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Sunday, 27 July 2014

More about glacial erosion


Two photos taken on the island of Svartloga the other day. The top one shows the streamlined whaleback forms typical of this part of the Stockholm Archipelago.  The rock surfaces here (heavily metamorphosed Pre-Cambrian granites) are on the up-glacier side of a larger roche moutonnee feature.  What is particularly interesting is the extent of fracturing on the rock surface -- the apparently random fracture pattern is obvious, except that it is not actually random at all, since there is a reason for everything in glaciated terrain.......

I'm intrigued by this top photo, since the fractures are on a rock surface which must have been under heavy compression when it was last glaciated.  Most cracks on roche moutonnee forms are seen where compression gives way to tension, near the highest point on the roche moutonnee surface.  Given a bit more time (ie a few thousand years more, maybe) and this rock surface would have been busted into bits.  Maybe we are looking at the result of pressure release, with these fractures appearing as a result of the removal of the ice load.

The bottom photo shows a moulded, smoothed and striated rock surface with some transverse fractures developing exactly where we would expect them, in readiness for the removal of the next slab or boulder as quarrying of plucking processes come into play.  The clean face at the top right of the photo is on the down-glacier flank of the roche moutonnee -- beautifully smoothed but not striated.  Sometimes these plucked faces are rough, and sometimes smooth.  The reasons for this are complex.  Another very striking feature is the colour difference between the up-glacier and down-glacier faces.  The striated (up-glacier) surface is very weathered, and is coloured grey.  The down-glacier or plucked face is a creamy pink colour -- quite unweathered and fresh.  It would be interesting to do some cosmogenic dating on faces like these, since their physical appearances suggest that on the striated face there has not been a huge amount of erosion since the last interglacial.......

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