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Thursday 8 October 2020

Quaternary Deposits at St Brides Haven (2): the Devensian sediments


Satellite image of the bay

Acknowledgement: RCAHMW Colour Oblique Digital Aerial Photographs taken by Toby Driver.  These show the setting of the little haven or creek.  It lies at the northern end of a shallow valley that runs across the peninsula past Mullock Bridge and on to the Gann estuary. The most interesting sequences of deposits lie on the eastern (west-facing) side of the bay.
To repeat what I said yesterday -- It's amazing that as far as I can see, nobody has homed in on this little creek on the north side of the Dale Peninsula to check out the glacial and related deposits. They are actually quite spectacular, and if anybody still has any doubts about Late Devensian ice getting this far south, please go and have a look. The till exposures are fascinating, and easy to examine, and there are erratics of all sizes all over the beach, above and below HWM.  Devensian ice must have come in here directly from the N and NW -- and the evidence is abundant.

On the western side of the cove the cliffline is very broken up, and there are multiple traces of old platforms, more or less well-formed.  There is one quite large triangular segment, just out of reach of storm waves except at times of very high tides -- but it is backed by rock cliffs and there is no trace of a raised beach.   The cliffs in this section are topped by up to 3m of blocky slope breccia.  In places there appear to be two layers, with more blocky breccia below and finer material above -- but there is no sharp break between them.  On places there appear to be traces of "churning" which might be an indication of the presence of permafrost. there are traces of till above the slope breccia here and there, but there are no good exposures.

Rock platform cut across ORS sandstones on the west side of St Brides Haven

The most important exposures occur in two groups, (1) on the clifftops running north from Cliff Cottage, and (2) on the Lime Kiln Peninsula between Eastern Beach and Southern Beach.  The main features are as follows:

1.  Cliff Cottage exposures

There is a more or less continuous exposure of Quaternary sediments at the clifftop, running from near Cliff Cottage northwards for several hundred metres.  Near the cottage, the exposures are easily accessible, but to the north of the gate on the coast path care must be taken in examining them, because there is a steep cliff face to be aware of.  This is the generalised sequence:

Again, there is no trace of a raised beach, and the shattered bedrock and slope breccia rests directly on a broken rock surface.  In some places the rock debris is very coarse, and in other places it is much finer, with traces of pseudo-stratification.  But a prominent characteristic is the convoluted and "churned" nature of the deposit -- normally taken as a sign of long-continued frost heaving in a permafrost environment.  The slopes hereabouts are quite gentle, and so I assume that the rate of downslope debris movement was slow and that the rate of accumulation or accretion was also slow -- compared, for example, with some of the exposures in north Pembrokeshire.  

Above a sharp break is a deposit that I did not expect to find here -- a stratified sandy layer c 30 cm thick, with occasional pebbly inclusions and gravelly layers.  It's uncemented (as are all of the deposits examined around the creek) and I am somewhat mystified by it!  Is it a layer of blown sand accumulated on a bare ground surface at some stage before the arrival of glacier ice from the north?  Or is it a "basal melting" layer associated with the arrival or melting of the ice cover?  It does appear to grade upwards into the till without a break, and I think there is quite a high silt content in some layers -- and that might exclude a "blown sand" origin.   I did not have enough time to examine it properly, but it will reward further study.  If there are traces of land shells or other organic materials in it, that might settle the matter.

The till here has the typical ORS colouring, and it is not particularly clay-rich.  The matrix is sandy and gravelly, and it is chock-full of erratic material derived from the north.  It look very similar to the clifftop tills found all around the south Pembrokeshire coast, and I would refer to it as a "meltout till" rather than a lodgement till.  The upper part of the till exposure has a high concentration of larger  blocks, and I speculate that this layer indicates an episode of meltwater action in which many of the fines from the matrix have been washed away.  But many of these blocks are quite angular, and they may have been derived from upslope rock outcrops. Again, further examination is needed.

Above the till there is no obvious "upper head" or slope breccia, but the till horizon passes upward directly into a typical sandy loam horizon, the upper part of which has been incorporated into the modern soil.

Here is a little photo gallery, which needs no labelling -- the horizons can be tied directly to the diagram above.

Not far from the big broken erratic on the Eastern Beach, below Cliff Cottage, there are fragmentary exposures which suggest that there is another till facies, at a lower level in the valley and at the head of the embayment.  A layer of grey clay-rich till is exposed; it's at least 30 cm thick, but its base is not seen. it grades upwards into the reddish stony and gravelly till which occurs in many locations around the bay.  I suggest that this grey till layer is a lodgement till, and that it is the equivalent of the Irish Sea till which we see at Druidston, Whitesands, Abermawr and many other locations.  There is no reason to separate the two till layers -- they must have been laid down during the same glacial "event". I do not think that the colour difference is simply a matter of weathering or discolouration associated with water penetration.

