Plans of roches moutonnées on Corstorphine Hill showing the variety of form and how it relates to joint patterns in the quartz dolerite sill.

Roches moutonnées and structure

Significance: the pattern and orientation of joints and fractures is important in determining the form of roches moutonnées

Classic roche moutonnée developed in orthogonally jointed quartz dolerite at Corstorphine Hill

Roches moutonnées range in size from 1-100 m or more in height yet all retain a classic asymmetric form which relates to the processes of abrasion and block quarrying beneath the glacier. The exact form of the ice-moulded hill is often however more an expression of bedrock structure than of glacial processes. Roches moutonnées are most clearly developed in strong rocks with orthogonal joint sets and where there are lateral variations in joint density. Even here however the attitude and spacing of the joints can give wide variations in form. Where rock structures dip into the direction of ice flow then formation of lee side cliffs is favoured. Where the dip is in the opposite direction then the hill may remain obversely asymmetric, with a cliff facing up the direction of ice flow. Salisbury Crags (below) provide a clear example of the primacy of rock structure over the form of glacially-eroded hills.

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