Macroscale Features Of Glacial Erosion

Macroscale features of glacial erosion are those features that are 1 km or greater in dimension. They are large enough to form significant landscape elements and may contain many of the smaller landforms already considered in this chapter. Five main landforms are recognised at this scale: (i) regions of areal scour; (ii) troughs; (iii) cirques; (iv) giant stoss and lee forms; and (v) tunnel valleys (Table 6.4).

6.3.1 Areal Scouring

The most commonly encountered landscape of glacial erosion is one of areal scour. It consists of an area of scoured bedrock composed of an assemblage of whalebacks, roches moutonnees, bedrock megagrooves and rock basins

Table 6.4 Macro-scale landforms of glacial erosion and their significance for the reconstruction of former ice masses

Landform

Morphology

Glaciological significance

Widespread areal scouring

Glacial troughs Fjords

Cirques

Giant stoss and lee forms

Areas of low relief smoothed into streamlined eminences and basins, taking the form of roches moutonnees and whalebacks. Rock surfaces may contain striae, bedrock gouges and cracks. Detailed morphology controlled by preglacial weathering characteristics and patterns of bedrock jointing.

Deep valleys with smoothed, polished and steep walls and flat floors. Fjords are drowned glacial troughs. Valley cross-sectional morphology can be described by empirical power-law functions and by second-order polynomials.

Large bedrock hollows that open downslope and are bounded upslope by a cliff, steep slope or arcuate headwall.

Large upstanding bedrock hills or spurs with both abraded and quarried faces. Detailed morphology controlled by preglacial weathering characteristics and patterns of bedrock jointing.

Warm-based ice carrying a basal debris load.

Percentage of landscape accounted for by streamlined bedrock features and by stoss and lee features may indicate former basal conditions.Quarried landforms indicate low effective normal pressures (0.1-1 MPa) inferred from the presence of basal cavities with abundant basal meltwater and regular fluctuations in basal water pressure. Quarried landforms also indicate rapid sliding velocity and thin ice; direction and orientation of ice flow.

Streamlined bedrock features and striae indicate high effective normal pressures (>1 MPa) inferred from intimate ice-bedrock contact and cavity suppression.

Warm-based ice, abundant meltwater and high ice velocities. The cross-sectional area of a trough or fjord and its longitudinal profile may become calibrated to discharge over time, enabling estimates of palaeo-ice discharge to be made.

Warm-based ice and abundant meltwater.

Elevation and aspect of cirques commonly used in palaeoclimatic reconstructions to provide information on height of the former regional snowline and to assess the mass balance conditions under which empty cirques may become occupied.

Warm-based ice carrying a basal debris load.

Low effective normal pressures (0.1-1 MPa) inferred from the presence of basal cavities. Quarried faces indicate abundant basal meltwater with regular fluctuations in basal water pressure. Rapid sliding velocity. Some evidence that roches moutonnees are deglaciation features formed under thin ice.

May indicate direction and orientation of ice flow. Some features may indicate relatively low levels of glacial erosion, because they are commonly associated with preglacial valley spurs or bedrock hills. Related to subglacial meltwater discharge beneath large ice sheets, either via rapid drainage of stored subglacial meltwater or surface meltwater-derived drainage. Tunnel valleys and channels invariably indicate the presence of a melting ice sheet overlying a poorly consolidated substrate. Calculations of palaeovelocity and palaeodischarge possible from measurements of channel shape, channel width and size of material transported.

[Modified from: Glasser, N.F. and Bennett, M.R. (2004). Progress in Physical Geography, 28, 43-75.]

(Figure 6.12A). Every part of the landscape is affected by glacial erosion, and depositional products are rare or absent (Figure 6.12B). The detailed morphology of the individual landforms within a region of areal scour is primarily controlled by the orientation, spacing and density of joints, foliations and other lines of weakness within the bedrock (Figure 6.12C and D). This type of landscape develops under extensive areas of warm-based ice. In Britain this type of terrain is sometimes referred to as knock and lochan topography. This term describes the upstanding rounded bedrock lumps (knocks) and the water-filled depressions (lochans) that separate them.

