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Saturday, 14 March 2015

Devensian till at Rhosyfelin





The winter rains in Pembrokeshire have cleaned up the surface of the till exposure at Rhosyfelin, and the till layer is now revealed in all its glory.  I don't think I have seen a better till in an inland West Wales exposure!  I'm pretty sure it's Devensian, although there is a remote possibility it could be older.....

This is the equivalent of the Irish Sea Till which we see on the North Pembrokeshire coast.  But that till is composed largely of recycled sea floor sediments, with a very large clay component.  These inland tills are stratigraphically equivalent,  but there is much less clay in the matrix, and sandy and gravelly materials make up the bulk of the deposits, with stone and boulder inclusions of many shapes, sizes and lithologies.  We can see some of this variety in the photos above.  Most of the strictly local stones and boulders (made of rhyolites) are bluish in colour, and they are angular or sub-angular in shape because they have not travelled far.  There are abundant dolerite boulders and pebbles -- some of them well-rounded.  As we can see, there are quartz pebbles too -- and if you click and enlarge these photos you can see a few rather interesting agglomerates with bluish or greenish inclusions.  Some of those pebbles are very beautiful -- and I assume they have come from the Fishguard Volcanic Series, somewhere in the vicinity. Advice from the geologists, please?

As for the directions of ice movement at the time of the "Rhosyfelin Glaciation" I'm currently not sure what went on.  Instinctively, one thinks that the Irish Sea ice much have come from the north or north-west, but I'm playing with the hypothesis that some ice at some stage of the Devensian might have come from a small and cold-based Preseli ice cap -- so there is a possibility of a short-lived glacial episode involving ice travelling across this area.  Maybe, when we have examined the erratics in more detail, things will become clearer.....




1 comment:

Constantinos Ragazas said...

Beautiful till!

Devensian no doubt. But the real question is when they got laid! And how they got laid.

Judging from how fresh and young they look, I'd say very recently! I bet it was a very wet affair.

Kostas