Mesoscale Features Of Glacial Erosion

Mesoscale features of glacial erosion are those between 1 m and 1 km in size and comprise a family of landforms which includes: (i) streamlined bedrock features; (ii) stoss and lee forms; (iii) rock grooves and basins; and (iv) meltwater channels (Table 6.2).

6.2.1 Streamlined Bedrock Features

At a mesoscale the most common effect of glacial erosion is to streamline bedrock protrusions to produce positive and upstanding landforms. These streamlined landforms, whalebacks, are also referred to by a wide variety of different terms

Table 6.2 Mesoscale landforms of glacial erosion and their significance for the reconstruction of former ice masses.

Landform

Morphology

Glaciological significance

Streamlined bedrock features ('whalebacks')

Stoss and lee forms ('roches moutonnees')

Subglacial meltwater channels

Streamlined bedrock eminences with abraded surfaces on all sides.

Upstanding bedrock eminence with both abraded and quarried faces.

Detailed morphology controlled by preglacial weathering characteristics and patterns of bedrock jointing.

Rock grooves, bedrock megagrooves and rock basins

Smooth-walled sculpted depressions and channels cut in bedrock.

Steep sided channels cut into bedrock or till. Channel orientation may be discordant with the local topography. Channels may have an irregular convex-up long profile.

Warm-based ice carrying a basal debris load.

High effective normal pressures (>1 MPa) inferred from intimate ice-bedrock contact and cavity suppression.

Thick ice.

Low sliding velocity with little available basal meltwater.

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 form under thin ice, for example possibly during ice-sheet build-up and decay.

May indicate direction and orientation of ice flow.

Warm-based ice carrying a basal debris load.

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 indicate rapid sliding velocity and thin ice. They may also indicate direction and orientation of ice flow.

Landforms occurring in association with striae indicate high effective normal pressures (>1 MPa) inferred from intimate ice-bedrock contact and cavity suppression.

Warm-based ice carrying a basal debris load.

Channel systems can be used to calculate former hydraulic potential gradient and therefore to infer the regional pattern of subglacial drainage, and to estimate ice-surface slope and ice thickness.

Ice-marginal meltwater channels

Either a complete channel cross-section or a channel floor and one wall wherever the other wall was formed by ice (half channel). Channels start and end abruptly.

Often associated with other ice-marginal depositional landforms.

Calculations of palaeovelocity and palaeodischarge possible from measurements of channel shape, channel width and size of material transported by meltwater flow.

Release of large quantities of supra-, en- or subglacial meltwater. Channels indicate the location of the former ice margin and patterns of ice recession.Gradient of channel long profile may indicate that of the ice margin.

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.]

such as rock drumlins, tadpole rocks and streamlined hills (Figures 6.6 and 6.7). Whalebacks are bedrock knolls that have been smoothed and rounded on all sides by a glacier. Individual whalebacks may be slightly elongated in the direction of ice flow, although the structural attributes of the bedrock (e.g., joints, bedding planes and foliations) may dramatically affect the morphology of their overall form. Whalebacks tend to have low height to length ratios. They are relatively high (1-2 m) in comparison to their length (1.5-3 m). Striations and other small-scale features of glacial abrasion may be superimposed on any surface of a whaleback. Striations on whalebacks are often continuous along the entire length of the whale-back. From this it is possible to infer that there were no basal cavities around the whaleback during its formation and the ice was everywhere in contact with

Global Warming The Earth Then And Now
Figure 6.6 Examples of streamlined bedrock features. (A) Whaleback in resistant granite, Norway (former ice flow left to right). (B) Roche moutonnee above Nant-y-Moch, Wales (former ice flow left to right). [Photographs: N.F. Glasser]

Whaleback

Ice flow

Whaleback

Roche moutonnée

Ice flow

Glacial till or softer rock

Pod (school) of whalebacks

Glacial till or softer rock

Crag and trail

Pod (school) of whalebacks

Crag and trail

Figure 6.7 The four main types of streamlined glacial erosional landforms.

the landform so that landforms were produced primarily by glacial abrasion. High effective normal pressures therefore must be present on both the proximal and distal faces of the whaleback in order to suppress the formation of basal cavities.

In contrast to whalebacks and other landforms dominated by glacial abrasion, stoss and lee features possess both abraded and plucked surfaces, and have therefore a pronounced asymmetry (Figures 6.6 and 6.7). They are defined as bedrock knolls or small hills with a gently abraded slope on the up-ice side (stoss) and a steeper, rougher plucked or quarried slope on the down-ice side (lee). The most common type of stoss and lee landform is a roche moutonnée. These landforms often occur in clusters or fields and may vary in size from several metres to tens or hundreds of metres. They form where high effective normal pressures occur on the stoss side of a bedrock hummock, but the pressure is sufficiently low on the down-ice side to allow a cavity to form (see Section 4.6). Consequently the up-ice side experiences glacial abrasion while the down-ice side is glacially plucked. The presence of a lee-side cavity is pre-requisite for the formation of a roche moutonnee and consequently they are restricted to areas where the ice flows fast enough and the effective normal pressure is sufficiently low to allow cavities to open. Roches moutonnees therefore form preferentially in areas of thin and fast flowing ice. As glacial quarrying is facilitated by regular and frequent fluctuations in basal water pressure, their formation is also facilitated by the presence of subglacial meltwater.

