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Sunday, 29 November 2015

Quarry face degradation

These two photos from Phil Morgan are interesting, since they show not only two of the key attributes of a modern quarry (a clean and preferably vertical rock face and a clean and tidy working floor) but also the beginnings of quarry face degradation.

The photos are from a Pennant Sandstone quarry in the Merthyr Valley.  The face is about 15m high.  Phil thinks that working stopped here in the 1800's -- so after maybe 150 years things are beginning to degrade.  The top photo was taken in September 2010, and the lower one in April 2011.  During the course of a very cold winter the boulder with the yellow line around it had been loosened by frost action at the top of the quarry face and had come crashing down.  It now rests about 4 ft from the rock wall, and its long axis is more or less at right angles to the face.  Effectively, of course, long axis alignment will be pretty random. 

Given enough time, with frost and biological processes operating intermittently over many thousands of years, this rock face will be gradually buried by an accumulating bank of rockfall debris and scree.  There have been many studies of natural rock cliffs (such as might be found on an abandoned sea cliff, or on the flank of a fresh glacial trough or on the outside bend of a meltwater channel) and the manner in which they are transformed by scree or talus accumulation.  The interesting thing is that the rock floor beneath the talus cone will survive because it is protected;  but the cliff will gradually be converted into a convex slope as the lower part is protected and the upper part continues to be degraded and retreats.  the physics are pretty simple.  I'll do another post on this in due course........

Here in the Merthys Valley we have a very simple situation, since this is high on a valley side with a long downward slope beneath the quarry -- so there is nowhere else for sediments to come from that might flood of transform the quarry.  At Rhosyfelin things are very different indeed, since slope accumulations can, and indeed have, come from various other directions and have got mixed up with the rockfall debris coming form from the crags above the rock face.

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