roche moutonnée
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In
glaciology Glaciology (; ) is the scientific study of glaciers, or more generally ice and natural phenomena that involve ice. Glaciology is an interdisciplinary Earth science that integrates geophysics, geology, physical geography, geomorphology, climato ...
, a roche moutonnée (or sheepback) is a
rock formation A rock formation is an isolated, scenic, or spectacular surface rock outcrop. Rock formations are usually the result of weathering and erosion sculpting the existing rock. The term ''rock formation'' can also refer to specific sediment ...
created by the passing of a
glacier A glacier (; ) is a persistent body of dense ice that is constantly moving under its own weight. A glacier forms where the accumulation of snow exceeds its Ablation#Glaciology, ablation over many years, often Century, centuries. It acquires dis ...
. 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 on the "lee" (downstream) side. These erosional features are seen on scales of less than a metre to several hundred metres.Douglas Benn and David Evans, ''Glaciers & Glaciation,'' Arnold, London, 1st ed. 1998


Etymology

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
wig A wig is a head or hair accessory made from human hair, animal hair, or synthetic fiber. The word wig is short for periwig, which makes its earliest known appearance in the English language in William Shakespeare's ''The Two Gentlemen of Verona' ...
s that were fashionable amongst French gentry in his era and which were smoothed over with
mutton Lamb, hogget, and mutton, generically sheep meat, are the meat of domestic sheep, ''Ovis aries''. A sheep in its first year is a lamb and its meat is also lamb. The meat from sheep in their second year is hogget. Older sheep meat is mutton. Gen ...
fat (hence ''moutonnée'') so as to keep the hair in place. The French term is often incorrectly interpreted as meaning "sheep rock".


Description

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 Glacial striations or striae are scratches or gouges cut into bedrock by glacial abrasion. These scratches and gouges were first recognized as the result of a moving glacier in the late 18th century when Swiss alpinists first associated them ...
. The rough and craggy down-ice (leeward) side is formed by plucking or quarrying, an erosional process initiated when 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 Frost weathering is a collective term for several mechanical weathering processes induced by stresses created by the freezing of water into ice. The term serves as an umbrella term for a variety of processes such as frost shattering, frost wedg ...
, ripping pieces 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. The side profile of a stoss and lee glaciated, bedrock knob (an erosional feature) is opposite to that of a
drumlin A drumlin, from the Irish word ''droimnín'' ("littlest ridge"), first recorded in 1833, in the classical sense is an elongated hill in the shape of an inverted spoon or half-buried egg formed by glacial ice acting on underlying unconsolidated ...
(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. The Swedish flyggbergs have been interpreted by Sten Rudberg and others as reshaped
inselberg An inselberg or monadnock () is an isolated rock hill, knob, ridge, or small mountain that rises abruptly from a gently sloping or virtually level surrounding plain. In Southern Africa a similar formation of granite is known as a koppie, a ...
s. Ice-smoothed bedrock bumps which lack the steep, plucked lee side faces are referred to as whalebacks or rock drumlins. 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. Roches moutonnées may not be entirely glacial landforms, taking most of their shape before glaciation. Jointing that contribute to the shape typically predate glaciation and roches moutonnee-like forms can be found in tropical areas such as
East Africa East Africa, Eastern Africa, or East of Africa, is the eastern subregion of the African continent. In the United Nations Statistics Division scheme of geographic regions, 10-11-(16*) territories make up Eastern Africa: Due to the historical ...
and
Australia Australia, officially the Commonwealth of Australia, is a Sovereign state, sovereign country comprising the mainland of the Australia (continent), Australian continent, the island of Tasmania, and numerous List of islands of Australia, sma ...
. Further, at Ivö Klack in Sweden, weathered rock surfaces exposed by
kaolin Kaolinite ( ) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is an important industrial mineral. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral ...
mining resemble roches moutonnées.


See also

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References

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External links


A roche moutonnée and perched boulders, northeastern Manitoba
image from
Geological Survey of Canada The Geological Survey of Canada (GSC; french: Commission géologique du Canada (CGC)) is a Canadian federal government agency responsible for performing geological surveys of the country, developing Canada's natural resources and protecting the en ...
Canadian Landscapes Photo Collection
A roche moutonnée, northern Abitibi, Québec
image from
Geological Survey of Canada The Geological Survey of Canada (GSC; french: Commission géologique du Canada (CGC)) is a Canadian federal government agency responsible for performing geological surveys of the country, developing Canada's natural resources and protecting the en ...
Canadian Landscapes Photo Collection
A roche moutonnée, Melville Peninsula, Nunavut
image from
Geological Survey of Canada The Geological Survey of Canada (GSC; french: Commission géologique du Canada (CGC)) is a Canadian federal government agency responsible for performing geological surveys of the country, developing Canada's natural resources and protecting the en ...
Canadian Landscapes Photo Collection {{DEFAULTSORT:Roche moutonnee Glacial erosion landforms