At the top of Sychnant Pass near Conwy, there are excellent exposures of the volcanic rock Rhyolite. Many of the exposures of the rock show faults or fractures and along some of the fault planes there are cavities which presumably once held hot gases. The fracturing probably occurred during the 'eruptive' stage when the rock was still plastic in nature. The Rhyolite also forms the bulk of the nearby mountain known as Alltwen. It is thought that the 'Alltwen volcano' erupted during the Ordovician period some c550 million years ago and would have formed an island in the sea with volcanic ash raining down on to the ocean floor.
Look for the tiny 20p coin below the cavity to indicate scale.