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Thursday, 11 September 2014

Glacial features on the Carn Goedog sill -- a photo gallery

 The "smashed up" tor at Carn Goedog.  Most of this damage has been done by periglacial frost action over a very long period of time, but damage will also have been done by glacial ice pushing in from the north and over-riding the crag.

Steeply-sloping ice-smoothed slabs and a litter of broken rock boulders -- on the edge of the Carn Goedog sill c 200m west of Carn Alw.

Mixed erratic litter of foliated rhyolite boulders and dolerite boulders -- to the west of Carn Alw

 Glaciated slabs, broken bedrock and perched blocks at the eastern end of the Carn Goedog tor

 Small meltwater channel at the eastern end of the Carn Goedog tor.  The main south-facing "cliff face" is just off the photo, to the right.  Note the distinct channel form and the distinct smoothing of bedrock on its right flank.  This smoothing may be partly down to glacial erosion and partly down to meltwater action.

Superb ice-smoothed slabs of spotted dolerite, about 200m east of the main Carn Goedog tor.

6 comments:

Constantinos Ragazas said...

Brian,

I am intrigued by the ”mixed erratic litter of foliated rhyolite boulders and dolerite boulders -- to the west of Carn Alw” . If the “foliated rhyolite boulders” can also be traced to Crag Rhosyfelin or thereabouts, the movement of these would then be from SE towards NW, if my sense of direction here is correct. Is this correct?

If this is correct, we would then have Rhosyfelin rhyolites transported in two opposite directions: SE towards Salisbury Plain and NW towards Carn Alw. Would such glacial transport be possible? Or there is another natural transport mechanism here that can explain this contradictory “mixed erratics litter”.

Can you please explain to a non-Brit the difference between Carn and Crag?

Kostas

BRIAN JOHN said...

Kostas -- you have your directions mixed up! Easy to do, when you are so far away! Rhosyfelin lies to the NW of Carn Alw -- so erratics could have been transported by Irish Sea ice from the one to the other. But I wouldn'd claim to have seen anything from Rhosyfelin up on the Preseli ridge. That sort of identification would need some quite sophisticated work from a proper geologist! There are lots of foliated rhyolites around Carnalw itself -- belonging to the Fishguard Volcanic series. and strange-looking volcanic rocks that I would hesitate to give a name to -- but there are certainly some ignimbrites and ashes mixed in there. Volcanic materials are messy! so all of the mixed up dolerites, rhyolites, ashes and so forth in the Carn Alw area look pretty local to me -- all from within a km or so of their outcrops.

A carn is an upstanding rocky crag or a pile of stones (which could be man-made) -- and craig means the same as crag -- a rocky eminence. So there is not much difference between the two.

AG said...

brian: You should be pleased, From your evidence, the Archeo's will have to at least acknowledge that a glacier removed rocks similar to the Bluestones from this Sill and transported them for some distance in the direction of Stonehenge? Although probably not!

Davey said...

I had a wander across the northern flanks of the Preseli's on Saturday afternoon visiting Carn Alw, Carn Bresb and Carn Goedog i saw about 5 other people on and around the rocks, the first time i have seen anyone there. Admirredly two people were bouldering on Carn Alw so not all had an archeological bent.

In advance of this weeks lecture at Castell Henllys i then popped across to Rhosyfelin and was surprised, latish on a Saturday afternoon, to see lots of digging activity going on and all the laybys choked by vehicles. There was one rental van/minibus from Wellingborough. No place to park up and investigate further unfortunately.

BRIAN JOHN said...

Thanks for the info, Davey. Not surprised to hear of people wandering around the rocks up on Preseli -- most days there are people up there, simply wandering and enjoying the peace and quiet. I imagine the archaeologists have finished up there for the time being, having filled in their holes.

Yes, lots going on at Rhosyfelin. Maybe they even have a film crew on the job just now, working on the latest blockbuster.....? I imagine that most of the diggers are from assorted university departments associated with some of the project partners.

chris johnson said...

I hope you or Brian are able to give us a report. Unfortunately I am not attending this year.