Box 62 Using Landforms Of Glacial Erosion To Reconstruct Glacier Dynamics

Landforms of glacial erosion are often used by glacial geologists to infer basic glaciological parameters such as ice-movement direction and change over time, but are seldom used for anything more complex. Sharp et al. (1989) demonstrated for the first time how glacial erosional landforms might be used to reconstruct parameters that affect the operation of basal processes and glacier dynamics. Their field study area is Snowdon, North Wales: an area that has been subjected to multiple glaciations of different duration and intensity. During the height of the last glacial maximum the Snowdon area was overrun by an ice sheet with a divide located in mid-Wales. At the close of the last glacial cycle, however, small cirque glaciers existed on Snowdon during a period known as the Younger Dyras (or Loch Lomond) Stadial. These two glacial episodes are marked by cross-cut striations. Detailed mapping of the glacial erosional landforms present, in particular of the size of striations and position of former lee-side cavities on bedrock surfaces, allowed Sharp et al. (1989) to suggest that the processes of glacial erosion were different during the two glacial events. Erosion beneath the ice sheet was dominated by lee-side fracturing of bedrock obstacles, by surface fracturing that created friction cracks, and by widespread abrasion. In contrast, erosion during the second phase, by the cirque glaciers, was confined to glacial abrasion and there is little evidence for lee-side cavities. Many of the surface features eroded on the bedrock surface by the ice sheet were not removed by the later cirque glacier, suggesting that relatively little erosion took place during this final episode of glaciation. From this, Sharp et al. (1989) infer that the cirque glacier had low sliding velocities which prevented cavity formation. On the basis of these inferences and mass balance estimates, Sharp et al. (1989) were able to calculate the dynamics of these former ice bodies. The cirque glacier was shown to have a low sliding velocity, around 10 m per year, high contact pressure at the glacier base, low basal water pressures and therefore few leeside cavities. In contrast the earlier ice sheet was shown to have a much higher sliding velocity (>35 m per year), lower basal contact pressures, higher basal water pressures and therefore widespread cavity formation. This illustrates the detailed inferences that can be made using simple observations of glacial erosional landforms combined with numerical estimates.

Source: Sharp, M., Dowdeswell, J.A. and Gemmell, J.C. (1989) Reconstructing past glacier dynamics and erosion from glacial geomorphic evidence: Snowdon, North Wales. Journal of Quaternary Science, 4,115-30.

Since striations are produced by glacial abrasion they indicate that: (i) the ice contained basal debris; (ii) the ice was warm-based and moved by basal sliding; (iii) there were moderate levels of normal effective pressure; and (iv) there was transport of rock debris towards the bed by basal melting. If any of these prerequisites for glacial abrasion ceases to exist, then the formation of striations will also cease. In this situation, the striated surface will become fossilised and the last ice-flow direction will be left imprinted on the bedrock surface. There are three situations where this may occur.

1. Where deglaciation occurs and the bedrock becomes re-exposed as the glacier margin retreats.

2. Where the basal thermal regime beneath a glacier changes from warm-based to cold-based. Once basal sliding stops, the basal ice becomes frozen to the bedrock beneath. In this situation new striations cannot form and the existing striations are preserved.

3. Where a layer of basal till is deposited immediately on top of a striated bedrock surface. In this scenario, the last ice-flow direction suggested by the striations will correspond approximately to the age of the overlying till. Striations therefore may be placed in a stratigraphical framework if the relative ages of the till units within an area can be established.

It is worth noting at this point that, under certain conditions, striations can also form under cold-based glaciers. Examples have been described from the Allan Hills area of Antarctica (see Box 6.9). Here the striations are shorter, less continuous and more irregular than those formed under warm-based conditions.

Striations can be used to reconstruct local patterns of ice flow, but their application to large-scale ice-flow reconstruction is more problematic. This is because basal ice conditions, and especially basal thermal regimes, change markedly over both time and space and consequently the pattern of striations beneath a glacier may be asynchronous (i.e. composed of a variety of different ages). Striations located in close proximity on the bed of a former glacier therefore may date from different time periods or relate to different ice flows with radically different flow directions. Furthermore, the preservation of striations on the bed of a former ice sheet depends upon the basal boundary conditions during deglaciation: for example, whether deglaciation occurs beneath cold-based ice (preservation of striations), or beneath warm-based ice (new striations forming constantly during deglaciation).

Striations formed during warm-based deglaciation will change direction as the orientation of the ice margin changes during recession. The youngest striations on a

Glacial Recession

Figure 6.3 Ice-smoothed valley walls next to the San Rafael Glacier in Chile. (A) Smoothed bedrock showing evidence of both polishing by glacial abrasion and the development of fractures by glacial quarrying. Former ice flow left to right. (B) Close-up of the valley walls showing smoothing by subglacial meltwater and glacial abrasion (note the striations). Former ice flow left to right. [Photographs: N.F. Glasser]

Figure 6.3 Ice-smoothed valley walls next to the San Rafael Glacier in Chile. (A) Smoothed bedrock showing evidence of both polishing by glacial abrasion and the development of fractures by glacial quarrying. Former ice flow left to right. (B) Close-up of the valley walls showing smoothing by subglacial meltwater and glacial abrasion (note the striations). Former ice flow left to right. [Photographs: N.F. Glasser]

bedrock outcrop will be orientated perpendicular to the ice margin, whereas older striations are related to more distant ice-marginal positions. Consequently, the further a striation is from the current ice margin, the more difficult it becomes to relate its formation to a particular ice-flow event. This situation is complicated further if cold-based deglaciation occurs, because this inhibits the formation of new striations and leads to the preservation of older striations that can be connected to the current ice margin. These complications mean that the interpretation of striation patterns over large areas, such as an entire ice-sheet bed, is a complicated task.

