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Tuesday, 21 March 2017

The South Pembrokeshire LGM - again

 Reddish Devensian till exposed in the wall of the Fopston Farm drainage reservoir, near Marloes. The cemented layer, stained with manganese oxide and iron oxide, may represent the position of the Holocene water table.

South Pembrokeshire really is a tough nut to crack.  I just wish that I had been around to keep an eye on all those elongated trenches that were dug across the area when they were installing the oil and LNG pipelines between the Milford Haven installations and the English Midlands.  Much will have been revealed..........  ah well, too late now......

When I looked at all the info from West Angle Bay over the past few weeks, I was reminded forcefully that there is abundant evidence of Devensian ice pressing into the mouth of Milford Haven from the west or north-west.  Along the coast to the north there are glacial and fluvioglacial deposits at Westdale Bay,  Mullock Bridge, Fopston (801096) and Fold near Marloes.   The Geological Survey field workers identified a sheet of "boulder clay" on the western side of the Dale Peninsula, and we are familiar with the evidence of Devensian ice affecting all of the coasts of St Bride's Bay.    We have mentioned Druidston and Sleek Stone (Broad Haven) on more than one occasion.  The surveyors identified strong evidence of glaciation on both Skokholm and Skomer Islands.  HH Thomas thought that there was a considerable expanse of "yellow loamy drift" with many foreign erratics to the west of Roch -- but very few patches to the east of the castle.  Cantrill said that the sheet of boulder clay on the coast between Nolton and Druidston extends "some little way inland."  He said there are good exposures of till at Madoc's Haven and Druidston, but that the drift keeps to the top of the cliff and thins out at Druidston Villa.  He said that to the east and north of the Villa there is a "small sheet of gravel" -- could this be a pro-glacial fluvioglacial deposit, laid down just beyond a static ice front?

Another spread of till with igneous erratics is described from the Talbenny area, and there is a giant erratic near the cliff edge at Mill Haven.  Small patches of gravel and sand occur at Rickeston Bridge and in the Walwyn's Castle Valley, and the surveyors seem to have thought that there was some glacial river diversion associated with a feature called "The Rock" c 300 yds NE of Rickeston Bridge.  Might this represent the maximum inland extent of Devensian ice in the SE corner of St Bride's Bay?  This might also be supported by the presence of a "conspicuous mound" of fine yellow sand, about 200 yds in diameter, to the NW of Orlandon, about 6 km to the west; this is linked to another smaller mound, and to exposures of gravel containing much ORS material observed to be about 15 ft thick.  OT Jones thought that these deposits might be linked with the kame terrace at Mullock Bridge (which I studied intensively during my doctorate fieldwork in 1962-65).   There is another patch of gravel on the edge of the cliff west of Ripperston Farm, with rounded ORS pebbles "probably picked up on the sea bottom".

The surveyors noted various small patches of reddish-yellow boulder clay (with striated pebbles) and also gravelly patches around Milford, Thornton and St Botolph's; but from the published descriptions it appears that these patches are well weathered and eroded, and that they lie stratigraphically beneath head.  Might they be the last remnants of a pre-Devensian cover of glacial and fluvioglacial deposits?  Much fresher ice-related deposits appear around the Dale Estuary, especially on the western flank.  On the eastern side of Gann Flat, almost a metre of gravelly and sandy head is seen in the cliff; this material seems to have originated in fluvioglacial sands and gravels like those at Mullock Bridge, but there has been "paraglacial rearrangement" similar to that at Westdale Bay.

My current impression is that the ice that came in from the NW across St Bride's Bay was not very thick, and that it found it very difficult to surmount the rampart of cliffs (which were of course at that time not sea cliffs, since sea-level was around 120m lower than it is now) between Newgale and the mouth of Milford Haven.  In many places these old cliffs or steep coastal slopes were in excess of 60 m high, and in places up to 80 m above present OD.  Beneath present sea-level the coastal slope continues to drop quite sharply down to -15m along most of the southern shore of the bay -- so incoming ice had to surmount an obstacle generally between 75 m and 100 m high between Little Haven and the western end of Skomer Island.  The stretch of territory to the north of Talbenny must have been a particularly prominent obstacle.  Lobes of ice may well have pushed inland both to the east of this rampart (towards Walwyn's Castle) and to the west (towards Orlandon and Mullock Bridge).  My impression is now that all of the land to the west of the St Bride's - Dale Estuary through valley was inundated by Devensian Irish Sea ice.



 Field sketch of the exposures at Westdale Bay, as recorded by Gillian Groom in 1957.  Note the till at the base of the exposure and the pseudo-stratified materials above.

Devensian glacier ice pressed into the mouth of Milford Haven, but what happened on the west and south coasts of the Castlemartin Peninsula?  There appear to be traces of massive clay till  beneath the beach at Freshwater West, and most of the coast between Frainslake and Broad Haven (South) is difficult to examine because of the presence of the Castlemartin Firing Range. Risking life and limb (there are unexploded objects lying around), I have looked at some of  the terrain and have seen scattered erratics but no expanses of fresh till.  


