Roche moutonnée

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Roche moutonnée near Myot Hill, Scotland

In

rock formation created by the passing of a glacier. The passage of glacial ice over underlying bedrock often results in asymmetric erosional forms as a result of abrasion on the "stoss" (upstream) side of the rock, and plucking (i.e. pieces cracked off) on the "lee" (downstream) side. Some geologists limit the term to features on scales of a metre to several hundred metres[1]: 324–326  and refer to larger features as crag and tail
, though they are formed in essentially the same way.

Etymology

The 18th-century Alpine explorer

mutton fat (hence moutonnée) so as to keep the hair in place.[1]: 323–324  The French term is often incorrectly interpreted as "sheep rock".[2][3]

Description

An oblique view of a roche moutonnée surface at Great Slave Lake, Northwest Territories, Canada; notice the contiguous, wavy rows of glaciated bedrock which resemble old-fashioned wigs as mentioned by Horace de Sassure

The contrasting appearance of the erosional stoss and lee aspects is very defined on roches moutonnées; all the sides and edges have been smoothed and eroded in the direction travelled by the glacier that once passed over it. It is often marked with

glacial striations
.

The rough and craggy down-ice (leeward) side is formed by

frost shattering, ripping chunks away from the rock formation. Studies show that the plucking of the lee side is a much more significant erosional process than the abrasion of the stoss side.[citation needed
]

The side profile of a stoss and lee glaciated, bedrock knob (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.[1]: 326–327  The Swedish flyggbergs have been interpreted by Sten Rudberg and others as reshaped inselbergs.[4] Ice-smoothed bedrock bumps which lack the steep, plucked lee side faces are referred to as whalebacks[5] or rock drumlins.[1]: 326–327 

Prest (1983) specifies a distinction between a glaciated "roches moutonnées surface" and a simple "stoss and lee" glacial feature. He says that the term "roches moutonnées surface" has been abused in the literature in which the term became interchangeable with the term "stoss and lee". He points out that a "roches moutonnées surface" is a continuous bedrock surface having a resemblance to the continuous, wavy or undulating rows of curls seen in French wigs at the time of Horace de Saussure while a simple stoss and lee feature refers only to a bedrock knob having a smooth stoss side and a plucked lee side appearance.[6]

Roches moutonnées may not be entirely glacial landforms, since they may have already had a similar shape

kaolin mining and then become weathered resemble roches moutonnées.[7]

See also

  • Glacial landforms
     – Landform created by the action of glaciers

References

  1. ^
    ISBN 9780340584316
  2. ISBN 978-0321714725
  3. ^ McInnes, Marnie (2017-07-07). "Why Geologists Think Glacial Mountains Look Like Sheep". The Atlantic. Archived from the original on 2017-07-15. Retrieved 2017-07-15.
  4. ^ Lidmar-Bergström, Karna; Olvmo, Mats (2015). Plains, steps, hilly relief and valleys in northern Sweden – review, interpretations and implications for conclusions on Phanerozoic tectonics (PDF) (Report). Geological Survey of Sweden. p. 13. Archived (PDF) from the original on October 26, 2016. Retrieved June 29, 2016.
  5. doi:10.1016/j.sedgeo.2023.106399
    . Retrieved 12 November 2024 – via Elsevier Science Direct.
  6. ^ Prest, V.K. (1983). Canada’s heritage of glacial features. Miscellaneous Report 28, Geological Survey of Canada, pp. 26–27, fig. 24 a, b and c
  7. JSTOR 521265
    .

Sources

Media related to Roches moutonnées at Wikimedia Commons

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