6.1.2 Micro Crag and Tails

Micro crag and tails are small tails of rock, which are preserved from glacial abrasion in the lee of resistant grains or mineral crystals on the surface of a rock. For example, in the slate rock of North Wales the presence of occasional pyrite crystals forms a point of resistance in an otherwise homogeneous rock. In the lee of these pyrite crystals small tails of rock are preserved. In many cases the pyrite weathers out on deglaciation (Figure 6.2D). Micro crag and tails are important because they provide clear evidence of both the orientation and direction of ice flow.

6.1.3 Friction Cracks

Friction cracks are a family of small cracks, gouges, chatter marks and indentations created in bedrock as larger boulders or clasts beneath a glacier are forced into contact with the bed. They vary in form from crescentic shaped gouges in which small chips of bedrock have been removed to fracture lines or cracks. Three main types of feature can be recognised: (i) crescentic fractures, which are a series of small cracks often forming a distinct line that are usually convex up-ice (Figure 6.2F); (ii) crescentic gouges, which occur where crescentic chips of rock have been removed and are normally concave up-glacier; and (iii) chatter marks, which are a series of irregular fractures. These features are not always consistently orientated in the direction of ice flow. For example, crescentic gouges are occasionally convex up-ice, when they are referred to as reverse crescentic gouges. In general there is much morphological diversity to these features and a wide variety of different forms have been recorded. They tend to form preferentially on crystalline or homogeneous bedrock lithologies.

Friction cracks differ from striations because they are not produced by the continuous contact between a clast and the glacier bed. Instead, they are formed by intermittent ice-bed contact. Local variations in effective normal pressure and bedrock topography are sufficient to make a clast 'bounce' or roll over a bedrock surface, creating small gouges or cracks when the clast comes periodically into contact with the bed. Friction cracks provide evidence of high effective normal pressures because considerable contact force between the clast and the bedrock is required to cause bedrock fracturing.

6.1.4 P-Forms and Micro Channel Networks

Smooth sinuous depressions and large grooves sculpted in bedrock are given the collective term plastically moulded forms or p-forms. The most commonly encountered types of p-forms are sichelwannen, potholes or bowls and channels (Figure 6.4). Sichelwannen are sickle-shaped bedrock depressions, usually occurring with an open end that points in the direction of ice flow. These open ends may be extended in the direction of ice flow as shallow runnels. Individual features are normally around 1 m in length but may occasionally exceed 10 m. Where they are particularly elongate in morphology they are referred to as hairpin erosional marks. These features are found at a wide range of different sizes from a few millimetres to several metres. Potholes are more rounded and deeper depressions which often occur in conjunction with sichelwannen. They may be up to several metres in diameter and depth. Sinuous or linear channels cut into bedrock, such as Nye channels, are also common (Figure 6.4A). These channels are usually less than a metre in width and depth. All these features may occur either in isolation or in close association and may be found with striations and other features of glacial abrasion. Striations are sometimes found superimposed on p-forms.

Hubbard Glacier Depth

Figure 6.4 Examples of p-forms and s-forms. (A) Deep Nye channel in front of Glacier de Ferpecle in Switzerland (former ice and water flow away from the camera). [Photograph: B.P. Hubbard] (B) Large grooves formed by a combination of glacial abrasion and meltwater erosion on a bedrock surface in Patagonia. [Photographs: N.F. Glasser]

Figure 6.4 Examples of p-forms and s-forms. (A) Deep Nye channel in front of Glacier de Ferpecle in Switzerland (former ice and water flow away from the camera). [Photograph: B.P. Hubbard] (B) Large grooves formed by a combination of glacial abrasion and meltwater erosion on a bedrock surface in Patagonia. [Photographs: N.F. Glasser]

The origin of p-forms is a source of debate. There are three main hypotheses: (i) formation by glacial abrasion; (ii) formation due to abrasion by a till slurry; and (iii) formation by meltwater. The presence of glacial striations on some p-forms has led many to argue for a mechanism involving glacial abrasion. The organisation of basal debris into distinct lines or streams at the base of a glacier (see Figure 7.8) would tend to concentrate glacial abrasion in certain areas, allowing the ice to sculpt grooves or channels. The alternative mechanisms involve either meltwater processes or a hyperconcentrated flow of till and water (a till slurry). For example, the formation of hairpin erosional marks and sichelwannen can be explained by flow separation around a small obstacle on a bedrock surface (Figure 6.5). The size of the obstacle controls the size of the erosional mark produced. They may form around single crystals, grains or nodules that protrude up through a bedrock surface, or alternatively around much larger bedrock knobs.

Flow velocity

Flow velocity

Horseshoe Vortices
Figure 6.5 Horseshoe vortices formed as meltwater is diverted around an obstacle on the glacier bed. [Modified from: Shaw (1994) Sedimentary Geology, 91, figure 8, p.276]
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