-velocity under the Laurentide Ice Sheet.

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Sources: Clark, C.D. (1993) Mega-scale glacial lineations and cross-cutting ice-flow landforms. Earth Surface Processes and Landforms, 18, 1-29. Clark, C.D. (1994) Large-scale ice-moulding: A discussion of genesis and glaciological significance. Sedimentary Geology, 91, 253-68. Stokes, C.R. and Clark, C.D. (2002) Are long subglacial bedforms indicative of fast ice flow? Boreas, 31, 239-49. [Modified from: Glasser et al. (2008) Quaternary Science Reviews, 27, figure 2, p. 371]

Figure 12.3 Rock-cored drumlin to the east of the Southern Alps of New Zealand. Former ice flow was right to left. There is a person in the left-centre of the photograph for scale. [Photograph:

Figure 12.3 Rock-cored drumlin to the east of the Southern Alps of New Zealand. Former ice flow was right to left. There is a person in the left-centre of the photograph for scale. [Photograph:

Figure 12.4 Satellite image of megascale glacial lineations (MSGL) formed under the Laurentide Ice Sheet in Canada. Former ice flow was from right to left. [Image courtesy: C.D. Clark]

2. Non-ice-moulded subglacial landforms, including geometrical ridge networks and crevasse-squeeze ridges. These features are believed to form by the squeezing of basal till into subglacial crevasses, and are commonly associated with surge-type behaviour.

3. Ribbed (Rogen) moraines, which are subglacial landforms comprising discontinuous, subparallel to parallel ridges oriented transverse to former ice flow. They are typically steep-sided, regularly spaced and made of diamict, sand or gravel. They have a variety of morphological forms and their precise mode of formation is debated. In some cases it can be demonstrated that they have been reshaped parallel to ice flow by ice overriding, indicating sediment deformation. In this case they can be regarded as ice-moulded subglacial landforms. It has also been argued that ribbed moraines reflect large-scale sediment deformation during inward-transgressive subglacial thawing at the ice-sheet bed.

4. Ice-marginal landforms, including ice-marginal moraines, which appear on satellite images as prominent cross-valley single moraines, in more complicated moraine systems with multiple ridges or as areas of hummocky moraine. They canbe linear, curved, sinuous or saw-toothed in plan. They are often associated with other ice-marginal features such as meltwater channels, ice-contact fans, kames, kame terraces, kettle holes and outwash plains (sandur). Ice-marginal land-forms generally reflect periods when the ice margin was stationary for some length of time. In marine or lacustrine settings, non-ice-moulded subglacial landforms include regularly spaced transverse ridges, De Geer moraines, delta moraines, moraine banks, grounding-line fans, trough-mouth fans and push moraines formed during ice retreat.

5. Eskers, which are sinuous ridges of sand and gravel, sometimes braided or beaded in planform, with surface pits and mounds. Most eskers form in icewalled tunnels where sediment is transported and deposited by englacial or subglacial meltwater. They tend to be orientated subparallel or parallel to the former ice-flow direction and often run down-valley. They indicate warm-based ice sheet conditions and form time-transgressively close to the ice margin during ice-sheet recession phases.

6. Meltwater channels, which appear as incised linear features with abrupt inception and termination. They often contain no contemporary drainage and individual channels may either follow or cut across the local slope direction. Variants include subglacial meltwater channels cut beneath the ice and ice-marginal meltwater channels, cut adjacent to the ice margin, for example during incremental recession.

7. Striae, which are millimetre-scale wide but metre-scale long linear scratches on bedrock surfaces, and other bedrock abrasional marks. These microscale landforms can be identified only in the field, but they are invaluable because they provide important information about former ice-flow directions and basal thermal regime and how these varied through time.

8. Unmodified preglacial landforms often appear in the core areas of former ice sheets, especially in upland areas. These landforms include plateau surfaces sometimes with well-developed tors and associated saprolites, boulder fields and boulder depressions, as well as fluvial valleys. These landforms are generally believed to develop by subaerial weathering and erosion processes. Their survival beneath an ice sheet indicates that former ice-sheet velocities were low and that the ice sheet was cold-based, at least for the majority of the time.

In areas lacking landform detail, sedimentary evidence may also be useful. Till-fabric analysis and till geochemical signatures may be used to infer former ice movement directions, as can directions of erratic dispersal.

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