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Monday, 13 December 2010

Glacier retreat and pressure release

I was zooming around on Google Earth the other day when I was reminded of the fantastic piece of "collapsed coastline" between Newport and Ceibwr on the North Pembrokeshire coast.  This piece of coast was invaded by ice coming in from the sea, from the NW, exerting great compressive stress on the old cliffline.  Here the rocks are quite soft, made up of shales and dark mudstones -- but rock is very strong when under compression.  But when, at the end of the Devensian, the glacier ice melted away in Cardigan Bay, suddenly the compressive stress was removed, and the response of this whole stretch of coastline was to collapse in a series of linked slips and landslides.  This is still happening today, with fresh landslides caused by slight undercutting at the base of the cliff by wave action.  The slips are cutting back into the land every year, causing the National Park to reroute the Pembrokeshire Coast Path further and further inland......

I have always referred to these landslides as "pressure release landslides" because they remind me of even bigger landslides in NW Iceland, where much stonger rocks (basalts etc) have collapsed in a very spectacular fashion following removal of glacier ice at the end of the last glaciation.

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