|models of ice flow
"On reaching Sulem Roe from the north, the eye at once fixes on a large roche moutonnée of diorite, which rises to a height of 200 feet above the sea-loch, and the surface of which is finely polished and striated, the marking pointing W.5˚S"
Ben Peach, 1879 p792
Hirda Field, 119 m OD
Lunna. Small ice-moulded rock knob. Ice moving R to L.
Roches moutonnées in
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 Brough, Ronas Voe. Ice movement from R to L, with plucking of lee side faces and formation of roches moutonnées.
moutonnées occur widely on Shetland. They
vary greatly in size. Some are only a few metres in height but in areas of
ice-moulded terrain these streamlined rock hills can reach heights of 25 m.
Fair Isle (HU 204706) Ice movement from left to right or from the west. Photo by Hamish Ross.
Roches moutonnées provide insights into a number of problems of the glacial history of Shetland:
Patterns of ice flow: ruches moutonnées are aligned roughly parallel to ice flow.
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 Shetland the orientation of roches moutonnées generally conforms to that other indicators of ice flow. More significantly, large roches 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.
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 ubiquity of roches moutonnées on Shetland requires that former glaciers were warm-based at all levels.