'Essentially the geology of most of the Brecon Beacons National Park consists of old Red Sandstone a sedimentary rock laid down more than 300 million years ago. The distinctive billiard table summits of mountain peaks in the Brecon Beacons such as Pen Y Fan, Corn Ddu Fan Hir and Fan Gyhirych are topped by "plateau beds". Plateau beds are tough gritty sandstones with occasional quartz pebbles and form a resilient capping. [...]
In stark contrast to the old red sandstone geological system the southern zone of the national Park consists of a Carboniferous limestone system that extends from the far west to the southernmost tip in the east. Limestone country is easy to recognize it is consistently grey in stark contrast to the reds and browns of the predominant old red sandstone. From Crickhowell the northernmost point of the limestone system is clearly visible to the south, overlooking the town in the splendid and striking form of Llangattock escarpment. One of the characteristics of the limestone system is its susceptibility to the action of acidic water. Some of the most splendid landscape features of the Brecon Beacons National Park have been created by the dissolution of the calcium carbonate content in the limestone by the acidic rain in streams and rivers over thousands of years. Such features include a dozen examples of limestone pavements with their distinctive clintes and grykes; Waterfall Country with its deep wooded river valleys and splendid cascades; a series of cave systems including the extensive Agen Allwedd that stretches underground a distance of from the quarried face of the Llangattock escarpment to the Clydach Gorge. [...]
The coal measures of such significance in the history of South Wales and one of the building blocks of the Industrial Revolution are only found in a few small areas within the National Park. [...]
In addition to the two great Carboniferous limestone and old red sandstone systems which in geological terms are relatively young much older rocks can be found in the north-west corner of the national Park between Llandeilo and Llandovery. Ordovician mudstones and grits and Silurians shale are over 500 million years old and are named after the Ordovices and Silures to two tribes inhabited in this part of Wales when the Romans arrived.'
'These interlinking complex systems of rock formation are the bedrock of the landscape we know as the Brecon Beacons National Park. Over geological time various factors and forces served to shape this underlying bedrock into the landscape we see and enjoy. Of particular significance is the period known as the ice age or more correctly glacial age during which expensive areas of northern Europe including Britain were covered by extensive ice sheets.'