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Saturday, 25 February 2012

Glacial deposits from the Early Devensian?

On looking through the records to see what there might be in the way of field evidence for an expanded Welsh Ice cap in the Early Devensian, the records are not easy to interpret.  Following the Ipswichian interglacial (O18 stage 5e) -- around 120,000 years ago, there was a long period of cool - cold - periglacial - glacial conditions, with lots of climatic oscillations, between 116,000 BP and 59,000 BP.  Some of these oscillations are given the O18 labels 5a - 5d, and then around 70,000 years ago came the sharply oscillating climate labelled as O18 stage 4.  That stage lasted for maybe 12,000 years -- see the earlier posts.

The coastal drift stratigraphy around Wales fits into this framework rather well -- with thick slope deposits (with much evidence of periglacial conditions and permafrost or frozen ground conditions now and then) sitting on top of the Ipswichian raised beach and beneath the glacial deposits assumed to date from the LGM of the late Devensian.  (See the stratigraphic columns for Pembrokeshire coastal sites in the earlier post.  Similar relationships are seen elsewhere in Wales.)

But then we have some rather mysterious exposures of tills that seem to pre-date the LGM -- at Criccieth,  Glanllybnnau,  Llanystumdwy,  Moranedd, and New Quay.  Do these tills date from an early expansion of ice from the Welsh Ice Cap which acted as a sort of precursor to the arrival of the Irish Sea Glacier?  (Big glaciers always take longer to react to climate change and then to build towards their maximum extent than smaller glaciers do.)  Or was there genuinely an interval between these two glacial "events" which coincided with the Middle Devensian interstadial episode conventionally labelled as O18 stage 3?

The "weathering layer" (assumed to have been formed during a warmer interlude)  seen above the Criccieth Till in the cliffs at Glanllynnau is overlain by fluvioglacial sands and gravels, with another till (called the Llanystumdwy Till) above that.  The weathered layer is a yellow-brown horizon on top of a blue-grey clay-rich till -- but we need much more evidence than a simple colour change before we can be sure that the top surface of the till has genuinely been affected by surface weathering under a warmer climate.  The upper surface of the Irish Sea till in Pembrokeshire is often decalcified and weathered in this way, simply because ground-water passes through overlying fluvioglacial deposits quite easily and then penetrates into the till beneath.  The colour change has nothing to do with surface exposure and nothing to do with a warmer climatic interlude -- it is simply a matter of chemistry.......

But on the other hand, in North Wales coastal sections  there are signs of solifluction features and also ice-wedge casts associated with this weathered horizon, and that has to mean there was an interval of non-glacial conditions before ice returned to deposit the upper layers in the sequence. 

This debate has some way to run.....

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