Pembrokeshire

Cliffs and sea cave at Stackpole Head, Pembrokeshire. The shattered limestone at the top of the cliff is probably due in part to intense frost shattering during the last glacial period, the Devensian, when this part of Pembrokeshire was free of ice, but subject to severe cold and permafrost conditions. At that time the sea level would have been much lower - perhaps by as much as 100 metres - and the coast line would have been some way out beyond the present day cliff line.

Cliffs at Stackpole Head, looking west to St Govan's Head (the far distant headland), Pembrokeshire. The shattered limestone at the top of the cliff is probably due in part to intense frost shattering during the last glacial period, the Devensian, when this part of Pembrokeshire was free of ice, but subject to severe cold and permafrost conditions. At that time the sea level would have been much lower - perhaps by as much as 100 metres - and the coast line would have been some way out beyond the present day cliff line. The flatness of the landscape round here is probably a much older erosion surface dating from the Tertiary.

Barafundle Bay near Stackpole in Pembrokeshire. A lovely spot, completely unspoiled. There is no road access to here, thank goodness. You have to walk at least a mile from the nearest car park. The distant headland is Stackpole Head. The sea arches in the nearer headland are the 'Lattice Windows'. This is a panorama constructed from 2 photos joined together using Canon's Photostich software.

Barafundle Bay near Stackpole in Pembrokeshire. A lovely spot, completely unspoiled. There is no road access to here, thank goodness. You have to walk at least a mile from the nearest car park. The distant headland is Stackpole Head. The sea arches in the nearer headland are the 'Lattice Windows'.

Another view of the rocks - Carboniferous Limestone - at Stackpole Quay, Pembrokeshire. In the left foreground, the crest of the Stackpole Quay anticline is just visible. In the left background, the northerly dip of the rocks does not match with the foreground. This is because the Stackpole Fault runs across the photo from L to R with different structures visible on either side. On the far side of the fault, the axis of the Stackpole Quay anticline is offset by 100 metres to the right (south).

The Stackpole Fault exposed in a small cove at Stackpole Quay, Pembrokeshire. The camera location of the previous photo in the photostream was in the notch at the top of the photo. Jan is standing just to the left of the fault plane, which consists of a jumbled smash-zone of fragmented limestone recemented with calcite. To the left of the fault plane, the rocks are dipping northwards at about 45°. There is a well-developed, nearly vertical set of fractures (parallel to Jan). These are axial planar cleavage developed around the Stackpole Quay anticline, but have probably been amplified as pinnate cleavage by the proximity and movement of the Stackpole Fault. (A bit like trying to plane a piece of wood against the grain - the plane 'chatters' and you get a rough surface, not a smooth one)

As well as being a very picturesque spot, Stackpole Quay is really interesting in terms of structural geology. The rocks are Carboniferous Limestone which have been folded and faulted. Of particular interest is the Stackpole Fault, which is a tear fault or strike-slip fault. This means that rocks on either side of the fault are shifted mainly horizontally relative to each other, rather than vertically. Here, the off-set across the fault is about 100 metres to the right (dextral). In some ways the Stackpole Fault is like a miniature version of California's San Andreas Fault, although the mechanism driving the fracture is different. In this southwards view, the Stackpole Fault can be seen in the quarry face in the distance. It runs towards the camera, causing the outcrop of rocks in the harbour just to the right of the boat to be truncated suddenly. The camera location (i.e. me) is directly on the fault. The main Stackpole Quay anticline axis is located in the slipway in the foreground on the RH side of the fault, but is truncated by the fault and off-set 100 m southwards to reappear at the far end of the harbour wall on the LH side of the fault. Hmm... it takes a lot of words to explain. It's much easier to understand when you go there and see it for yourself :-)

Another view of the Middle Cove islet at Stackpole Quay, Pembrokeshire, formed of Carboniferous Limestone folded into a plunging syncline. In this photo, the axis of the syncline crosses the islet from L-R with a gentle plunge easterly (R). Cliffs of Devonian age Old Red Sandstone are visible in the background.