6.3.2 Glacial Troughs and Fjords

Glacial troughs and fjords (drowned troughs) are deep linear features carved into bedrock (Figure 6.13). They represent the effects of glacial erosion where ice flow is confined by topography and is therefore channelled along the trough or valley. Troughs may be cut beneath ice sheets as well as by valley glaciers and larger outlet glaciers. Glacial troughs are formed by a combination of both glacial abrasion and quarrying. Both these processes are required to produce steep-sided troughs, although the effects of glacial abrasion are more obvious due to the smoothed and polished trough walls. The processes of glacial quarrying are most evident on the valley floors, where quarried bedrock landforms such as roches moutonnees and rock basins are common.

Tunnel valleys Large, sinuous, steep-sided valleys or and tunnel depressions that may contain enclosed channels basins in their floor.

Tunnel channels are incised into bedrock, glacigenic sediment or other pre-existing materials. Tunnel valleys are usually infilled with sediment and occur both on the continental shelf and in lowland areas.

Pre Dharwar Landscape Cycle

Figure 6.12 Landscapes of areal scouring. (A) Areal scouring comprising whalebacks, roches moutonnees and rock basins in the Harlech Mountains of North Wales. (B) Ice-scoured bedrock dominated by ice-smoothed bedrock in Norway. Note person in centre of photograph for scale. (C) Glacially quarried bedrock blocks scattered around the lee side of a roche moutonnee in the Harlech Mountains of North Wales. The size of the blocks is directly related to the orientation and spacing of joints in the bedrock. (D) Ice-scoured bedrock on a hillside in western Ireland. Note how the glacial erosion has exploited the geological structure within the bedrock.

[Photographs: N.F. Glasser]

Figure 6.12 Landscapes of areal scouring. (A) Areal scouring comprising whalebacks, roches moutonnees and rock basins in the Harlech Mountains of North Wales. (B) Ice-scoured bedrock dominated by ice-smoothed bedrock in Norway. Note person in centre of photograph for scale. (C) Glacially quarried bedrock blocks scattered around the lee side of a roche moutonnee in the Harlech Mountains of North Wales. The size of the blocks is directly related to the orientation and spacing of joints in the bedrock. (D) Ice-scoured bedrock on a hillside in western Ireland. Note how the glacial erosion has exploited the geological structure within the bedrock.

[Photographs: N.F. Glasser]

The cross-sectional form of individual glacial troughs is often described as being 'U-shaped', but their true morphometry is more accurately described by empirical power-law or quadratic equations. If a slope profile is surveyed from the centre of a trough up one of its sides then this profile can be described by mathematical equations. In this way the cross-profiles of individual troughs can be compared to examine, for example, the role of lithology in determining trough morphology. The simplest equation used to describe the cross-sectional morphology of a trough is a power-law equation such as:

Y = aXb where Y is the vertical distance from the valley floor, X is the horizontal distance from the centre of the valley, a is a constant and b is a measure of the profile curvature.

Glacial Erosion Features

Figure 6.13 Photograph of a glacial trough hanging above Golfo Elefantes, a deep fjord on the western coast of Chile. Note the steep valley sides and parabolic cross-section. [Photograph:

Figure 6.13 Photograph of a glacial trough hanging above Golfo Elefantes, a deep fjord on the western coast of Chile. Note the steep valley sides and parabolic cross-section. [Photograph:

Most glacial troughs have values of b of between 1.5 and 2.5. A parabola would have a value of 2. Alternatively the cross-sectional shape of a glacial trough can be described by a quadratic equation, such as:

where Y is the vertical distance from the valley floor, X is the horizontal distance from the centre of the valley and a, b, c are coefficents determined stastically for each trough.

The choice of equation used in studies of troughs depends on the aim of the study. If the aim is to compare the variation of trough profiles from a standard shape such as the parabola then the power law equation is most applicable. However, if the aim is to compare the shape of individual troughs with one another then the quadratic equation is most appropriate.

The morphometric description of troughs is a powerful tool, because it allows the variation in trough form to be examined objectively. For example, we might expect trough morphology to vary with the lithology, or the strength of the rock mass, into which it is cut. By comparing mathematically the shape of troughs cut in one type of bedrock with those cut in another such hypotheses may be tested (Box 6.4).

It has been suggested that glacial troughs represent equilibrium landforms, such that once the initial morphology of a trough is established it changes little during erosion. This suggests that troughs represent the adaptation, by erosion, of

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