Roche Moutonnée & Crag and Tails

Roche Moutonnée

Between ten and twenty metres in length and four metres high, roche moutonnée are resistant areas of bed rock formed as a glacier abrades the surface of the rock, carving it into a gentle upstream slope (known as the stoss slope), therefore creating a smooth convex shape as the ice travels over it (with a few polished striations and truncated end). The downstream side of the bedrock (known as the lee slope) however is a lot steeper and rougher than the upstream side due to the glacier having plucked the rock away. It is believed that plucking may have occurred on this side due to a reduction in pressure of the glacier moving over the stoss slope, therefore providing the opportunity for water to refreeze on the lee side and pluck the rock away. 

However, care must be taken when using the term roche moutonnée as it only refers to those landforms created by glaciation rather than pre-existing hills that were simply polished smooth by it. Additionally, roche moutonnée must also not be confused with that of a whaleback. Similar in size and shape (as well as containing striations), a whaleback is an erosional landform that has a similar stoss slope to that of a Roche moutonnée, but has a smooth lee side (compared to the cliffed lee side of the Roche moutonnée). The lack of plucking on the whaleback has a variety of theories, some of which being the ‘physical characteristics of the bedrock, thickness of overriding ice and the speed of ice movement’. The most widely accepted theory however is the thickness of the overriding ice. It is argued that once the ice is over one to two kilometres thick the underlying erosional feature will no longer undertake plucking due to the vast pressure preventing cavities from forming, thus creating a whaleback. If however the overriding ice is less than the one to two kilometres thick plucking can occur, therefore creating a roche moutonnée. Nevertheless, whalebacks with small cavities but no plucking are known as rock drumlins.

 Crag and Tail 

A crag is an isolated hill or mountain (from other high ground), made of a larger rock mass than a roche moutonnée, but is formed in the same way. The lee side of the rock (softer rock known as the tail) however was protected from the erosional power of the glacier by the crag and as such appears as an ‘elongated tapered ramp extending downstream’. Some crag and tails however that have been latterly surrounded by sea often lose the tail due to post-glacial erosion.

An example of a crag is that on which Edinburgh Castle is built, with the Royal Mile being located on the tail. Looking at examples like this (or local examples if possible) may help students understand the concept better.