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Sunday, 5 June 2011

Roches Moutonnees

 Above -- a very old sketch of a roche moutonnee hill in Sweden.  Below -- a very large roche moutonnee in Yosemite National Park, USA

 Those pictures on Google Images are a real hotch-potch of all sorts of things, only some of which I would classify as roches moutonnees or streamlined forms. Above are two illustrations that DO fit the bill.  The up-glacier side is on the right in both photos -- with a much gentler slope that has been smoothed and abraded by ice under pressure as it rises.  In contrast, the down-glacier (left) sides have been steepened and broken up by plucking and quarrying on a grand scale.  The processes are still being debated, but here the basal ice will have been under tension -- there may well have been a cavity, and freezing and thawing on the glacier bed might have been instrumental in the processes of dragging large blocks of stone away from the hillside.

The assymetric long profile is typical -- features which do not look like this are better referred to as whaleback forms.  They are much more irregular and "hummocky", and in whaleback territory it is sometimes difficult to discern the direction of ice movement.

I have seen roches moutonnees ranging in size from whole mountains to very small features no more than 5m long and 2m high -- but the processes responsible for them are essentially the same in all cases.

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