Pwllcrochan -- much confusion, and a raised beach

The shelly beach at Pwllcrochan, west of the spit. Bedrock exposures of red marls and pebbly Ridgeway conglomerate.  Typical salt water weathering phenomena on rock surfaces.

A nice piece of Ridgeway Conglomerate, picked up off the beach

Close-up of the edge of a detached block of conglomerate, showing (at the tip of the trowel) the junction between the red marl and the stony and flaky conglomerate.  This has been coated with a veneer of manganese oxide.  Does this indicate very long exposure?

Yesterday I spent two or three hours at Pwllcrochan (Martin's Haven) on the south chore of Milford Haven -- a pretty cove with extensive mud flats and a colourful shoreline, dominated by the oil and gas installations.  An interesting place, not least because if you are a geomorphologist looking for Pleistocene deposits you have to cope with some very complex stratigraphy in the outcrops of the ORS Ridgeway Conglomerate.  In the bedrock there are alternating beds of red marl and conglomerates of several different types, including some which really should be called breccias, because they are packed with broken pebbles and flakes and fragments of many different lithologies.  Some of them look like tillites or ancient frost-shattered slope breccias -- except, of course, that they were supposedly formed at a time of hot and dry desert-like conditions interspersed with torrential floods which broke up and redistributed older sand and gravel beds and even boulder beds.  On the shoreline the conglomerate beds are broken up into many detached blocks into something of a chaotic jumble, made even more confusing by the black manganese oxide veneer which coats many surfaces and the litter of shells, beds of seaweed and other shoreline litter.

It took me quite a while to get my eye in -- on several occasions I thought I had discovered an exposure of the cemented raised beach close to HWM, but had to revise my opinion when I realised I was looking at yet another detached and damaged chunk of Ridgeway Conglomerate.

However, on walking westwards towards the Texaco jetty I discovered that there are many exposures of a raised beach made of typical estuarine gravels at around present HWM.  It's up a metre thick, with a silty and sandy matrix, and is solidly cemented with iron oxide and manganese oxide cement.  We see the same association as in many other South Pembrokeshire localities -- rockfall slabs and boulders, and coarse slope breccia incorporated into the beach material.  So at the time of beach formation, bedrock fragments were falling down from the rock cliffs above -- exactly as happens today.  Above this bouldery coarse layer there are up to 2m of coarse pseudo-stratified slope breccia deposits, and then -- in just a few places -- up to 4m of finer slope breccia containing much colluvium. On the clifftop, a silty modern soil layer.

Approx grid reference:  SM 91707 03449.

I did not see anything that could be interpreted as glacial or fluvio-glacial in origin.

So this is the sequence:

Modern soil

Finer slope breccia containing colluvium -- c 4 m max (uncemented)

Coarse blocky slope breccia -- c 2m (uncemented)

Rockfall debris (in places incorporated in the beach) c 1 m (cemented)

Raised beach of pebbles and gravels - c 1 m (cemented)

Conglomerate or red marl bedrock

Colluvium-rich reddish slope breccia exposed in one of the thicker sequences

An exposure of the cemented raised beach (incorporating a block of Ridgeway conglomerate) exposed at the head of the shelly modern beach.  Note the incorporation of blocks of bedrock, some of them smoothed by wave action.  The black colouring is manganese oxide cement.  Above, blocky uncemented slope breccia.

Another exposure of the cemented raised beach, incorporating large angular rockfall boulders of ORS red marl.

In the entrance of the creek called Martins Haven, on the foreshore, there is an exposure of a flattish sheet of cemented gravels overlain by solid grey sticky clay.  I did not have time to examine this exposure in detail.  It is submerged beneath every high tide.  It might be fluvio-glacial in origin -- or the gravels might just have been laid down by the small stream that here reaches the sea.

And what are we to make of that very peculiar natural spit that projects out from the shore northwards into the waterway?  It has to be based in something  --- could it be an old moraine or a bedrock reef?  More investigations needed......

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The significance of all this?  Well, it confirms the presence of the Ipswichian (??) raised beach inside Milford Haven, albeit at a very low level.  So it is not really "raised" at all..........  It also confirms that there was a prolonged period of slope breccia accumulation after the formation of the raised beach.  We can assume that this latter phase occupied the whole of the Devensian glacial episode.

Did the Devensian glacier ice affect this area?  More evidence from elsewhere is needed in order to sort that one out.  There are certainly glacial deposits at Picton Point and at Landshipping, and around the mouth of the waterway -- but how they relate to the events recorded in the outer coastal Quaternary stratigraphy remains to be elucidated.