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Wednesday, 16 September 2015

Meltwater channels: Afon Teifi and Afon Nyfer catchments




Many thanks to Neil Glasser for sending me a copy of this interesting paper, which we have not previously discussed on this blog.  The paper looks at the meltwater channels in the Cardigan - Newport area, and confirms what I have been saying for 45 years or more -- that the channels cannot be attributed to one glacial episode -- the Late Devensian.  They are far more interesting than that, and this paper reveals just what complexities there are -- identifying different channel types and using contained sediments as one of the means of working out relative ages and origins.  Many of the channels are originally subaerial and fluvial, formed in conditions when there was no glacier ice anywhere in the vicinity -- as one would expect. Then we have the effects of at least two glacial episodes -- the Anglian and the Late Devensian -- with channel segments and sometimes whole channel systems being modified when used by large volumes of meltwater.  Sometimes the water flowed subglacially, and sometimes in the open.  Many of the channels are humped -- I would have liked the location of the humps to have been shown more clearly on the above map.  But it's a great map -- and a very useful reference for future work.

The authors have concentrated on a classification of channel types and on explanations of the phases of erosion which have given them their present forms.  What the authors don't do, however, is tie these channels (or even segments of them) in to the ice bodies responsible, and the directions of ice movement or hydrostatic gradients within the ice.  That's for another paper, maybe.  But the interesting thing, as far as I am concerned, is that the channels have alignments that are very "inconvenient" when we try to relate them to the current consensus on the movement of the Irish Sea Glacier across this area, both in the Anglian and Devensian glacial episodes.  The ice is supposed to have moved from NW towards SE across this area on both occasions -- maybe N-S during some phases.  But these channel systems, like the Gwaun-Jordanston channel system further to the west, are aligned for the most part E - W -- and that would mean that prevailing meltwater flow would have been against the hydrostatic gradient in the ice......  and that would break the most basic of the physical rules.

https://www.qra.org.uk/top-50-quaternary-websites/        See p 52
http://brian-mountainman.blogspot.co.uk/2012/02/those-meltwater-channels.html

So the possibility exists that these channel systems  -- all of them -- were originally created at a time when Welsh ice covered this area, moving approximately east - west, with meltwater flow during a deglaciation or ice wastage phase then following the normal rules of physics and flowing westwards with assorted swings influenced by pre-existing topography.  Alternatively, if Welsh ice was not involved, it is possible that pre-existing (pre-glacial?) channels were such significant features of the landscape that they directed subglacial meltwater flow during a phase -- or several phases -- of very rapid ice wastage.   That would only work if the collapse of the Irish Sea Glacier in Cardigan Bay was truly catastrophic.  Somebody else needs to work all that out..........  maybe with some modelling thrown in.

So where did that huge volume of meltwater go?  One last thought on this -- could that extraordinary chasm in Ramsey Sound have anything to do with it?


I have done several posts about Ramsey Sound and the images kindly provided by Dr Paul Evans.  There is talk of catastrophic floods being responsible for the draining of impounded lake water through the Straits of Dover -- could it be that the Ramsey Sound trough was formed in a similarly catastrophic fashion, by meltwater that had been forced westwards along the North Pembrokeshire coast (off the present coastline) and which then found a convenient escape route southwards between the mainland and Ramsey Island?


Glasser, N. F., Etienne, J. L., Hambrey, M. J., Davies, J. R., Waters, R. A. & Wilby, P. R. 2004 (August): Glacial meltwater erosion and sedimentation as evidence for multiple glaciations in west Wales. Boreas, Vol. 33, pp. 224–237. 

Abstract

This article presents the results of a geomorphological and sedimentological investigation of former glacial meltwater drainage in the region of the lower Afon Teifi, one of the major rivers of southwest Wales. Former drainage characteristics in the region are reconstructed concentrating on palaeo-drainage routes associated with successive Pleistocene glaciations and their role in the Quaternary evolution of the lower Teifi. Mapping of these features throughout a c. 100 km2 area reveals a complex evolution in the establishment of the present-day drainage system, with evidence for the following surface channel types: (i) type 1 channels of primary subglacial origin cut during the late Devensian (late Wisconsinan/late Weichselian) glaciation; (ii) type 2 channels representing either pre-late Devensian subaerial fluvial run-off, unconnected to the course of the preglacial Afon Teifi, or originating as subglacial chute channels; (iii) type 3 channels developed as subglacially modified pre-late Devensian tributaries of the Afon Teifi. Two further features are also described: (iv) type 4 channels are drift- plugged abandoned preglacial courses of the Afon Teifi, and (v) type 5 channels formed as lateglacial and post- late Devensian gorges which bypass type 4 channels. A relative chronostratigraphy based on channel geomorphology and sedimentology reveals an evolutionary sequence considerably more complicated than identified in previous studies, with extensive modification of the lower Afon Teifi region by glacial meltwater during at least two periods of Pleistocene glaciation.

1 comment:

BRIAN JOHN said...

By the way, if you are wondering where the Afon Brynberian meltwater channel is, and Craig Rhosyfelin, it's located just off the bottom of the map, to the right of the number 10. The valley runs approx northwards to join the valley of the Afon Nyfer.