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Terrestrial Glacial Features - Erosional Landforms
Introduction Ice that moves may have a considerable effect in altering the features of the surface, either unconsolidated or solid rock, over which it passes, particularly if 'armed' with loose rock. Ice that is free of such debris has little erosive effect on resistant unjointed rock, although on softer and well-jointed rock, ice movement results in fragments of rock being eroded and transported. A range of glacial landforms are produced by processes of glacial erosion. Although the processes and mechanics of glacial erosion are poorly understood, there are three principal mechanisms of glacial erosion recognised; glacial abrasion, glacial plucking and glacial meltwater erosion. The primary or regional pattern of glacial erosion within a glacier is controlled by the basal thermal regime. Only when a glacier is warm based meltwater is produced in large quantities, and only when meltwater is abundant can basal sliding and therefore glacial abrasion occur. Glacial plucking is also facilitated by the presence of meltwater. The erosional processes have left distinct landforms. These can be organised on the basis of scale. Common micro-scale landforms are striations, micro crag and tails, friction cracks and p-forms. At meso--scale the following can be recognised; stream lined bedrock landforms, stoss and lee forms, grooves and basins and meltwater channels. Macro--scale landforms include areas of areal scour, glacial troughs, cirques, giant stoss and lee forms and tunnel valleys. Although there are many erosional landforms, the subject of Roche Moutonnée will form the main focus for this poster.
It is necessary to examine all three processes, before specifically looking at the process that produces Roche Moutonnées. Glacial abrasion is when debris is transported along the base, causing striations, see Plate 2. Many variables effect the process of glacial abrasion, including basal contact pressure, which is the amount of pressure the glacier exerts; basal sliding, which is how fast the glacier is moving; and the amount of debris available, as the glacier cannot erode unless there is material present, this material is often produced from frost shattering (Bennett & Glasser, 1996). Glacial plucking occurs when the ice freezes around an obstacle and then the subsequent movement causes the obstacle to be pulled out and carried along (the obstacle will have been weakened by frost shattering before it can be pulled out). Pressure release cracking occurs when the ice moves away from the rock and as it slowly expands it cracks (Clowes & Comfort, 1997). Glacial meltwater erosion can be either chemical or mechanical. This process relies on many factors, including
the structure of the bedrock, the amount of meltwater discharge and the quality of the sediment in the meltwater
(Bennett & Glasser, 1996)
Roche Moutonnées Roche Moutonnée, derives its name from early European and British Courts. Where 'moutonnée' was
a smoothed, curled wig worn by barristers. There is also the analogy of the back end of a sheep, hence 'mouton'.
Therefore, the smoothed flank is supposed to resemble a wig or a sheep!!!! (Sharp, 1992) Roche Moutonnée is an example of a Stoss and Lee feature. It is regarded as a hallmark of glacial erosion (Sugden, 1976). A Roche Moutonnée is a bedrock feature that is rounded, smoothed, grooved, striated and polished by abrasion
on one flank, and made steep, jagged and irregular on the opposing flank by plucking (Sharp, 1992). An example
of a Roche Moutonnée can be seen in Plate 1, Cwm Idwal, Wales.
The depth of joints and the space between them contribute to the size of the blocks that are plucked from the original bedrock hummock. The properties of the parent bedrock determine the morphology of the Roche Moutonnée. They are best developed on well jointed granites and other crystalline rocks. As the process of plucking takes place, facilitated by meltwater and variations in basal water pressure, the rock is plucked away from the hummock from the lowest point, down-ice in the cavity. Gradually the plucking moves higher up the jagged face, in a manner resembling that of steps, Figure 1. Plucking dominates on the down ice side - the ' lee 'side, where the pressure is sufficiently low to allow a
cavity to develop and the rock is plucked away. This 'lee' face is usually higher than the 'stoss'. The up-ice
side experiences abrasion and because of the two different processes an asymmetric profile is developed.
Roche Moutonnées can vary in size from just a few metres up to hundreds of metres. Their width and length
are usually 2 or 3 times the height. As the 'lee' side is a characteristic feature of a Roche Moutonnée, they can only form in areas where
the ice velocity is high enough and the effective normal pressure low enough to allow cavities to open. They are
found in areas of thin and fast flowing ice and an abundance of meltwater. Roche Moutonnées are often found
in clusters or fields (Bennett, 1999). Examples of Roche Moutonnées can be found in Cwm Idwal, Wales; Lochinver,
Scotland; Heinabergsjokull, Iceland; Sondre Stromfjord, West Greenland.
As a feature of glacial erosion, the specific subject of Roche Moutonn6e is a neglected part of glacial research. For example it is assumed that Roche Moutonnées are a product of erosion beneath thick ice during glacial maxima, (Sugden & John, 1976). This however, is just a generalisation, as it is untested. Gordon (198 1), highlights how the fracturing and quarrying of stoss side layers has generally taken place on their down-ice slope sides, so that relatively smooth profiles result. His research concerning the relationships of bedrock structure and ice movement (conducted in Northern Scotland and West Greenland) indicate that bedrock layers have also been fractured on their up-ice flanks creating relatively rough profiles, Figure 2. Upper edges of the rises are generally rounded off into convexities, but the lower junctions between the rises and treads are frequently angular. Sugden et al (1992) conducted research to investigate the Evolution of Large Roche Moutonnées'. The study took place in Upper Deeside, in the Grampian Mountains, Scotland, where he compared the evolution to the morphology predicted by theory. His results indicated a 'good fit' to the existing theories. Some glaciologists, e.g. Sharp (1992) are of the opinion that Roche Moutonnées are elongated in the direction
of ice flow. However, Bennett & Glasser (1999) suggest that the orientation of the surfaces is not a reliable
indicator of the direction of ice flow; as the morphology is a factor depending upon patterns of joints or foliations
in the rock.
In conclusion, it can be seen that glacial erosion has formed distinctive landscapes and each landform tells a story about the glacier that formed it. The Roche Moutonnée is a predominant landform, that is dominated by glacial abrasion and therefore, according to Glasser (1999) have pronounced asymmetry. This is especially true of the Roche Moutonnée in Plate 1, in Snowdonia. However, erosional processes and mechanics of glacial erosion are poorly understood and therefore, there has
been little research on the formation of Roche Moutonnées which has led to the debate of their formation. Bennett, M.R & Glasser, N.F (1996/1999) Glacial Geology Ice Sheets and Landforms, Wiley, Chichester
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