In a howling gale (mercifully no rain) I have been back to look at the assemblage of erratic boulders on the edge of the moorland near Crosswell around grid ref SN119351. The nearest cottage is Glan-yr-Afon, currently being renovated. Due south, a couple of hundred metres away, is the house called Glan-yr-Afon-uchaf. There is an interesting piece of grassy and hummocky common land near here -- used every year for the start and finish of Ras Beca, a very tough fell race which takes runners up onto the top of the Preseli ridge and back again across boggy moorland.........
I have done one previous post about this site:
https://brian-mountainman.blogspot.com/2017/10/the-spotted-dolerite-enigma.html
... and I am seriously intrigued by it. There are so many heavily abraded and faceted boulders here (some disturbed but most in situ) that I'm increasingly convinced that this is a moraine. There is actually a back slope facing towards Carn Goedog, which is about 2 km away, although the land is generally falling away northwards towards the lower lands of the Brynberian and Nevern valleys.
A little stream runs north towards Pont Saeson, where it joins the Afon Brynberian. It's incised into the surface of what appears to be a morainic accumulation more than 10m thick. There are a number of exposures of stony till in the vicinity, full of cobbles and larger erratics of dolerite, rhyolite, spotted dolerite and quartz. There are some other igneous rocks too that appear to be volcanic ashes and gabbros. As I mentioned in my earlier post, we are right on the edge of the Fishguard Volcanics outcrops here, where ashes and lavas are supposed to predominate. Igneous intrusive rocks are not supposed to be here -- there are supposedly no intrusive outcrops until we reach Carn Goedog. But the great majority of the erratic boulders here are made of spotted dolerite. Where have they come from? Could they have come from the west, carried by ice from the outcrops around Waun Mawn and the headwaters of Cwm Gwaun? That is possible, but I'm not aware of any spotted dolerites in that neighbourhood. The nearest source of spotted dolerites would be Carn Goedog -- but that would involve ice from a Preseli ice cap flowing towards Brynberian and Crosswell. I have speculated on that in the past, and there is some evidence in support, but more detailed mapping is needed.
Close-ups of two spotted dolerite boulders from the common near Glan-yr-Afon
I'm confused. But leave it with me. I'm working on it.........
Well spotted.
ReplyDeleteYou should invite some of the MPP adherents up there, in or out of a howling gale, with or without rain, and provide them with a talk on landscape changes and glaciation, as understood by an academic physical geographer.
ReplyDeleteNear that location is a big slab stone footbridge, despite being marked on the OS map it isnt easy to spot as usually surrounded by undergrowth. Id like to think it has been there for a long time.
ReplyDeleteI have another place for you SN102347 not to far away. Parking at the large layby (old gravel storage) near Brynberian and walk up the bridleway, cross the main road and down the track to Penllyn. Head towards Sinar and there is a gate and footpath into a very streange shaped field, full of large hollows and rocks and odd shaped terrain. Alwys puzzled me as to what has gone on here.
Its the way i access Beddyrafanc before meandering across the hills.
Saw you once with a group near Carn Alw.
There is a footbridge marked on the map near Hafod Tydfyl at SN113344, the bridge has long since collapsed, but the stone supports are still there. Any traffic i guess now uses the track and crosses the rhyd/ford just to the east.
Thanks for the info, Cysgod! Yes, that stone footbridge is lovely -- actually there are two, less than 50m apart. I think the slabs are probably rhyolite. I know that patch of rather chaotic lumps and bumps -- just to the east of the Penyllyn track. My old friend Chris Day used to live there. This is beginning to look like quite a substantial belt of morainic features -- watch this space....
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