Quaternary sediments at Marloes

 

Annotated BGS map for the Marloes Bay area

Here we go again.  Yet another important coastal site displaying abundant till, glaciofluvial sands and gravels, and slope breccia accumulations that appear to date (yet again) to the Devensian.  I can find no reason why they should be considered older -- and they tie in very nicely to the exposures at St Bride's, Westdale Bay and Mullock Bridge.

Today my wife and I walked along the coast path and checked that the sediments shown on the geological map do indeed outctrop along the cliffline.  Indeed they do, in abundance -- but the mapping of some areas as sands and gravels and other areas as till is pretty arbitrary.  In reality, the sediments are very mixed up with the deposits grading into one another laterally.  We see nothing here that looks like the clay-rich blue-grey Irish Sea till of the North Pembs coast --  but that's not surprising since the ice here must have come from the NW, flowing across land and generating what I have called elsewhere a "land facies" of the till which is an exact stratigraphic equivalent.  I have referred to the sticky clay till with shell fragments and lignite as the "marine facies" -- formed where the ice has entrained sea-floor sediments before crossing the coast (which of course was not there at the time).

Glaciofluvial sands and gravels exposed on the clifftop near Raggle Rocks

Gravelly till on the clifftop west of Raggle Rocks.  This till is seen along the whole of the clifftop between Gateholm and Hooper;s Point -- a distance of c 2 km.  It is always underlain by coarse slope breccia and sand and silt layers; and it is overlain by sandy loam and colluvium, incorporating modern soil.

The focus of attention at Marloes must be the stream valley which is followed by the footpath called Sandy Lane.  It's remarkably similar to the stream cutting at Druidston -- but there the valley's "drift fill" comprises Irish Sea till for the most part.

Bing satellite image of the stream exit, showing the locations of the two key exposures.

The "Sandy Lane Valley" seen from the west.  Note that behind the cliffline there is a reverse slope with stream cuttings exposing up to 20m of slope breccia and till

Western exposure

If you walk down to the beach via Sandy Lane, this exposure is on the clifftop to the right of the concrete steps.  One can see a broken section of sediments c 4-5m thick, in an inaccessible position.  the sequence is as follows:

4. Sandy loam with modern soil -- c 1 m thick

3. Diamicton with a large variety of stone types, colours and sizes and incorporating much broken bedrock material,  interbedded with irregular layers of sand, silt and gravel.  These layers are discontinuous. Thickness c 1.5m.

2. Slope breccia made of Silurian sandstone and shale fragments and incorporating some sandy horizons.  Several different facies with irregular junctions.  Direction of downslope debris travel uncertain, but there may be signs of debris flows?  In the eastern part of the exposure, the sandy layers are concentrated near the base.  Greatest thickness c 3.5m.

1.Very coarse rockfall debris c 50 cm thick, in contact with a steeply sloping sandstone rockface.

Western part of the western exposure, showing slope breccia overlain by interbedded till and sandy and gravelly layers (lighter colouring).  Brownish sandy loam near the surface.

Eastern part of the same exposure, showing c 3.5 m of accumulated slope breccia interspersed with irregular and discontinuous sandy and silty layers.  Total thickness c 3.5m.

Interpretation -- a long period of slope breccia accumulation with climatic oscillations.  Prevailing climate -- probably periglacial.  Some episodes of sand and silt accumulation (as colluvium or hillwash, or maybe as blown sand?).  Early / Middle Devensian?  Then a relatively short-lived glacial incursion by ice that has travelled across the Dale Peninsula from the NW.  Accumulation of meltout till or flowtill with interspersed layers of sands and gravels in a chaotic ice wastage environment.  Late Devensian?  Finally accumulation of Holocene sandy loams and blown sand in an interglacial environment.

Eastern Exposure

This exposure can be seen if you turn left on reaching the beach via the concrete steps.  It's quite spectacular, and the sediments rest on a very prominent but broken-up rock (raised beach) platform c 5m above the beach.  You can climb up onto the platform if you take care. Once there, the Quaternary sequence can be examined closely without danger to life and limb.

This is a seriously interesting exposure.  I have not seen anything quite like it anywhere else on the Pembrokeshire coast.  

The essential features are as follows:

6. Modern soil c 20 cm

5. Sandy loam c 80 cm -- discontinuous

4. Blown sand? c 30 cm

3. Diamicton with varied stone types and sizes in a sandy and gravelly matrix and interbedded discontinuous sand and silty layers. Complex internal structures, assumed glaciotectonic in origin. Greatest thickness c 3m, but to right of section, above slope breccia less than 1m thick.

2. Silt and clay band, discontinuous and irregular, c 10 cm.

1. Flaky sandstone slope breccia with some larger fragments. c 80 cm thick -- but thickens to right of section to more than 2m thickness.

Rock platform eroded across steeply dipping Silurian sandstones.

Interpretation -- The diamicton has to be interpreted as a meltout till or flowtill, resting on a thin silt-clay (basal ice?) layer and interspersed with apparently water-lain sandy and silty lenses.  These are similar to the features seen at Abermawr, in the upper till layer above the Irish Sea till.  

The silt and clay band which lies between the slope breccia and the overlying gravelly till.

Interbedding of silty and sandy layers with horizons of gravelly till with abundant stone inclusions mostly of local origin

Interpretation and stratigraphic relationships around the vertical contact between tectonized till to the left and block slope breccia to the right. 

Sediments at the top of the section, accumulated following ice wastage.  

Because of the vertical contact between gravelly till and relatively undisturbed slope breccia, it is assumed that overriding glacier ice has eroded away most -- but not all -- of the slope breccia in this relatively low-lying enviromnent.  The structures, and the complex and variable stratigraphic sequence, suggests that there was a complex environment here, as ice descended from the Dale Peninsula plateau down towards the base of the cliffline, with much removal of preexisting sediments.  Later, there was chaotic ice wastage and an accumulation of sediments that were saturated and highly mobile.  In the modern jargon, this was a "paraglacial" environment in which sediments were mobilized and rearranged.   In that respect, the environment was very similar to that of Westdale, less than 2 km away, to the SE.

The only other place where till is seen at a low level, at the base of the Marloes cliffs, is in the little valley called Mathew's Slade.  It's a typical till for this area, but the lower part of the valley is filled with landslide material and fallen blocks of bedrock, and I suspect that the till has been carried downslope on a "raft" of landslide debris:

I was intrigued to find that none of the sediments examined is cemented, and even the lowest layers in the sequence are easy to excavate with a trowel.  To me, this suggests a recent (Devensian) age for all of the sediments currently exposed.

Is the raised beach visible hereabouts?  It does not appear to be present on the rock platform which I examined, but adjacent to the path at the top of the concrete steps, there is a poor exposure at the base of a grassy slope in which a number of large well-rounded boulders appear to be in situ.   This might be an exposure of the beach, at about the same level as the rock platform at the mouth of the valley -- but I did not have the time for excavations to check this out.

I'd very much welcome other opinions on the Marloes exposures from somebody with the time and resources to undertake detailed study........