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Wednesday, 29 March 2017

The edge of the Preseli Ice Cap?




Wizard wheeze for today.  New theory for us to mull over.

For some time I have been intrigued by what has gone on in the past in the southern foothills of Mynydd Preseli, in the zone that includes Rosebush, Maenclochog, Llandilo, Llangolman, Efailwen and Llanfyrnach.  There is a line in the landscape -- irregular, but quite distinct.   The thing that puzzles me in particular is the fact that to the north of this line there are no deeply cut valleys or gorges, but to the south of it we see a string of deeply cut winding valleys with some interesting features including connecting valleys and blunt-headed small tributaries. I cannot see any geological or structural reason for this contrast.  Could it be that the line connecting these "valley heads" represents a stillstand along the edge of a small Preseli ice cap?  We know that there are meltwater gravels at Rosebush and Llangolman -- I shall check out whether there are any others........  watch this space.

The valleys do not seem to be similar to those of the Gwaun - Jordanston meltwater channel system, which displays two features indicative of subglacial meltwater flow: an anastomosing pattern and humped long profiles.  The channels between Rosebush and Efailwen appear to be subaerial ones, which carried large torrents of meltwater southwards into the Eastern Cleddau catchment.

The satellite image, by the way, is from the Bing satellite coverage, with a slightly distorted "landscape"view.  It shows up the river gorges very well.

Postscript:

Here is a superficial geology map of the area, from the Geology of Britain Viewer.  It shows the occurrence of patches of sands and gravels mostly inside the line drawn on the image above.  Significant, or not?  We shall see......


POSTSCRIPT

Adding here the model by Henry Patton and his colleagues showing the theoretical extent of the Preseli Ice Cap at 23,850 yrs BP -- at the time of its proposed maximum extent.  The southern margin is very similar indeed to that which I am now proposing on geomorphological grounds.  What will be much more difficult to work out is the relationship between this little ice cap and the powerful Irish Sea Glacier which came in from the N and NW.  When did the two ice masses make contact, and what happened in the contact zone?  Did that contact zone oscillate over space and time?


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