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Friday, 4 November 2022

The glaciation of Milford Haven


Looking west along the eastern part of Sandy Haven, some 5 km in from the mouth of the waterway.  Red marls and other rocks belonging to the ORS series dominate -- and the clifftop sediments are also predominantly red in colour.

The glaciation of Milford Haven is one of the great neglected topics.  The geological maps show plenty of glacial and fluvioglacial deposits scattered in patches across the landscape, but I am not aware of any comprehensive study.  And it is widely assumed that the glacial deposits that have been mapped are all pre-Devensian and pre-Ipswichian, originally designated "Older Drtift" and more recently assigned to the Wolstonian or Anglian glacial episodes.

In the BGS Memoir relating to "The Country around Milford" (1916) TC Cantrill and his colleagues (including HH Thomas) mention many locations where isolated erratic boulders and Quaternary deposits may be found, but they were not very consistent with their labelling; they used the term "boulder clay" for clay-rich glacial deposits, but many other exposures of till were simply referred to as "gravel", and they did not distinguish between water-lain or fluvioglacial gravels and fine-grained gravelly slope deposits.  We should not be too critical -- the BGS surveyors were working under great pressure, and not one of them was a glacial geomorphologist.  Nonetheless, the Memoirs are invaluable resources, summarising a multitude of field notes.

The Momoir is here:
227 Milford

We already know that the LGM Irish Sea Glacier or ice stream entered Milford Haven, since there is fresh till above interglacial sediments at West Angle and till and thick fluvioglacial deposits at Mullock Bridge, not far from Dale.  It's amazing that this evidence is ignored by so many Quaternary scientists even though it is well documented.  They insist on portraying an LGM ice edge somewhere out to the west, sometimes 20 km or more out into St George's Channel.

There are also classic exposures of glacial and other sediments at St Bride's Haven, Marloes, and Westdale Bay --  all tying the late Quaternary sediment sequence in with that of St Brides Bay and the coast of North Pembrokeshire.

If Irish Sea ice pressed into the Haven, how far to the east did it extend? Today I managed to get over to Sandy Haven (SM860070) while the sun was shining and the tide was low!  It's "the one that got away"  -- for a variety of reasons I have never examined it properly before, in spite of having walked past it on the Coast Path on many occasions.  The rocks (belonging to the Devonian Milford Haven Group) are predominantly bright red marls and sandstones, but there are also greenish and buff-coloured sandstones and conglomerates exposed along the shoreline.  There are complex structures too, making the 1 km of cliffline somewhat chaotic and interesting, with abundant faults and crossing fractures and little anticlines and gullies everywhere -- but hardly any caves.  The cliffs here are seldom more than 20m high, and rockfalls and slope processes are more significant  than shoreline processes associated with wave action.  This is a much more sheltered coastline than that of the open Atlantic coast to the west.

On the clifftops between Ferry Cottage and the eastern end of the bay, two patches of Devensian sand and gravel are shown, but I think these are periglacial rather than fluvioglacial in origin --  and in a close examination of the cliff exposures I only found one small patch of gravel that I would link with the presence of glacial meltwater.

The Quaternary sequence in Sandy Haven is dominated by rockfall debris and slope breccia with so much internal variety that generalisations are hazardous!  The sequence in one place seems to bear no relation to the sequence 20 m away, just around the next corner; I conclude therefore that the relatively steep slope inland of the cliffs has generated complex rockfalls and debris flows over many thousands of years, with the details of downslope stratigraphy determined by the nature of the rock outctops rather than by changes of climate. I may be wrong, but currently I can see nothing consistent in the stratigraphies of the exposures from west to east.  Here are a few glimpses of what can be seen:

To the west of the steps:

To the east of the steps:

It's tempting to correlate some of these layers with the stratified "rearranged till" at West Angle on visual grounds alone, and to match up the buff-coloured colluvium with similar materials at other South Pembrokeshire sites, but I will hold back on that for the time being.

So we have plenty of evidence for a prolonged period dominated by rockfalls,  slope processes and probably a periglacial climate dominated by frost-shattering or breccia production and by downslope debris creep and catastrophic debris flows or slope collapses.  The slope breccia is in some places so "churned" that permafrost processes should be invoked.

"But is there any Devensian till here?  And where is the raised beach at the bottom of the sequence?" I hear you cry......

