500 million years old
The volcanic lava that came to be these rocks contains a lot of silica. Because of this, the lava flowed slowly, frequently crystalizing before leaving the volcanic vent. Under the influence of increasing pressure the volcano would explode, throwing blobs of lava as well as clouds of dust, ash and gasses into the atmosphere. The eruption of Mount St. Helens in the USA happened in this way in May 1980. When the hot fragments collected together, the load was welded, forming a rock known as ash-flow tuffs.
This silica-rich rock and its angular fragments, weathers slowly and produces very acidic soils, of little use to farmers but great for heather Calluna vulgaris.
Can you see the crusty lichen Buella aethalea? This slow-growing acid-loving lichen can be used to date rocks, a bit like tree rings can date trees. Its size has been used to estimate the retreat of glaciers and the dates of ancient tsunamis.
Where do the rocks come from? Haverfordwest, Pembrokeshire