Presumed Irish Sea till exposed on the Eastern Beach, passing upwards into more stony and sandy reddish till typical of ORS locations.  There is no major stratigraphic break between the two layers.

2.  Lime Kiln Peninsula exposures

There are three small projections from the shoreline in the vicinity of the Lime Kiln, and Quaternary sediments are exposed in the cliff face in positions where they are easily examined.  But great care has to be exercised in interpretation because of the presence of cist graves in the upper metre or so of the sediments -- and there is other disturbed ground too.

Nonetheless, what we see here is interesting.  For the most part, brecciated slope deposits predominate, and in places they are almost 3m thick.  In places they are very blocky and clast-supported, and elsewhere they are flaky, with much smaller particle sizes, where there has been more downslope travel.  the flaky horizons seem to overlie the more blocky ones.   But "churning" features are commonplace, and there are traces of at least two fossil ice wedges.  Again, these are signs of a prolonged and severe permafrost environment prior to the arrival of the ice.

There is no raised beach, bit in one location where red gravelly till rests on bedrock, there is a high concentration of well-rounded pebbles -- and this might well indicate that pre-existing pebble beach debris has been incorporated into the basal till layers by overriding ice.  Thus is seen elsewhere in Pembrokeshire, and in the Isles of Scilly.

Till incorporating well-rounded beach (?) pebbles.  The basal layer here appears to be undisturbed, but -- beware! -- at top right we see the remains of yet another cist grave.

The reddish till caps the slope deposits, but in this part of the bay it is seldom more than 1m thick.  The cist graves are almost exclusively cut into this till layer.  The till is packed with erratics, mostly less than 30 cm in diameter -- but as we can see from the makeup of the adjacent pebble beach, there are many larger erratics (including boulders up to 2 m in diameter) that have been washed out of the till over the years.  Many of these appear to be derived from Ramsey Island  (and maybe the offshore skerries) and from the Cambrian and Pre-Cambrian rocks exposed at the western end of the St Davids Peninsula.  There are a lot of granite pebbles on the beach -- I am mystified as to where they might have come from......

Churned ORS slope breccia overlain by reddish till with remnants of cist graves exposed by coastal erosion.

In one location there is a sandy layer lying on top of a churned slope breccia layer c 1m thick.  The sand layer is c 20 cm thick, and is overlain by reddish till and disturbed ground.

Broken bedrock and slope breccia (c 80 cm) with signs of churning, overlain by c 15 cm of layered sand, overlain by c 25 cm of reddish till (disturbed).

The full sequence of sediments here seems to be as follows:

Soil and sandy loam
Reddish sandy and gravelly till
Sand layer
Fine brecciated slope deposits (churned)
Coarser brecciated slope deposits and broken bedrock
Raised beach (??)



St Brides Haven turns out to be an important Quaternary site, since the sequence of deposits here is easily accessible.  It demonstrates -- yet again -- that late Devensian ice travelling from the NW overwhelmed the south coast of St Brides Bay and penetrated onto -- and possibly right across -- the Dale Peninsula.  So the site should be interpreted in conjunction with the evidence from West Dale, Mullock Bridge, Broad Haven and Druidston.  On the geological maps till is shown on the plateau around Talbenny -- and it is reasonable to assume that this till is of Devensian age.

So my interpretation of the sequence of events here is as follows:

Holocene -- creation of sandy loam layer and soil formation
Ice wastage -- some redistribution / transformation of till layer
Late Devensian Glaciation -- deposition of reddish meltout till
Late Devensian Glaciation -- deposition of sand layer by basal melting?
Late Devensian Glaciation -- Irish Sea Glacier moves in from the NW -- deposition of Irish Sea till
Early / Middle Devensian -- accumulation of finer slope breccia with permafrost effects
Early Devensian -- rock breakdown and coarse slope breccia accumulation
Ipswichian interglacial -- raised beach? (traces only)
Erosion of assorted raised beach platform remnants -- age unknown
Creation of St Brides - Mullock Bridge - Gann through valley, possibly by subglacial meltwater during earlier glacial episodes.

Earliest events at base -- newest at the top.

One important point is that none of the visible deposits at St Brides Haven is cemented.  There are signs that cementation has started in some of the lower exposures of slope breccia, but even in these it is still easy to extract stones.  Cementation is a difficult issue, but I think this does tend to confirm the relatively recent age of the deposits.

Other relevant posts:

PS.  Where did the granite come from?  Well, very old igneous rocks belonging to the Pre-Cambrian Johnston series outcrop on the very stable coastline between Little Haven and Howney Stone.   Stack Rocks are also made of the same rock types (quartz diorite, quartz albite and dolerite -- and also granite) so we can assume that there are outcrops also on the sea floor well out into St Bride's Bay.  This may well be a source for at least some of the igneous pebbles and boulders to be found in the St Brides till and on the beaches.

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