Geograph - photograph every grid square

Craig-y-ddinas or Dinas Rock is a popular spot just inside the boundaries of the Brecon Beacons National Park and the Fforest Fawr UNESCO Global Geopark. The name, which can be translated as 'rock of the citadel', is a reference to the Iron Age defensive site which was established on its flat top.

The quarry which now serves as the car park was opened up in the C19th to work the Carboniferous Limestone which can be seen dipping steeply to the north at the back of the quarry. The vertical wall is a geological fault plane whilst the sloping wall to its right (now partly covered in netting for safety reasons) is a bedding plane.

The Dinas Silica Mine, again opened in the C19th, and worked into the middle of the C20th lies around 1/2 mile to the east. It worked the Basal Grit (of the 'Millstone Grit series'), often referred to as the 'silica rock' on account of its purity (and hence its suitability for the manufacture of refractory bricks for lining furnaces). Geologists refer nowadays to the Basal Grit as the Twrch Sandstone.

The car park is managed by Natural Resources Wales (formerly Forestry Commission Wales) in co-operation with the owners of the mineral rights, Natural Amenities Ltd, an independent body established to safeguard the mineral workings for mine enthusiasts.