Roche Moutonée

TO begin, if you have never seen nor heard of a roche moutonée, fear not! You have come to right place, here at Fell within the Fell Words section we look at the meanings behind some of things hillwalkers come across all the time but sometimes go unnoticed or just never get researched. One thing is for certain we guarantee you will have seen one of these!

A roche mountonêe is a glacial trough formation and it’s name originates from the french words for rock sheep, because some look like a sheep in a field from a distance. 

The distinctive feature of a roche moutonée is it’s shape, it has a smooth inclining stoss (up valley) surface and a rough jagged steeper lee (down valley) slope. 

A brilliant example of a roche moutonée in the
Nant Ffrancon glacial valley.

The formation occurs as a glacier advances down valley and it comes into contact with a mass of resistant hard rock. It can't erode the resistant rock so the glacier has to move over it. Due to the increase in pressure some of the ice melts at the glacier bed and causes basal sliding. As the glacier moves up and over the resistant outcrop debris or moraine scratch the surface of the rock and leave striations that are still visible today.

On the downslope of the the roche moutonée the surface changes dramatically, the gradient steepens and the smooth striated facia of the stoss side is all gone. 

Now as the pressure that previously melted the ice at the bed of glacier reduces the water refreezes while in contact with the lee side. Plucking then occurs leaving a jagged, sharp and angular surface to this side of the formation. 

In terms of size, the formations vary greatly, some can be one or two metres in length others can be around 100. Keep your eyes peeled when out and about in area of glacial history as once you are familiar with the standout shape you will recognise them frequently.

A diagram showing the formation of a roche moutonée.

A good place to spot loads of these features is in the valley leading up to Easedale Tarn from Grasmere, Cumbria. Littering the valley floor either side of the Easedale Beck you will see an array of examples large and small so be sure to check that out. 

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