Geograph - photograph every grid square

The mass of Dinas Mawr comprises outcrop of a rhyolitic tuff known as the Garth Tuff (part of the Capel Curig Volcanic Formation, of Ordovician, Caradoc age). A nodular structure is frequently developed, but reasons for its formation are not properly understood. One factor that everybody agrees upon is that it relates to the proximity of water or wet sediments at the time of its original emplacement (tuffs are volcanic ash which may be very hot indeed when deposited). The nodular structure here has been replaced by massive, milky quartz suggesting that mobilisation of silica from the hot rock has occurred. Etrex 10 GPS for scale, length 10cm.