roches moutonnées



A low whaleback viewed from the SW along the line of ice flow











Smooth schist surfaces on a whaleback. Note the adjacent joint-aligned cleft running down towards the road and the crossing fractures, filled with soil and vegetation which define the detail of the form













Whalebacks in SW Sweden


Definition: a bedrock knoll smoothed and rounded on all sides by a glacier (Bennett and Glasser, 1996)

A fine group of ice-moulded rock forms can be seen at the excellent interpretative site at Dulnain Bridge in Strath Spey. Although labelled on the map as roches moutonnées, both whalebacks and roche moutonnées are represented. The whalebacks have a smooth lee side, whilst roche moutonnées have a cliffed lee side. Both features are 10-20 m long and up to 4 m high.

The rock type here is mica schist. The main structures in the rock are a strong foliation or cleavage which dips to the SE and a set of joints which trend N-S and W-E. Former ice flow was down Strath Spey towards the northeast. The size and shape of these landforms in plan is controlled by the distribution and spacing of joints. The shape on the up-ice face is determined by the foliation. The occurrence of whalebacks and roche moutonnées side by side at Dulnain Bridge probably reflects the importance of rock structure in determining the form ice-moulded bedrock surfaces.

These landforms lie on the up-ice side of a rock knoll that protrudes from the floor of the strath. Whalebacks often show striations on all surfaces, indicating that abrasion has scoured the entire surface of the rock bump. Whereas a lee-side cavity developed in the glacier ice flowing over roches moutonnées, the absence of plucking implies that ice remained firmly in contact with the bedrock across the entire surface of whalebacks.

Such ice-moulded surfaces require that the ice which flowed down Strathspey was of a considerable thickness. It is difficult to estimate precisely what this was but hills on either side of Strathspey at Aviemore are ice-moulded to a height of  500 m OD and ice moulding in the Cairngorms extends to elevations of over 1000 m. As much as 1 km of ice may flowed over this spot during the maxima of the Quaternary ice sheets