I was coming to that.

First, the raised beach.    

It's exposed in just one locality, in the cliffs near the car park.  We can see it in a very messy exposure, in a sandy deposit dominated by well-rounded pebbles generally less than 10 cm in diameter.  There are many erratic pebbles.  The layer is only about 20 cm thick, and it is not cemented.  It rests on a rough rock platform just out of the reach of storm waves, but it appears to rest on a brecciated layer of red marl fragments, and for this reason I think it is in a secondary position and that the in situ raised beach is hidden behind the exposure face.  Above the raised beach we see about 1m of redddish sandy and gravelly brecciated slope material.

The raised beach at Sandy Haven.  Note the high concentration of rounded pebbles -- probably displaced from their original positions.  The beach rests on a broken surface of red marls. 

Next, the Devensian till.  

This is exposed in several places, mostly around 100m - 80m to the west of the steps. Grid ref SM 859072. There is one large mass of till exposed in a small embayment.  It's at least 4m thick, with a reddish colour and a sandy matrix.  It's packed with rounded, faceted and broken stones and cobbles of all sizes and many rock types -- including some igneous boulders almost 1m in diameter.  This is not a deposit dredged up by overriding ice from an old sea bed -- this is a "land facies" of the Irish Sea till, with its characteristics determined by the nature of the land surface of the south shore of St Bride's Bay.  The ice responsible for this till has travelled for the last 6 km of its journey from the NW across the S Pembs coastal platform.  It is thus very similar in composition to the till studied at Ogof Golchfa on the south shore of the St Davids Peninsula.

The till exposures are very varied and discontinuous.  The base is not seen in the main exposure, but in one locality 1m of till incorporating many slope breccia fragments is seen resting directly on bedrock. The till grades up into about 2m of red flaky gravels, with colluvium and soil above.  In the main exposure the till is overlain by a layer of fine red colluvium about 30 cm thick, with about 1m of stony slope breccia above that.  

Two exposures of the red stony till at Sandy Haven.   Igneous rocks are included in the erratic assemblage, and there are some large igneous boulders on the beach below.

Stony till with abundant local fragments of slope breccia, with a layer of sandy and silty colluvium above, and a layer of slope breccia (upper head) above that.

A small exposure of stony gravels that may be associated with meltwater activity alongside melting ice.


This is not the easiest site to interpret, and there is nowhere that we can cite as showing a full stratigraphic sequence.  However, piecing it all together it looks like this:

6.  Modern soil
5.  Fine-grained colluvium (aeolian? slope wash?) up to 1m thick
4.  Thin slope breccia generally c 1m thick, incorporating glacial debris
3.  Sandy gravelly till up to 4m thick, incorporating much slope breccia
2.  Slope breccia and colluvial deposits -- many different facies
1.  Raised beach cobbles -- c 20 cms thick (disturbed?)

The precise placing of the till in this sequence is difficult to establish.  Maybe all we should say is that there was a prolonged cold / periglacial episode in which many slope breccias and gravelly deposits were accumulated on an old and complex cliffline, with glacier ice affecting the area for a limited time and incorporating slope breccia, soil and other materials as it flowed over the landscape, probably from the NW.  Glacial deposits were thin and discontinuous.  There may have been somewhat catastrophic ice wastage, with much debris flowage and mixing of deposits.  The till is in any case more like a meltout till than a lodgement till.  Meltwater seems to have played only a minor role in the redistribution of glacial and periglacial materials.  


1.  There is no doubt that the raised beach (Ipswichian?) does extend inside the mouth of Milford Haven, although it was probably thin since this is not a high-energy storm wave environment.  Its altitude is not much above present HWM.

2. There is no doubt that this bay was affected by glacier ice which probably flowed across an old land surface, from the NW.  This was probably at the time of the LGM, around 26,000 years ago.

3. The fact that there are erratics in the raised beach suggests at least one earlier glacial episode.

4. The glacial deposits at Sandy Haven appear to be stratigraphically related to those at West Angle, Westdale, Marloes and other sites.

This is the stretch of cliffs examined during my recent visit.  On this old map the location of the caravan site, camping field and car parks have been added.  The crucial exposures are to the west of 
the access steps.  There is a good path from the picnic area and car park to the top of the steps.  This is a part of the Pembrokeshire Coast Path.


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