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Tuesday, 27 September 2011

More on the Rhosyfelin outcrops

With reference to the recent dig at Craig Rhosyfelin, conducted by MPP and his team,  where did that big slab of rhyolite actually come from?  All will be revealed in due course by the geologists, but in looking back at my photos I am struck by the fact that there are teetering pinnacles and precarious slabs all over the place on this rocky spur.  Just look at these pics:

Slabs like the one discovered in the hole are probably littered around all over the place, around all of the flanks of the spur, incorporated to a greater or lesser degree into scree and rockfall debris.    The top photo above was taken on the "back" side of the spur (ie the south side)  -- note that the rock slabs are almost overhanging here, because there is a steep dip of the rock layers towards the NW.  You can see that reasonably clearly on the bottom photo, and ever more clearly here, on the photo taken by Dylan Moore:

Dylan's photo was taken from a bit higher up, in the little channel that runs up towards the bend in the road -- and indeed past it, towards the SW.  This little channel is a couple of hundred metres long, and I would guess that the big slab could have slipped down from the overlooking crags almost anywhere along its length, and might even have slid down a snowslope before ending up in its present position.

As far as the nature (or precise "fabric" and "provenance") of the rock is concerned, note that the strike of the rhyolite slabs runs straight out across the valley here -- on the alignment shown on my previous post. 


If you take the second black line from the top, one might expect to find fairly consistent fabric along that line, because the rhyolites as they cooled might have all experienced the same conditions.  In the same way, the fabric along lines 1, 3 and 4 might have experienced consistent conditions too, but different from those along line 2 to a greater or lesser degree.  The only thing that might screw up this situation would be later metamorphism in which heat and pressure came from a quite different direction that had nothing to do with the strike of the rocks.  Complicated?  Quite possibly......

But I still think we need to know what the rocks are like on the other side of the valley, in the top right corner of the photo.  Straight question to the geologists:  Have you guys sampled the rocks on that side of the valley?  And if so, what have you found?

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