Inglaciology, aroche moutonnée (orsheepback) is arock formation created by the passing of aglacier. 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 ascrag and tail, though they are formed in essentially the same way.[citation needed]
The 18th-century Alpine explorerHorace-Bénédict de Saussure coined the termroches moutonnées in 1786. He saw in these rocks a resemblance to thewigs that were fashionable amongst French gentry in his era and which were smoothed over withmutton fat (hencemoutonnée) so as to keep the hair in place.[1]: 323–324 The French term is often incorrectly interpreted as "sheep rock".[2][3]
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 withglacial striations.
The rough and craggy down-ice (leeward) side is formed byplucking 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 tofrost 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 adrumlin (a depositional feature). In a drumlin, the steep side isfacing the approaching glacier, rather thantrailing it.
Even larger examples are known from Sweden where they are referred to asflyggbergs.[1]: 326–327 The Swedishflyggbergs have been interpreted bySten Rudberg and others as reshapedinselbergs.[4] Ice-smoothed bedrock bumps which lack the steep, plucked lee side faces are referred to aswhalebacks[5] orrock 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 shapebefore glaciation.Jointing that contributes to their shape typically predates glaciation, and roche moutonnée-like forms can be found in tropical areas such asEast Africa andAustralia. Further, atIvö Klack in Sweden, rock surfaces that have been exposed bykaolin mining and then become weathered resemble roches moutonnées.[7]
Media related toRoches moutonnées at Wikimedia Commons