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Saturday, 31 December 2016

The Cardigan Bay Conundrum

 Fresh surface of Welsh till exposed beneath HWM on the east side of the Nevern estuary, Newport

Washed surface of Welsh till in the same area -- incorporating fluvioglacial pebbles and cobbles.  The lithologies are dominated by grey and grey-brown gritstones and coarse sandstones that appear to have come from the Upper Ordovician or Silurian rocks of Ceredigion

 Close-up of a washed till surface nearby, with a more typical variety of stone shapes, including many faceted cobbles.  Again the grey and grey-brown gritstones and coarse sandstones dominate

Every time I go for a walk on the estuary in Newport,or even on the beach, I come across exposures of till that do not quite tie in with the view that this coast was overwhelmed, on two occasions at least, by Irish Sea Ice that came in from the N or NW.   Actually, the evidence does not contradict the idea that the Irish Sea Glacier was dominant here -- rather, it argues for a situation that was more complicated, with phases during which Welsh ice was dominant.

I have not done any stone counts or other careful forms of monitoring, but my impression is that 70% - 80% by weight of the erratic content in the exposed till consists of grey and grey-brown gritstones and sandstones which are quite unlike the mudstones, shales and fine sandstones that outcrop in the cliffs on either side of Newport Bay.  These rocks belong to the Cwm-yr-Eglwys Formation of Caradog age, about 450 million years old.  They overlie the igneous rocks of the Fishguard Volcanic Group which outcrop a little further inland.  On the outer headlands the outcropping rocks belong to the Dinas Island Formation, of Upper Caradoc age.  The point is that all of these sedimentary rocks are deep-water sediments -- quite unlike the shallow-water facies (including coarse gritstones) of the Lower Silurian rocks which outcrop between Llangranog and Tywyn and which underlie much of the Ceredigion landscape.

The assumption has to be that the till in the Newport Estuary has been deposited by Welsh ice flowing across the coast from the NE, or (more likely) by Welsh ice pushed far out into Cardigan Bay and then diverted southwards by a more powerful Irish Sea ice stream.  A third possibility is that thick glacial deposits far out in Cardigan Bay have been re-worked or picked up and incorporated into the load of the Irish Sea Glacier as it woved across the North Pembrokeshire coast from the north and north-west.

Whatever the truth of the matter, things were certainly more complicated than suggested by the models of the Devensian Welsh Ice Cap created by Henry Patton and colleagues:

Henry and his colleagues of course admit that their models do not take any account at all of the interactions that occurred along the margins of the Welsh Ice Cap between Welsh Ice and Irish Sea Ice, and we hope that that is something covered in the next stage of the modelling work.

But what is now clear is that at some stage in the Devensian the ice flowing westwards from the Welsh Ice Cap was much more extensive than shown in the models, probably by at least 20 km.  That would have sufficed for Silurian gritstones in vast quantities to be carried onto the North Pembrokeshire coast in Irish Sea till or in a special "hybrid facies" of the till.

So the maximum extent of the Welsh Ice Cap must have pre-dated the maximum extent of the Irish Sea glacier.  That makes sense.  We have some rather mysterious exposures of tills that seem to pre-date the Devensian LGM -- at Criccieth,  Glanllynnau,  Llanystumdwy,  Moranedd, and New Quay.  These tills might well date from an early expansion of ice from the Welsh Ice Cap prior 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.)  On the other hand, was there an interval between these two glacial "events" which coincided with the Middle Devensian interstadial episode conventionally labelled as O18 stage 3?

As I have pointed out in another post, 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.......

Then we come to the matter of the North Pembrokeshire meltwater channels, which we have always assumed to be associated with Irish Sea Ice coming in from the N and NW.  That doesn't make sense any longer -- but I'll deal with that issue in another post.

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