Devensian stratigraphy of Druidston Haven

Generalised sequence seen in the cliff today.  Much of the face is subject to slumping and is masked by vegetation.  In the north (left of the diagram) the slope breccia is invisible, having been covered by slumping and "flowage" of saturated Irish Sea till.

Druidston Haven, to the south of Rickets Head on the west-facing coast of St Bride's Bay, has been studied for many years and is one of the classic Quaternary sites of Wales.   The sequence of sediments is very similar indeed to that of Abermawr -- as indeed it should be, if it has experienced a similar Quaternary history.  I went down there yesterday to take a look, and examined it quite closely after enjoying a leisurely picnic in the sun.

The sequence shown in the diagram above is nowadays a little difficult to discern, because of extensive slumping of the clay-rich sediments and because of vegetation growth.  The raised beach is currently not visible, but I have seen it here in the past.

Wave-cut platform remnants at the northern edge of the pebble beach, near Priests Vault.  There is strong structural control associated with fracture planes.  Note the igneous giant erratic......

Cemented breccia (iron oxide and manganese oxide cement) resting on the rock platform.  Note the abraded inclusions.

One new feature, never recorded before, is a solidly cemented breccia made of sharp-edged fragments of black shale, resting on a remnant of an old raised beach or wave-cut platform, exposed in the northern corner of the main bay, where the present pebble beach buts against the cliffline.  There are two distinct platform remnants here, both affected by storm waves around present HWM.

I'm puzzled by the breccia, because it does not look like the fault breccia which is prominent just around the corner in the cliffs -- marking the edge of the Ordovician horst flanked by Coal Measures to the south and to the north.  The amount of displacement on the Simpson fault is c 1800m, so it is not surprising that there is a wide band of smashed-up rock or fault breccia here, prominently visible in the cliffs.  But I am confused because the limited exposures of this breccia suggest that it rests on a wave-cut platform, and it contains abraded stones and smaller pebbles which clearly have nothing at all to do with the tectonics along the fault line.  So I think this is a Quaternary deposit, made up mostly of shattered shale and mudstone fragments from the faulted debris exposed in the cliffs and with added inclusions which may have come from interglacial deposits -- either raised beach or slope breccia.

All of which brings us to comparisons with Broad Haven, not far to the south, and Whitesands, to the north-west, out at the tip of the St David's Peninsula.

https://brian-mountainman.blogspot.com/2019/02/the-full-whitesands-quaternary-sequence.html

At Whitesands, the cemented breccia is quite prominent, and elsewhere it can be seen to be associated with the raised beach.  So the most parsimonious explanation of it is that it is an interglacial deposit, laid down some time before the onset of the Devensian cold phase.  I see no evidence of a periglacial origin -- and I think the debris has accumulated as a result of "normal" rockfall and slope degradation processes.

I am getting more and more interested in these cemented deposits, as readers of this blog will know!

https://brian-mountainman.blogspot.com/2018/11/on-significance-of-cementation.html

At the southern end of the drift cliff, a sharp contact between slope breccia (the lowest visible stratum), made of broken sandstone and shale blocks from the adjacent cliffline, and stony Irish Sea till, coloured grey.  Modern pebble beach in the foreground.

Classic massive, stiff Irish Sea till exposed near the northern end of the cliff. There are relatively few pebbles and larger erratics in this till, assumed to be a lodgement till made mostly from dredged up sea floor materials.

The brown slope breccia is between 2m and 3m thick, and could be thicker in some places.   It does contain some erratics, but consists largely of broken sharp-edged fragments of Ordovician sandstones, quartzites and shales in a sandy and gravelly matrix.  There is no clear stratification, but the deposit is matrix-supported -- so it is best interpreted as a periglacial deposit accumulated over many thousands of years of cold climate.

It is overlain by clay-rich Irish Sea till, which has all the characteristics of a lodgement till in the northern part of the section but is more stony and gravelly elsewhere.  There is no clear junction between the lodgement till and the overlying stonier facies -- so the sedimentation process seems to have been unbroken, with the  facies change related to sediment supply and gradually changing glaciological conditions.  The overall thickness of the grey Irish Sea till is between 15m and 20m.  Here and there we see inclusions of patches of very stony till, suggestive of a dynamic ice wastage environment -- but there are no horizons of true fluvioglacial deposits.

A very stony facies of the Irish Sea till -- still grey in colour, but with an abundance of stones of all shapes and sizes, and many different lithologies including Cambian red and purple sandstones from near Caerfai and Caerbwdi.  This appears to be a lens of stony till surrounded by more clay-rich horizons.

Above the Irish Sea till we see 15m - 20m of brown melt-out or ablation till, full of stones of all shapes and sizes and containing many "super-erratics", many of which have fallen onto the pebble beach below.  Many others are exposed in the stream valley running inland from the coast.  There is no sharp junction between the Irish Sea till and the melt-out till.  In the latter there is considerable variation -- and signs of rough stratification in places, but the internal variations are quite subtle and the deposit cannot really be broken down into any meaningful sequence.  Some horizons are sandier and others are stonier, but there are no recognizable fluvio-glacial layers that might match those at Abermawr, and the till extends all the way to the clifftop, with no overlying slope breccia deposit.

Melt-out till at the top of the Druidston section.  There are some traces of stratification, and some lenses of stony till, but the stratification is discontinuous.

The top part of the exposure, showing true glacial deposits extending to the ground surface.  There is no sign here of periglacial rearrangement or redeposition.  The soil horizon contains some sandloess, and is c 20 cm thick.

What we have at Druidston is an impressive sequence of deposits representing a single glacial episode, which must be late Devensian in age, underlain by a slope breccia that probably represents the onset of cold conditions.  There is less variety here than at Abermawr, but the basic features of the story are the same.  And this site would be very rewarding indeed for studies of glacial sedimentation.

 Large erratic boulders (mostly igneous, from the outer part of St David's Peninsula) exposed in the stream cutting; and (below) the top of a super-erratic which I estimate to weigh more than 100 tonnes.

Note:  the deposit which I now refer to, with a degree of confidence, as "melt-out till"  was assumed for many years to be a soliflucted and rearranged  mixture of till and fluvioglacial sands and gravels and brecciated slope deposits.  It was referred to very often as "rubble drift" -- and indeed I used that label very frequently myself.  However, I am now convinced that it is a true glacial deposit in its original stratigraphic position, and that it is largely unmodified.    It was not formed by lodgement processes on a glacier bed, but by a much more complex set of englacial and supraglacial processes, probably in a dead-ice environment.