Roche moutonnée

Roche moutonnée near Llyn Cau on Cadair Idris, Wales. Direction of glacial movement was from left to right.

In glaciology, a roche moutonnée (or sheepback) is a rock formation created by the passing of a glacier. When a glacier erodes down to bedrock, it can form tear-drop shaped hills that taper in the up-ice direction.

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. The French term is often incorrectly interpreted as meaning "sheep rock"

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", 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.

Note that the side profile of a roche moutonnée is opposite to that of a drumlin. In a drumlin, the steep side is facing the approaching glacier, rather than trailing it.

See also

• Glacial landforms

References

• 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. 

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