Dixon and his colleagues were sure of the presence of till in a "pipe" in the limestone at Catshole Quarry, Pembroke, capped by head and other deposits.  From the published description, it appears that this may be a very old deposit.  A fresher till may be present in the St Florence and West Jordanston area, and the GS field mappers recorded a thickness of 7 ft of gravelly reddish clay, with occasional igneous erratics.  They observed till overlying the Tertiary gravels and conglomerates in the Flimston clay-pits, and remarked on the presence of scratched and facetted stones and igneous fragments as well as large angular fragments of chert.  On page 199-200 of the 1921 Pembroke and Tenby GS Memoir there are abundant references to erratic boulders, many of which are derived from the St David's Peninsula and Ramsey Island.  Sandy reddish till with igneous erratics is also recorded in a deep fissure or pipe in the NE corner of Sandtop Bay on Caldey Island -- but when I visited this bay a couple of years ago no exposure could be seen.  On the other hand, I have observed fresh till at the other end of the island, in Ballum's Bay, which has to be of Devensian Age (it rests in a Carboniferous Limestone fissure, and if it was older it would certainly be solidly cemented....




Some of the faceted and rounded erratic stones taken from the fresh till in Ballum's Bay, 
Caldey Island.

The coast between Tenby and Pendine appears not to have been affected by Devensian ice, and DQ Bowen has presented evidence from Marros to support this contention. In my own research at Marros I did not observe any till or related deposits either.

The most parsimonious explanation of the distribution of ice-related deposits on the Dale Peninsula and Castlemartin Peninsula is that there are three zones:

1.  A zone close to the cliffline and a little way inland which appears to have been affected by Devensian ice pressing onshore.

2.  A zone further inland where erratic boulders are abundant but where glacial deposits appear to be scarce, heavily weathered and eroded, and mostly restricted to interfluves.

3.  A coastal strip running from Tenby to Pendine which appears to have escaped from any direct Devensian glacial effects.

As suggested by the GS surveyors in 1921 and by JC Griffiths in 1940, the last incursion by Irish Sea Glacier appears to have been by ice that was thin and not powerful enough to overcome the coastal slope and to push far inland.  My current thinking is portrayed on the map below.  I wonder how long this map will survive before it needs to be modified?  What was I saying the other day about falsification......? 


An attempt to portray the Devensian (LGM) limit for Pembrokeshire.  The ice from the north was powerful enough to progress inland as far as Mynydd Preseli, Wolfscastle and Roch; but further south it was nowhere powerful enough to progress far inland after encountering the coastal slope / old cliffline.









4 comments:

Myris of Alexandria said...

Remind us again of the significance of faceted and rounded cobbles with regard to their being glacially transported please. Where is the Devonian Sst coming from (if it be Devonian)??
Good to see all this new work. Into the mainstream lit??
M

BRIAN JOHN said...

Faceted cobbles are a good indicator of a glacial origin, if they are found in a sediment. Striations are good too. Facets are sometimes fracture faces, showing where a chunk has been sheared off because of irregular pressure exerted beneath or within glacier ice. Stones may be tumbled over, with four or five different facets on them -- some influenced by the fabric or bedding or crystal structure within the rock, and some apparently not. Some facets may be abrasional, where a cobble or boulder is stuck on the bed and then rasped or worn away by overriding ice carrying tools. Poor sorting is an even better indication -- I always look for a mixture of stone sizes, shapes and lithologies, and a very irregular particle size distribution. Tills are often described as "unsorted" or "poorly sorted" -- but those terms are not really very good...... Rounded pebbles (from old beaches or from fluvioglacial or river deposits) survive rather well in till, but sharp-edged fragments tend not to survive very long before their sharp edges are rounded off. I have done posts in the past on stone shapes in till.

Lots of ORS sandstones and red marls in South Pembs -- so much of the material in these reddish coloured tills has not moved far.

Publications? Working on it -- but most of the big journals nowadays ask for CASH in large quantities for articles to be published, or they are rather snooty about articles (such as mine) which depend on old-fashioned observations rather than on whizzo techniques!

Myris of Alexandria said...

Yes is the same for detailed petrographical papers, long out of fashion.
M
Mis- and short- guided/sighted.

BRIAN JOHN said...

Other fields too, no doubt. But one advantage of the "digital / commercial" era is that we have things like Researchgate and Academia -- which will accept databases, illustrative material, detailed field or lab findings or supporting information. It can be made available to all, and anybody interested can get at it and can -- if they choose -- cite it. I have set up a "working papers" category on Researchgate, to which I have already submitted stuff. May well upload more......

Why don't you do the same, Rob? Better than having stuff festering on your computer hard drive and never seeing the light of day......