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Thursday, 24 March 2011

Teetering tors, damaged tors and denuded tors

On my visit to Dinas Mountain the other day I was forcefully struck by the differences between these three types of tors in North Pembrokeshire, in the proximity of the Devensian ice limit.

Where overriding ice has been more than 100m thick, old tors and monadnocks (and there are scores of them) in the St David's Peninsula area, around Pen Caer and in the Fishguard - Dinas - Newport area have been so heavily denuded that some are now reduced to low rounded knolls.  Some have craggy rock outcrops on them, but even these are seriously damaged.  Typical is Garn Fawr, near Pwllderi on the western side of Pen Caer:

In the next zone, further inland and on the uplands of Dinas Mountain and Carningli, where the ice was tens of metres thick rather than hundreds of metres thick,  tors have survived much better, and some are very craggy and spectacular, but most are very seriously damaged and "rearranged" by the pressure of ice and by demonstrable glacial erosion.  Glaciated ice-smoothed slabs are common, and here and there we can see deep gouges (although striations are extremely rare, because of the coarse and crystalline nature of many of the rock surfaces).  Occasionally we can see extensive areas of ice-moulded slabs, as at Garn Fawr (yes, another one!) on Dinas Mountain:

Finally, beyond the ice edge, we have the teetering tors which are in some cases so delicate and unstable that they could not have survived the assault of moving glacier ice 20,000 years ago.  There is only one of these left in Pembrokeshire -- Maiden Castle, a fragile and grotesque tor made of Ordovician rhyolite (once thought to be Precambrian) overlooking Treffgarn Gorge in the centre of the county:

Now obviously these three categories incorporate a lot of variation, depending on local rock-type and the original tor configurations etc, but by and large the morphology of these tors does confirm the conclusions drawn -- on a variety of other grounds -- as to where the ice edge was, and the intensity of glaciation during the Devensian glacial episode.

1 comment:

Anonymous said...

"Teetering tors" Batman!!

Have you seen the film about the Stuttering Sovereign, starring DDDD.......D.D'Arcy??