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September 3, 2017 at 2:51 pm #9243AnonymousGuest
At 10.30 am a group of 15 members plus 2 dogs assembled at the National Trust car park [SR976938] at Broad Haven, near Bosherston. Our first task was to cross the sandy beach to the Carboniferous Limestone outcrop on the north east side of the bay where the beds are dipping southwards and are planed off about 3 metres above the high water mark. This was the height of sea level when the raised beach was deposited during the Ipswichian Interglacial (128-122,000 BP) The rock platform is covered by the remnants of a raised beach deposit of rounded pebbles and cobbles set in a sandy matrix with patches of cemented shelly deposits in places. However, the raised beach is much dissected and eroded by gullies, but it grades upwards into periglacial solifluxion deposits composed of limestone clasts up to 20 cms across. There are also patches of sand rock up to 2 metres thick that appear to be incorporated into the solifluxion deposit. At the top of the section there are exposures of sandy loam and blown sand that form extensive dunes around Broad Haven. The lack of till (boulder clay) at this locality suggests that the area experienced only periglacial conditions during the late Devension (26-10,000 BP) since it was located to the south of the ice front at that time.
We scrambled up over the much weathered Carboniferous limestone to a vantage point on the cliff top and looked across Broad Haven to the sand dunes that extend around the head of the bay. These ridges of sand are formed where there is a wide foreshore that dries out between the tides. Onshore winds blow the dry sand inland so that the resulting dunes tend to migrate landwards until anchored by vegetation such as marram grass. Stackpole Warren is a zone of wind blown sand over a kilometre wide that extends from Broad Haven across to Barafundle Bay. The sand dunes mask the underlying limestone plateau surface which extends across the Pembroke peninsula and represents the marine planation surface eroded during Pliocene times (5-2 Ma).
Broad Haven forms the drainage outlet from what are known as the Bosherston Lakes (also called the Lily Ponds) that were established by the owners of the Stackpole Estate as an ornamental landscape feature in the mid 18th century. This man made landscape of freshwater lakes with dams and sluice gates, was created from the lower reaches of the Broad Haven valleys. Today the area is managed by the National Trust and the Countryside Council for Wales as a National Nature Reserve. Yet this semi natural environment has been created from a geomorphological feature left over from the late Devensian, around 10,000 years ago. The valleys were flooded as sea level rose during the Flandrian marine transgression following the melting of the ice sheets. Deciduous woodlands gradually developed around the tidal creeks and salt marsh backed by sand dunes covered much of the Broad Haven estuary.
Our last stop before lunch was on the far side of Saddle Point where we viewed the limestone cliffs towards Stackpole Head. The coastline is deeply cut by several narrow inlets such as Sandy Hole and Raming Hole. We could see how the overlying terra rossa (orange sandy clay) had been washed down into the limestone fissures masking the limestone cliffs.
After lunch we gathered at St Govan’s free car park and walked a short distance to view the cliffs extending towards St Govan’s Head. Here the limestone is massively bedded and dips gently seawards. This means that when large blocks are fractured from the cliffs they slide downslope and accumulate at sea level. Thus the cliff profile tends to be around 45° whilst further west along the coast where the beds are horizontal or dipping inland, the cliffs are vertical and undercut.
However, our main objective for the afternoon was to visit the deep inlets between St Govan’s and Saddle Head [SR958928]. When the sea erodes the limestone along faults that are aligned at right angles to the cliff line it produces steep narrow inlets known as geos. The three major geos are Stennis Ford, Huntsman’s Leap and Devil’s Barn. The most famous of these is Huntsman’s Leap, a 30 metre deep vertical chasm that is only a few metres wide at its entrance; narrow enough to be jumped by a galloping horse! However, this is not a recommended practice since the drop to the sea below is over 40 metres. The next inlet to the west is the Devil’s Barn that includes a blow hole and a natural arch; and as these features eventually collapse, the geo will be extended inland. Around the head of this geo we examined a thick deposit of terra rossa exposed in the pathway. This orange coloured sandy clay is derived from the slow chemical weathering (solution action) of the limestone leaving a residual deposit of insoluble iron compounds.
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