Volcanic activity was widespread in the Ordovician period, and evidence of this can be seen throughout Wales, from Llandwyn Island in Anglesey to Porth Maen Melyn in Pembrokeshire. Pillow lavas are formed when red hot blobs of lava cool rapidly as they are forced out of underwater volcanoes. When these blobs fall onto previously cooled blobs, they create the distinctive bumpy pillow-like forms for which they are named. Cooling rims are visible on the cliff-top pillow lavas of Porth Maen Melyn. These formed where the lava reacted with sea water, forming a skin which enclosed the still molten lava.
The site also boasts excellent exposures of rhyolite agglomerate. These formed when broken blocks of mafic lava, which contains dark coloured minerals such as iron or magnesium, were thrown into the air by a volcanic explosion and then engulfed in the molten lava flow as they fell back down.
Grid reference: SM889393 (within the National Park)
Directions: There is a small car park cut into the side of the road at Pwll Deri (OS Grid reference: SM892388). From here it's a 20 minute walk along the Coast Path to reach the pillow lavas which are located 10 metres off the path on the north side of Porth Maen Melyn.
Note: the nearby cliffs are very dangerous. There are no facilities at Pwll Deri car park or in the immediate area.
Porth Maen Melyn
