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Saturday, 5 June 2021

Outwash gravel sheets in Central Pembrokeshire


Did a braided river like this once flow from the Trefgarn Gorge exit and then southwards across the site of Haverfordwest?

Extract from the geology map showing the main gravel occurrences in pink.  The present course of the Western Cleddau river can be picked out by the yellow band on the image, showing where the present flood plain is located.

I have long been intrigued by the extensive sheets of glaciofluvial gravel found in the Haverfordwest area, especially on the "plateau" incorporating Prendergast, on the NE edge of the town, and running up through the site of the Withybush Aerodrome (where of course they are massively disturbed) towards Rudbaxton and thence towards the southern exit of the Trefgarn Gorge.

There was an outcrop of these gravels on the bank that dropped away beneath Cherry Grove, where I lived as a child, and I was intrigued by them even at that tender age! 

But why does this "gravel sheet" not run along the present route of the Western Cleddau river and to the west of the A40 road?  Was there a braided outwash river to the east of the road?  That would have been a strange situation, since much of the land to the east of the road is above the 50m contour whereas much of the land to the west of the road is below 50m.  Or might the river have migrated westwards and removed vast quantities of glaciofluvial material in the process?

Various attempts have been made to recognize river terraces in the Haverfordwest area, but they have never been very successful, partly because of the extent of the built-up area and partly because terrace remnants are small and difficult to identify.  Traditionally, the gravels in the Haverfordwest area have been interpreted either as pre-glacial river gravels laid down at a time of higher sea-level, or as glaciofluvial gravels associated with one of the earlier glaciations (Anglian?).  This would make sense, if the Gwaun-Jordanston channels are assumed to have been formed by vast torrents of meltwater flowing under the ice and flowing first westwards and then southwards, through Trefgarn Gorge and towards Milford Haven.  This too makes sense, with an ice surface gradient sloping down from NW towards SE.   If the gravels are stained and even rotten -- and hence very old -- this would support the Anglian glacial hypothesis........ and the assumption that Devensian or LGM ice could not have affected central Pembrokeshire.

On the other hand, I have been thinking for some time that Devensian ice did not simply skid to a halt along the coast of St Bride's Bay, but pushed well inland.  To his credit, Prof Dai Bowen was the only person to have suggested this in his Devensian / Weichselian ice limits maps:

Recently some big exposures have been opened up in association with the building of a new Haverfordwest High School on the site of the old Sir Thomas Picton School.  Near Prendergast Cemetery and the County Archives building there is a huge mound of excavated gravels, incorporating large boulders which suggest to me either a very powerful meltwater torrent or the close proximity of glacier ice.  Sadly, I have not been able to examine any in situ gravels which might indicate the direction of meltwater flow.........

Another braided river plain (sandur), showing the complexity of anastomosing channels and suggesting the frequent lateral displacement of the dominant water routes.  We should not strictly refer to this as a "flood plain" since the "flooding" is going on for much of the time during the melting season, here, there and almost everywhere........  Multi-channel rivers like this are a nightmare to cross on foot with a heavy pack!

Kaldalon braided river and sandur in NW Iceland.  We crossed this one many times in 1960, usually in the middle of the "night" when the water level was low.

When I examined the apparently fresh glaciofluvial gravels at Picton Point, I became convinced that they were Devensian in age, and that they were laid down nor far from an ice edge:

It would be logical for the gravel sheet north of Haverfordwest to be the same age, and to have formed in similar circumstances.

If we look at the topographic map of north Pembrokeshire we can see where the main drainage routes are.  The Gwaun-Jordanston meltwater channel system has flummoxed researchers for well over a hundred years, but most people nowadays accept that they are very old, having been cut (and then modified) by sublgacial meltwater during several glacial episodes.  But they must have been used by huge torrents of meltwater at the end of the Devensian (LGM) glaciation as well.  If Lake Teifi and the other features in the Teifi Valley were created in the WAXING phase of the LGM ice advance, that makes it quite likely that meltwater escaped westwards through Cwm Gwaun and the other big rock-cut channels prior to the LGM and after it as well.  All very confusing.  That means a complex history of meltwater flow and landform development, between 30,000 and 20,000 years ago.

Main meltwater routes used during the Late Devensian.  It is assumed that meltwater flowed westwards and then southwards through Trefgarn Gorge when the ice was far advanced, and then escaped northwards into Cardigan Bay once the ice edge had retreated to the north of the present coastline.

So where should we draw the Devensian ice edge at the time of the LGM?  I am inclined to think we are talking of retreat phases here, rather than a terminal of "end moraine" position.  Watch this space.......


Tony Hinchliffe said...

My older brother had his 2 covid vaccinations in the County archives building you mention! He has more than a passing, amateur interest in geomorphology, e.g. Trefgarn Gorge, so I will tell him what you have observed.

mukesh sharma said...

Thank you very much for your very detailed understanding happy birthday meme