September 10, 2026 Post by : Editorial Glacial Erosion

Some of the most useful clues about the direction of past ice flow are written on bare bedrock. Roche moutonnées are asymmetrical rock hills that have been smoothed and sculpted by passing ice. Their gentle, rounded upstream side and their steep, jagged downstream side make them look a little like sheep lying down, which is roughly what the French term means. Once you learn to recognise them, you can walk across a glaciated landscape and read the direction the ice was travelling.

The asymmetry of a roche moutonnée comes from the way ice erodes bedrock. On the upstream side, the ice is thick and presses down, and it abrades the rock with embedded sand and gravel, smoothing and polishing it. As the ice passes over the top and descends the downstream side, it pulls away from the rock and plucks out blocks along fractures and joints. This plucking creates a rougher, steeper face. The result is a hill that is smooth on one side and craggy on the other, with the smooth side facing the direction from which the ice came.

What Roche Moutonnées Tell Us

Because they record ice flow direction, roche moutonnées are valuable in reconstructing former ice sheets. By mapping the orientation of many such features across a region, geographers can build up a picture of how ice moved, where it diverged around high ground, and how it converged into valleys. They are especially useful in areas where other directional indicators, such as striations or drumlins, are absent or ambiguous.

Roche moutonnées also provide information about the nature of the bedrock and the speed of the ice. Hard, massive rocks tend to form large, well-defined roche moutonnées, while softer or heavily fractured rocks may be broken up more quickly. Fast-flowing ice tends to produce more elongated, streamlined forms, whereas slower ice may leave stubbier hills. The size of the feature can also reflect how long the ice sat over the same spot, with larger roche moutonnées forming where erosion was sustained.

Seeing Them in the Field

In the field, roche moutonnées are often found in groups, sometimes forming a 'whaleback' landscape of rounded rock ridges. They may be partially covered by a thin veneer of till, but their shape is usually visible in the topography. In coastal areas, they can form small islands or skerries, smoothed by ice and later modified by waves. In upland areas, they stand out as bare rock knolls among peat bogs or moorland.

For students of geography, roche moutonnées are a perfect introduction to the idea that glaciers erode differently on different sides of an obstacle. They are also a reminder that ice is a powerful agent of change, capable of sculpting solid rock into forms that persist for thousands of years. Next time you see a rounded rock hill with a steep, broken face, check which way the smooth side points. You may be looking at a signpost left by an ice sheet that vanished long ago.

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