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Tuesday, 15 December 2015

Ceibwr -- three meltwater channel generations



On clearing out some old files today, I found this splendid old oblique aerial photo showing the Ceibwr channels.  Remember that when these were cut there would certainly have been no coast here, and no sea.  When meltwater was flowing in these channels, it was flowing along a hydrostatic pressure gradient, seeking out a lower and easier route beneath and out of a melting glacier.

The big channel -- flowing from the village of Moylgrove -- is on the left. In the centre of the photo we see a substantial rock ridge, and beyond it is Channel 2, a subsidiary arcuate and humped channel with a clearly defined intake point overlooking the present beach in Ceibwr Bay.  It's a rather spectacular small channel running out to the coast.  And incised into the floor of that one is Channel 3, a small one which is made up of a series of linked pot-holes.  That has to be the most recent of the three features.  The chutes which must have acted as feeders for the subsidiary channel may be contemporaneous -- there is no way of telling.

Note the obvious parallel with Craig Rhosyfelin -- big channel, rock ridge, and smaller channel........

I think these channels might be rather old -- maybe going all the way back to the Anglian Glaciation, around 450,000 years ago.  If they were Devensian, we would not find these old cemented and stained gravel deposits within the channel walls.  On the other hand there does not appear to be much Devensian till or meltwater deposits here either -- which suggests that the channels were used by meltwater again while the Devensian ice was melting away from this area around 18,000 years ago.  The best exposure of the Devensian till is within Channel 3.

More work must be done here -- meltwater has clearly been flowing subglacially from SE to NW.  But that is precisely opposite to the expected meltwater flow direction during the Devensian, since the ice gradient must have been NW - SE.  So we come back again to the thought that maybe these channels date from a time when Welsh ice, rather than Irish Sea Ice, was the dominant force in North Pembrokeshire........

The big channel now occupied by Ceibwr Bay.  The rock ridge is on the left.


 The rock ridge and the subsidiary channel to the right of it.  View inland from the coast.

Channel 3, which contains both cemented and uncemented deposits.

See also this post:

6 comments:

ND Wiseman said...

Hi Brian,

I can't figure out how to reach you privately, so I'll ask my qweschun here ...
Do most glaciers typically leave moraines?

Neil

BRIAN JOHN said...

Neil -- valley glaciers normally do, because they get a good rock supply from below and above scree off valley sides). But ice sheet edges are another matter entirely -- in some places you get huge moraines where very active warm ice has crossed thick soft sediments, and in other places where cold ice has crossed cold solid bedrock, you may get nothing. it all depends on the glaciology!

Dave Maynard said...

Would there also be further meltwater channels further out to sea, if the sea level was lower at that time? I guess though that they would be filled with more recent deposits, so it might be a tricky process identifying them.

Dave

BRIAN JOHN said...

I see no reason why not, Dave. I think there is a huge meltwater channel running through Ramsey Sound -- do a search via the search box.

Dave Maynard said...

I looked at the black and white photo of Ceibwr more closely and seem to remember that the back of the bay is formed of boulder clay/till. Am I right, or am I confusing it with Mwnt?

What would be the sequence here, did this material get deposited before the meltwater channel was cut, or after? It would have had to survive the water if before, and if it was laid after, I can't quite see the mechanism for doing so. Would this constitute evidence for different periods of glaciation?

Dave

BRIAN JOHN said...

Dave -- Mwnt is the place where there is thick and contorted till in the drift cliff at the back of the beach. Here, at Ceibwr, there is till in the valley that runs to Moylgrove, but it is difficult to see decent exposures of it. We have both fresh till and cemented till and fluvioglacial deposits inside these valleys, so I reckon they are very old, and re-used every time a new glaciation comes along.