Etymology

The term “roche moutonnée” was introduced by French geologist Horace-Bénédict de Saussure (1740-1799). The origin of the word lies in its resemblance to the shape of a wig worn by 18th century French aristocrats whose hair was shaped like a wave. Sheep fat was used to hold the wig in place, thus explaining the term moutonné (Benn and Evans, 2010). In Sweden, very large roches moutonnées are called flyggbergs, some being up to 3 km long and 350 m wide (Rudberg, 1954; 1973; Iverson et al., 1995; Benn and Evans, 2010).

Genesis

Roches moutonnées are products of glacial erosion and explained by the presence of an obstacle creating a constraint to the ice flow (irregularity of bedrock topography). The two main erosional processes that produce roches moutonnées are abrasion on the surface up-ice and the plucking of rock fragments down ice. The plucking process is responsible for the erosion of a larger volume of rock than the abrasion process (Jahns, 1943; Boulton and Jones, 1979; Cohen et al., 2006). It is dependent on the orientation of joint and fracture systems, which is intrinsic to the bedrock structure (Glasser and Warren, 1990; Sugden et al., 1992).

Once the glacier comes in contact with a rocky obstacle, a succession of processes leads to the formation of roches moutonnées:

1. Increase in cryoclastic pressure at the point of contact with the obstacle. Particles move towards the footwall and increase abrasion (Boulton et al., 1974; Glasser and Warren, 1981), causing the formation of striations and grooves. The increase in cryostatic pressure lowers the melting temperature of the ice, leading to partial melting at the base;
2. Naturally flowing towards low-pressure zones, water bypasses the obstacle down ice;
3. A low-pressure zone (cavity) is present down ice of the obstacle, causing an increase in melting temperature so that the water refreezes in rock fractures. Also, the low pressure allows new fractures to form (Carol, 1947; Iverson, 1991; Hallet, 1996). As water seeps into cracks, it causes rock fragments to break off, creating a plucking face.

Cavity formation plays a major role in the formation of roches moutonnées. Cavities and, by extension, roches moutonnées preferentially develop in temperate, thin-glacier regions, in marginal positions and where meltwater is abundant (Sugden et al., 1992; Roberts and Long, 2005; Benn and Evans, 2010). The presence of striations suggests warm-based glacial dynamics and a basal deformation flow mode.

It has been suggested that roches moutonnées are preglacial in origin (Lindström, 1988; Patterson and Boerboom, 1999) and that the fluvioglacial processes of Carol (1947) presented above merely accentuated pre-existing bedforms. Although these models are consistent, the ubiquity of roches moutonnées must be explained by a simple process that can occur anywhere. According to Boulton (1982), the difference in up-ice/down-ice pressure alone would be sufficient to cause the formation of fractures on rock reliefs in subglacial conditions.

Spatial Distribution

Roches moutonnées are ubiquitous bedforms that are, in Quebec, listed in almost all regional works dealing with Quaternary geology. Roches moutonnées are generally concentrated in areas of competent bedrock such as granitoids (Glasser, 2002).

Synonyms

Sheepback rock, sheep rock, embossed rock, mounded rock, glaciated rock knoll.

References

Contributors