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Wednesday, 11 November 2015

Steep rock faces are doomed to fail

I have long been mystified by the fact that when the archaeologists turned up at Craig Rhosyfelin in 2011 they were thinking the word "quarry" rather than accepting that the rock face was a perfectly natural one.  The rock face on the NW flank of the crag, partly covered with a bank of rockfall debris, is as natural a phenomenon as one is likely to find anywhere in the world -- simply because steep rock faces are doomed to fail.

Such things are created in the first place because certain natural processes concentrate erosional activity into very narrow bands -- for example glacial erosion on the outside bends of troughs, river bank undercutting in areas of high velocity turbulent flow, or wave erosion at the foot of cliffs in the intertidal zone.  The key fact is that the entrained debris is efficiently removed. Once these processes cease, other factors come into play, and it is the ambition of every cliff to become a gentle slope.  So once the erosional process stops, and once the process of debris removal stops, debris will start to accumulate at the foot of the cliff, building up inexorably until the whole cliff is destroyed or buried under its own debris.

Look at any part of the British coastline and you see illustrations of these principles:




Obviously high vertical cliffs are more vulnerable than others, especially if they are made of soft, fractured or jointed rocks which are susceptible to freeze-thaw processes, rainwater or groundwater rotting,  or biological processes.  If you look at any cliff, anywhere, its shape is unique -- influenced by geology and environment.  In some cases, where undercutting has created overhangs, they survive for many thousands of years -- and in other cases overhangs collapse very quickly.

The sloping face of the Rhosyfelin crag was a very little cliff made of heavily fractured foliated rhyolite bedrock where rockfalls on a considerable scale were, and are, absolutely inevitable.  The appearance of the rock face, and the characteristics of the debris bank, are entirely as any geologist or geomorphologist would expect them to be.  Occam's Razor.  Why would anybody want to look for a complicated explanation of what we are looking at, when a perfectly simple one will do?



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