Other RMs

Corstorphine Hill

Cairngorms

Shetland

Central Park

RMs and structure

Roche moutonnée in the Garleton Hills. Ice movement from left to right

 

 

 

 

Roche moutonnée form of The Lamb, off North Berwick. Ice movement from left to right

 

 

 

 

 

 

 

Lobeck's diagram of roches moutonnées on the edge of the Canadian Shield. Ice movement from left to right

Roches moutonnées

Definition: rock hills shaped by the passage of ice to give a smooth up-ice side and a rough, plucked and cliffed surface on the down-ice side. The upstream surface is often marked with striations.

The roche moutonnée is one of the classic landforms of glacial erosion. These are hills of resistant rocks from 1-100 m or more high that show a strong asymmetry. The up-glacier side tends to be smooth and gently-inclined and to carry polished, scratched and chipped surfaces (or glacial microforms). The down-ice side often shows a stepped cliff, especially where the rock has well-developed joint systems.

The form and features of roches moutonnées point to the processes of subglacial erosion that shaped these landforms. Glacier ice moving against rock bumps is forced to deform around the obstruction. Any projecting edges are subject to high stresses and to highly effective abrasion as the rock-studded sole of the glacier rasps by. Frictional heating generates a film of meltwater which flows to the low pressure zone in the lee of the bump. The water may refreeze, welding the ice to the rock and promoting pull away (or plucking) of blocks. The generation of loose blocks may be encouraged by freeze-thaw or by the loading and unloading and progressive fatigue of the rock in the lee-side cavity or low pressure zone.