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, clim ...
, a roche moutonnée (or sheepback) is a
rock formation
A rock formation is an isolated, scenic, or spectacular surface rock (geology), rock outcrop. Rock formations are usually the result of weathering and erosion sculpting the existing rock. The term ''rock Geological formation, formation ...
created by the passing of a
glacier
A glacier (; or ) is a persistent body of dense ice, a form of rock, that is constantly moving downhill under its own weight. A glacier forms where the accumulation of snow exceeds its ablation over many years, often centuries. It acquires ...
. 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
[Douglas Benn and David Evans, ''Glaciers & Glaciation,'' Arnold, London, 1st ed. 1998 ] and refer to larger features as
crag and tail
A crag (sometimes spelled cragg, or in Scotland craig) is a rocky hill or mountain, generally isolated from other high ground.
Origin
Crags are formed when a glacier or ice sheet passes over an area that contains a particularly resistant r ...
, though they are formed in essentially the same way.
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 covering made from human or animal hair, or a synthetic imitation thereof. The word is short for "periwig". Wigs may be worn to disguise baldness, to alter the wearer's appearance, or as part of certain professional uniforms.
H ...
s that were fashionable amongst French gentry in his era and which were smoothed over with
mutton
Lamb and mutton, collectively sheep meat (or sheepmeat) is one of the most common meats around the world, taken from the domestic sheep, ''Ovis aries'', and generally divided into lamb, from sheep in their first year, hogget, from sheep in thei ...
fat (hence ''moutonnée'') so as to keep the hair in place.
The French term is often incorrectly interpreted as "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 w ...
.
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, 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.
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 ("little 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 till or groun ...
(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, an ...
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, since they may have already had a similar shape
before glaciation.
Jointing that contributes to their shape typically predates glaciation, and roche moutonnée-like forms can be found in tropical areas such as
East Africa
East Africa, also known as Eastern Africa or the East of Africa, is a region at the eastern edge of the Africa, African continent, distinguished by its unique geographical, historical, and cultural landscape. Defined in varying scopes, the regi ...
and
Australia
Australia, officially the Commonwealth of Australia, is a country comprising mainland Australia, the mainland of the Australia (continent), Australian continent, the island of Tasmania and list of islands of Australia, numerous smaller isl ...
. Further, at
Ivö Klack in Sweden, rock surfaces that have been exposed by
kaolin
Kaolinite ( ; also called kaolin) is a clay mineral, with the chemical composition Al2 Si2 O5( OH)4. It is a layered silicate mineral, with one tetrahedral sheet of silica () linked through oxygen atoms to one octahedral sheet of alumina (). ...
mining and then become weathered resemble roches moutonnées.
See also
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References
Sources
*
*
External links
A roche moutonnée and perched boulders, northeastern Manitoba image from
Geological Survey of Canada
The Geological Survey of Canada (GSC; , CGC) is a Canadian federal government agency responsible for performing geological surveys of the country developing Canada's natural resources and protecting the environment. A branch of the Earth Science ...
Canadian Landscapes Photo Collection
A roche moutonnée, northern Abitibi, Québec image from
Geological Survey of Canada
The Geological Survey of Canada (GSC; , CGC) is a Canadian federal government agency responsible for performing geological surveys of the country developing Canada's natural resources and protecting the environment. A branch of the Earth Science ...
Canadian Landscapes Photo Collection
A roche moutonnée, Melville Peninsula, Nunavut image from
Geological Survey of Canada
The Geological Survey of Canada (GSC; , CGC) is a Canadian federal government agency responsible for performing geological surveys of the country developing Canada's natural resources and protecting the environment. A branch of the Earth Science ...
Canadian Landscapes Photo Collection
{{DEFAULTSORT:Roche moutonnee
Glacial erosion landforms