Once a lee-side cavity has opened and glacial plucking is initiated, the properties of the parent bedrock determine the detailed morphology of the resulting roche moutonnee (Figure 6.8). Bedrock jointing is particularly important because joint depth and spacing determine the size of the blocks that can be quarried from the lee of the original bedrock hummock. It is possible to predict the evolution of the plucked surface of a roche moutonnee (Figure 6.9). Block removal will begin at the furthest point down-ice in the cavity and as successive blocks are removed the

6.2.2 Stoss and Lee Features

Massive rock

Foliation dipping up ice

Layering dipping down ice

Layering Layering dipping Layering dipping dipping down ice gently up ice steeply up ice

Direction of ice flow

Figure 6.8 Schematic representation of the relationships between geological structure and roche moutonnée morphology. (A) Quarried lee-side slopes. (B) Abraded lee-side slopes. (C) Abraded and quarried lee-side slopes. [Modified from: Chorley et al. (1984) Geomorphology, Methuen, figure 17.18, p. 449]

Ice flow

Figure 6.9 A theoretical model of the evolution of a roche moutonnee. Blocks are removed successively from number 1 to number 10. [Modified from: Sugden et al. (1992) Geografiska

Figure 6.9 A theoretical model of the evolution of a roche moutonnee. Blocks are removed successively from number 1 to number 10. [Modified from: Sugden et al. (1992) Geografiska

quarried surface will migrate further up-ice. This results in a quarried lee-side face that resembles a staircase (Figure 6.9). The spacing of the horizontal and vertical joints within the bedrock will determine the dimensions of each step within the staircase. In bedrock that is not heavily jointed the glacier may create its own

Pictures Features Glacier Erosion

Figure 6.10 The distribution of microscale features of glacial erosion on the surface of roches moutonnées. (A and B) Plan and profile views of typical roches moutonnees. (C) A stereographic model of the distribution of microscale features of glacial erosion across the surface of a typical roche moutonnee. [Modified from: Chorley etal. (1984) Geomorphology, Methuen, figure 17.17, p. 448]

Figure 6.10 The distribution of microscale features of glacial erosion on the surface of roches moutonnées. (A and B) Plan and profile views of typical roches moutonnees. (C) A stereographic model of the distribution of microscale features of glacial erosion across the surface of a typical roche moutonnee. [Modified from: Chorley etal. (1984) Geomorphology, Methuen, figure 17.17, p. 448]

fractures due to basal pressure fluctuations. In this case the staircase formed by the removal of blocks may be more varied.

A variety of other microscale erosional landforms often occur on the surfaces of roches moutonnees (Figure 6.10). Although roches moutonnees are characterised by abraded stoss slopes and quarried lee sides the orientation of these two surfaces is not always a reliable indicator of ice movement direction because their detailed morphology is controlled in part by the pattern of joints or other weaknesses within the rock mass (Figure 6.8). This reduces their reliability as palaeo-ice-flow indicators.

6.2.3 Grooves and Rock Basins

Grooves can be formed by either glacial abrasion or by meltwater erosion. Grooves are similar in morphology to striations, except for their greater size and greater depth. They range from tens of metres to hundreds of metres in length and may be up to several metres wide and a metre deep. They are probably the product of glacial abrasion although the flow of meltwater can also be important. The location and orientation of individual grooves may also be influenced by the presence of structural weaknesses within the rock mass.

Rock basins are individual depressions carved in bedrock. They are often found in association with roches moutonnees and may fill with water on deglaciation to become lakes. Rock basins range in size from several metres to hundreds of metres in diameter. The development of these basins is controlled by the distribution of structural weaknesses within a rock mass, which can be exploited by glacial quarrying. The size and density of the basin is therefore usually a function of the spacing of joints, or other lines of weakness, within the rock mass.

