Tor modification to a roche moutonnée-like form, Beinn Mheadhoin
Sheet jointing and form of a roche moutonnée on the N side of Loch Avon after Sugden and John (1976)
An explanation of the origin of the term roche moutonnée from the excellent interpretative site at Dulnain Bridge
Sugden et al. (1992) have provided a useful model of how spurs in the Dee valley have been modified beneath thick ice to form large roches moutonnées.
Stac an Fharaidh on the plateau above Loch Avon. The preglacial spur has been plucked on the left (down-ice) face to give cliffs. Weathering pits >10 cm deep on the upper surface of the roche moutonnée indicate formation before the last interglacial
Definition: A roche moutonnée is a rock hill shaped by the passage of ice to give a smooth up-ice side and a rough, plucked and cliff-girt surface on the down-ice side. The upstream surface is often marked with striations. Whalebacks are sister forms.
Cnapan a’ Mheirlich, Glen Avon
Roches moutonnées occur widely in the Cairngorms, both on the granite and on the surrounding metamorphic country rocks. Like other landforms of glacial erosion, roches moutonnées are usually absent from the Cairngorm plateau, although good examples occur at the head of Glen Avon. The roches moutonnées vary greatly in size. Ice-moulded hills in Strathspey, such as Ord Bàn, and in Glen Dee, such as Craig Leek, have a relative relief of 200-300 m. In contrast, the granite roches moutonnées in the valley of the Garbh Uisge Mhor, on the slopes of Ben Macdui, are only a few metres in height.
The roches moutonnées of the Cairngorms provide insights into a number of problems of the glacial history of the area:
Patterns of ice flow: roche moutonnées are aligned roughly parallel to ice flow. The mega-forms of Strathspey and the Dee valley mark the passage of thick ice along these corridors, whilst the roches moutonnées above Glen Avon reflect ice flow towards the northeast.
Several factors complicate this basic relationship between orientation and patterns of former ice flow. Rock structure can favour the development of ice-moulded hills that do not parallel ice flow (Gordon, 1981), although in the Cairngorms the orientation of roche moutonnées generally conforms to that other indicators of ice flow. More significantly, large roche moutonnées have been shaped during multiple phases of glaciation. The direction of ice flow may have been similar in each phase but, in some places, it was not. The spurs of the southern Cairngorms, such as Carn Crom, show a dominance of lee-side plucking on eastern faces but also signs of plucking on southern faces. This duality may reflect two directions of ice flow; one from the Lairig Ghru and the other, more restricted in time or erosional capacity, from Glen Derry. Similarly, on Cnapan a' Mheirlich (above) there is a scatter of metamorphic erratics which reflects a late phase of ice flow that is not related to the many phases of northeasterly flow that shaped the hill.
Depths of glacial erosion: at a few locations, small roches moutonnées occur downslope of tors and represent the glacial modification of pre-existing forms. One of the best examples occurs on the northern spur of Beinn Mheadhoin.
The evolution of large roches moutonnées can be traced by using the curved sheet joints of granite spurs as indicators of the preglacial form. Where remnants of tors occur upslope, the bites taken out from the spur by leeside plucking represent the sum of Quaternary glacial erosion (Sugden et al., 1992).
Former glacier basal thermal regime: both the abraded stoss side and the plucked lee side of roches moutonnées require the former presence of sliding ice and meltwater. The presence of roches moutonnées means therefore that the ice masses that progressively shaped these bedrock hills were warm based, with ice at the glacier bed above its pressure melting point. The valley-floor roches moutonnées reflect the passage of thick, fast-moving and sliding ice streams. The high-level roches moutonnées above Loch Avon indicate that during some phases of the Quaternary warm-based ice crossed the edges of the surrounding plateau.
Age of features: low-level roches moutonnées show only shallow weathering pits, indicating significant erosion by the last ice sheet. Some high-level ice-moulded rock surfaces have deep weathering pits indicating formation before the last ice sheet.