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Monday, 28 April 2014

The other Rhosyfelin rhyolite outcrops


In addition to the main Rhosyfelin rock ridge, adjacent to the archaeological dig site, there are a number of other rhyolite outcrops in the valley, some of them quite prominent.

The photo above shows one of these outcrops, to the SSE of the main ridge, and about 100m up-valley.  I think this has been referred to as "sampling point number 11" by Richard and Rob.  The crag is not a very big one, but it illustrates how important slope processes are here.  On the downslope side of the crag there are occasional exposures of bedrock, but the main features of interest, to a geomorphologist, are the slabs of bedrock that have broken off and rolled or slid down the slope, to end up resting on the valley floor.  there are five or six of these big elongated slabs, including the two shown in the foreground of this pic.

Some of the rocks that started off higher up the slope and ended up lower down have probably moved very slowly indeed, over many thousands of years, simply by being incorporated into slope debris moving inexorably downhill under (initially) periglacial slope processes and later under temperate-climate solifluxion processes. 

Message?  Big stones and slabs move downhill, especially if there is a good gradient, occasionally ending up 50m or more from their places of origin, so long as there is nothing obvious to stop them....

1 comment:

chris johnson said...

Very interesting to read all your posts.

Next time I visit Rhosyfelin I will put a spirit level on the "altar stone" It seemed to my eye to be surprisingly horizontal and had it arrived in its current position 100% naturally then it might well be conforming with the angle of the slope, or even drifted downhill towards the flatter land.

Of course I might be mistaken, as my wife always told me when I hung pictures using my eye as a spirit level.