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Tuesday, 27 November 2012

Rhosyfelin crags


Another nice pic from Rhosyfelin -- thanks to Chris. He says this was taken about 40m upstream from the "quarry" -- presumably on the other side of the ridge, where the crags are "overhanging" and more precarious.

OK -- Chris wonders how crags as delicate as this can survive glaciation and assorted other destructive earth surface processes.  Fair enough -- this is one of the great questions of glacial geomorphology.  I won't pretend that I have an easy answer on tap.  There are delicate crags all over the place, in areas which HAVE been glaciated quite recently.  There are crags like this at Felin y Gigfran, at Nant y Bugail neat Trecwn, and in Tycanol Wood -- not to mention all those delicate crags up on the top of Preseli.  In other places the crags are not so delicate -- they are smoothed off, and have obvious traces of glacial erosion on them.  This is the classic beetling fragile precarious craggy rock in Pembrokeshire -- the Precambrian rhyolite crag at Maiden Castle, near Treffgarne:


Parts of this crag are so fragile and delicately balanced that they look as if a decent storm might blow them down -- let alone an Irish Sea Glacier.  We geomorphologists assume, therefore, that this crag was not affected by Devensian ice, and that the central part of Pembs was beyond the ice limit during the last glacial episode.

On the other hand, if a glacier is cold-based rather than warm-based it has the extraordinary capacity to PROTECT landscapes and even very delicate tors from the processes of glacial erosion, which are concentrated into zones where streaming occurs -- where the ice moves at high velocity.   This has happened in parts of the Cairngorms, in North Wales, on Dartmoor, and on the uplands of Preseli.

Back to Rhosyfelin.  There are several possibilities.  

One, the crags as we see them today might have been fashioned by frost-shattering and other periglacial processes after the Devensian ice retreated from the area around 20,000 years ago.  We have quite a long period to think about here -- about 10,000 years in fact, when the climate was warming only very gradually.  The climate was still very severe -- probably permafrost was present for much of the time, and it got even colder during the Older Dryas and Younger Dryas episodes before the real Holocene climate warming set in.

Two, the crags might indeed be very old, having survived beneath the Devensian ice because the valley was filled with largely stagnant ice, with more active glacier ice shearing or streaming over the top.

Three, we might be looking here at the fractured remains of a crag which has been extensively broken up by plucking and entrainment processes during the Devensian.  (This is the sort of mechanism which I think will have affected this crag during the Anglian Glaciation, leading to the entrainment of material from Rhosyfelin into the ice which moved SE and E towards Somerset and Wiltshire.)

Then we have the other complicating factors of snowmelt, fluvio-glacial streams and even possible lake waters affecting this valley.

Work in progress...........

1 comment:

chris johnson said...

Yes,

My question is triggered by the discussion you are having with Kostas in the previous thread. I'll paste it again here:

"The back of this crag has many rock outcrops which look precarious and anything but smooth - I sent Brian a photo I took some 40 metres from the site along today's river bank.

As a layman on all things geomorphological I have difficulty imagining how these outcrops were NOT demolished by a glacier or eroded by meltwater torrents.

I hope Brian can explain."