Geology Field Trip to Strumble Head & Abercastell 24 June 17

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    Saturday 24 June 2017

    Strumble Head [SM896414]

    We assembled at the free car park near to the lighthouse on the headland. The weather was relatively calm with light cloud. We walked  to the WW2 look out building which is now used by bird watchers and tourists, then make our way down the grassy slope to the rocky foreshore. Here we were able to examine the type locality of the Strumble Head Volcanic Formation of Middle Ordovician age (Llanvirn Stage). The volcanic formation is over 1000 metres thick on Strumble Head and formed of basaltic pillow lavas. Each pillow is enclosed in a thin layer of glassy lava representing a chilled margin. Internally the pillows show concentric zones of vesicles, some of which are filled with calcite or dark green chlorite. The vesicles were originally gas bubbles in the molten lava. The spaces between adjacent pillows are sometimes filled with hydroclastites that are aggregates of volcanic glass produced by rapid quenching of molten basalt as the lava flow came into contact with sea water. It is thought that the lava was extruded from submarine fissures and was rapidly cooled under water producing the glassy skin which prevented gas from escaping. Eventually pressure built up within the pillows as new lava was extruded and the outer skins ruptured producing new pillows.
    From the lookout building we followed the coast path eastwards to Pen Caer that is formed of a large dolerite intrusion where the cliffs show strong vertical jointing. On the east side of the headland the headland you can look over Pwll Bach [SM 902413] to the Carreg Gybi promontory where there is another dolerite intrusion. A well bedded sequence of light coloured flinty mudstones, felspathic sands and ash bands containing spilite pebbles dip steeply seawards into the bay. These beds are considered to be formed of the disintegration products of contiguous lava flows that have been subjected to deep weathering and more recently differential erosion has resulted in a strong ribbing effect.

    We drove along the country lanes to Mathry where refreshment facilities were available in the Farmer’s Arms and the local teashop. From Mathry a minor road leads directly to Abercastell where a small car park overlooks the inlet.

    Abercastell [SM852336].
    We Followed the coast path on the west side of the estuary. In several places the track is cut through outcrops of the Penmaen Dewi shales that display strong cleavage dipping about 700NE. After about 500 metres the path starts to climb steeply over a massive dolerite intrusion that is continued on the opposite side of the inlet on Ynys y Castell. The north side of this island displays the dolerite  cross cutting  bedded rhyolitic agglomerate. Next we followed the path across the fields  towards Ynys Deullyn where the Aber Mawr Shale Formation is well exposed in the steep sided faulted cleft that separates the island from the mainland. About 300 metres along the coast to the west there is a spectacular natural arch where the cleavage planes are dipping steeply NW but the bedding planes can clearly be seen above the arch dipping at about 300SE.

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