Roche moutonnée
In glaciology, a roche moutonnée (or sheepback) is a rock formation created by the passing of a glacier. The passage of glacier ice over underlying bedrock often results in asymmetric erosional forms as a result of abrasion on the 'stoss' (up-ice) side of the rock and plucking on the 'lee' (down-ice) side. These erosional features are seen on scales of less than a metre to several hundred metres.[1]
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[edit] Name
The 18th-century Alpine explorer Horace-Bénédict de Saussure coined the term 'roches moutonnées' in 1786. He saw in these rocks a resemblance to the wigs that were fashionable amongst French gentry in his era and which were smoothed over with mutton fat (hence 'moutonnée') so as to keep the hair in place.[2] The French term is often incorrectly interpreted as meaning "sheep rock" [3]
[edit] Nature
The appearance of the erosional stoss and lee feature is very defined on roches moutonnées as all the sides and edges have been smoothed and eroded in the direction that the glacier that once passed over it. It is often marked with glacial striations.
The rough and craggy down-ice side is formed by "plucking" or quarrying, the erosional process in which ice melts slightly by pressure and seeps into cracks in the rock. When the water refreezes, the rock becomes attached to the glacier. But as the glacier continues its forward progress it subjects the stone to frost shattering ripping strips away from the rock formation. Studies show that the quarrying of the lee side is a much more significant erosional process than the abrasion of the stoss side.
Note that the side profile of a roche moutonnée (an erosional feature) is opposite to that of a drumlin (a depositional feature). In a drumlin, the steep side is facing the approaching glacier, rather than trailing it.
Even larger examples are known from Sweden where they are referred to as flyggbergs. Ice-smoothed bedrock bumps which lack the steep, quarried lee side faces are referred to as whalebacks or rock drumlins.[4]
[edit] See also
[edit] References
- ^ 'Glaciers & Glaciation' (Arnold, London 1998) Douglas Benn and David Evans, pp324-326
- ^ 'Glaciers & Glaciation' (Arnold, London 1998) Douglas Benn and David Evans
- ^ (Lutgens Tarbuck Tasa Essentials of Geology 11th Edition)
- ^ 'Glaciers & Glaciation' (Arnold, London 1998) Douglas Benn and David Evans, pp326-7
- Tarbuck, E.J.; F.K. Lutgens (2002). Earth: An Introduction to Physical Geography, 7th ed.. Upper Saddle River, NJ: Prentice Hall. p. 342. ISBN 0-13-092025-8.
- Trenhaile, Alan (2007). Geomorphology: A Canadian Perspective. Don Mills, Ontario: Oxford University Press. pp. 180–1. ISBN 0-19-542474-3.
[edit] External links
Media related to Roches moutonnées at Wikimedia Commons
- A roche moutonnée and perched boulders, northeastern Manitoba, image from Geological Survey of Canada Canadian Landscapes Photo Collection
- Roche moutonnée, northern Abitibi, Québec, image from Geological Survey of Canada Canadian Landscapes Photo Collection
- Roche Moutonnée, Melville Peninsula, Nunavut, image from Geological Survey of Canada Canadian Landscapes Photo Collection
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