This is a view of the plunging syncline in the small islet in Middle Cove, just north of Stackpole Quay, Pembrokeshire. The fold is developed in Carboniferous Limestone, with a plunging axis trending approximately ESE-WNW, dipping easterly. The fold style is mostly concentric, but the 'space problem' in the centre of the fold has been accommodated by small-scale thrusting and crumpling, and additional movement taking place along bedding planes.

This is a view of the Eight-Arch Bridge over the eastern arm of the Bosherston Lakes or 'Lily Ponds', in south Pembrokeshire. Only seven arches are visible in this photo. The Lily Ponds were originally tidal inlets but were dammed off from the sea at Broad Haven South during the 19th century to create a branched freshwater lake.

The Strumble Head lighthouse is located on Ynys Meicel ('Michael's Island') which itself is composed of the Strumble Head Volcanic Formation, mainly pillow lavas.

A general view eastwards over the pillow lavas on Strumble Head, Pembrokeshire. Can you spot the seal?

Another photo of the Ordovician age pillow lavas on Strumble Head, Pembrokeshire. This photo shows the slaggy texture of the pillows. Unfortunately no scale as this particular exposure was not readily accessible to allow the placing of the lens cap or similar. However, the individual pillows are around 20 cm - 40 cm long. Pillow lavas are formed when molten basalt lava is erupted onto the sea floor. If the water is deep enough, the hydrostatic pressure stops the lava from exploding the sea water into steam. Instead, the liquid rock and liquid water co-exist together in an uneasy truce. The lava is extruded like toothpaste into successive oval globules, resembling pillows or sacks, hence the name. See the video clip here: www.youtube.com/watch?v=y7CtJNO14YI

Another photo of the Ordovician age pillow lavas on Strumble Head, Pembrokeshire; a close-up view of one of the pillows. Radial cracking/jointing is visible, as is a hint of a finer-grained chilled margin on the edge of the pillow. Pillow lavas are formed when molten basalt lava is erupted onto the sea floor. If the water is deep enough, the hydrostatic pressure stops the lava from exploding the sea water into steam. Instead, the liquid rock and liquid water co-exist together in an uneasy truce. The lava is extruded like toothpaste into successive oval globules, resembling pillows or sacks, hence the name. See the video clip here: www.youtube.com/watch?v=y7CtJNO14YI

Another view of the Ordovician age pillow lavas on Strumble Head, Pembrokeshire. Pillow lavas are formed when molten basalt lava is erupted onto the sea floor. If the water is deep enough, the hydrostatic pressure stops the lava from exploding the sea water into steam. Instead, the liquid rock and liquid water co-exist together in an uneasy truce. The lava is extruded like toothpaste into successive oval globules, resembling pillows or sacks, hence the name. See the video clip here: www.youtube.com/watch?v=y7CtJNO14YI

A closer view of the line squall over the 'Bishops and Clerks', a group of rocks and small islands just west of St David's Head, Pembrokeshire. Within a couple of minutes this squall was over St David's Head, bringing heavy horizontal rain and strong winds for about 15 minutes, followed by clear, sunny but windy conditions.

A line squall rapidly approaching St David's Head, Pembrokeshire. The small islands and rocks are part of the 'Bishops and Clerks'. Part of Ramsey Island is visible in the extreme left. Within a couple of minutes this squall was over St David's Head, bringing heavy horizontal rain and strong winds for about 15 minutes, followed by clear sunny but windy conditions.

Blustery conditions in Whitesands Bay / Porth Mawr, viewed from the coastal path leading to St David's Head, Pembrokeshire. The headland in the foreground is Trwynhwrddyn ('Ram's nose') - a narrow peninsula comprised of steeply dipping uppermost Cambrian and basal Ordovician shales and tuffs. The hills in the distance are on Ramsey Island.