The formation of rock basins beneath a glacier provides a good illustration of the role of positive feedback within landform development. The situation is as follows: as a glacier flows over an irregular bed it develops zones of compressional and extensional flow (see Figure 3.8). Zones of compressional flow will tend to transport material away from the bed, whereas extensional flow will cause ice to move towards the bed. Erosion by glacial quarrying will be limited in areas of extensional flow because bedrock blocks cannot be transported away from the bed easily, although abrasion may be facilitated by the increased contact pressure between basal clasts and the bed. Net erosion by quarrying is favoured in areas dominated by compressional flow because the eroded blocks can be transported away from the bed. Consequently, as a glacier flows over a slight depression it will first experience extensional flow on the up-ice side and then compressive flow on the down-ice side of the depression. The extensional flow component will increase basal pressure and abrasion of the basin floor whereas the compressional phase will facilitate block removal and plucking. The form of the basin, therefore, will be accentuated by erosion. This will in turn increase the degree of extensional and compressional flow experienced by the glacier as it flows over the basin, which will in turn accelerate its erosion. In this way the basin will grow as consequence of the positive feedback between basin erosion and compressional flow.

6.2.4 Meltwater Channels

The final group of mesoscale landforms of glacial erosion are meltwater channels (Figure 6.11). Meltwater channels can form in five environments (Table 6.3): (i) subglacially (beneath the ice); (ii) laterally (along the ice margin); (iii) in proglacial locations (in front of the ice), associated with the flow of water away from the glacier or out of ice-contact lakes; (iv) in supraglacial (ice surface) or englacial (within the ice) environments; and (v) from subglacial lake outbursts.

The orientation of subglacial meltwater channels is controlled by the hydraulic potential gradient within the glacier (see Section 4.6.2). This gradient is determined primarily by the glacier surface slope and secondarily by the topography beneath the glacier. As a consequence, subglacial meltwater channels may cut across or be orientated transverse to the surface contours and drainage patterns of the present-day topography. Subglacial meltwater is also able to flow uphill if driven by the hydraulic gradient within the glacier. Consequently, subglacial meltwater channels may not have constant gradients but may have an 'up and down', or 'humped' long profile.

In contrast, ice-marginal and lateral meltwater channels run parallel to the glacier front and are most commonly found in the ablation area of glaciers where rates of surface ablation and meltwater production are highest. The morphology of these channels is variable: for example they may stop suddenly as meltwater is diverted into the glacier via a crevasse or moulin. This process may be recorded by a chute or sudden right angle bend in the channel. Ice-marginal channels sometimes start in large bowl-shaped depressions, interpreted as plunge pools formed by water falling from the glacier into the channel in the same way as plunge pools form on rivers beneath waterfalls. Some lateral meltwater channels have a distinct

Figure 6.11 Photographs of typical meltwater channels: (A) Ice-marginal meltwater channels next to a small valley glacier in Svalbard. (B) Subglacial meltwater channel near Helsby in Cheshire, England. [Photographs: N.F. Glasser]

Table 6.3 Diagnostic criteria for classification of meltwater channels.

Subglacial

Lateral

Marginal

Submarginal

Proglacial

Supraglacial/englacial

Undulating long profile

Descent downslope may be oblique

Descent downslope may form steep chutes

Complex systems - bifurcating and anastomosing

High sinuosity

Abandoned loops

Abrupt beginning and end

Absence of alluvial fans Cavity systems and potholes

Ungraded consequences

Variety of size and form within the same connected system

Association with eskers

Parallel with contemporary contours

Form 'series' of channels parallel to each other

Approximately straight

Perched on valley sides

May terminate in downslope chutes

Absence of May form networks networks

Gentle gradient Steeper gradient

(oblique downslope)

Parallel for long Sudden changes in distance direction

May terminate abruptly

May be found in isolation from all other glacial features

Regular meander bends

Occasional bifurcation

Flows direct downslope

Large dimensions-wide and deep

Meander forms crescentic valley on face of hill Low gradient

Sinuous

Approximately constant width

[Modified from: Greenwood et al. (2007) Journal of Quartemary Science, 22, Table 1, p. 641]

Ln Ln cross-sectional form, whereas others may simply have a cross-section that resembles a bench or step cut into the hill side. The longitudinal gradient of ice-marginal channels approximates that of the glacier surface at the time of their formation.

In proglacial settings, powerful streams draining the glacier may cut distinct channels or gorges as the water flows away from the ice margin. Aggradational landforms, including sandar and braided outwash channels, are also common. Ice-dammed lakes may drain over cols or ridges via overspill channels. These are usually channels cut into cols or in notches within ridges or hills surrounding an ice-dammed lake.

Finally, meltwater channel systems can also be cut by subglacial lake outburst floods, where large volumes of water drain catastrophically from beneath a glacier or ice sheet towards the ice margin. One of the most famous examples of this is the Labyrinth in the Dry Valleys of Antarctica (Box 6.3), but there are also examples in the northwestern North America associated with the drainage of Glacial Lake Missoula and examples associated with subglacial volcanic activity in Iceland (jokulhlaups).

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  • michael
    What are examples of mesoscale landforms?
    1 year ago
  • Sandra
    What are features created by glacial erosion?
    9 months ago
  • lillo greece
    How do glacial whalebacks form?
    7 months ago

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