300 million years ago the Brecon Beacons area was submerged beneath a huge estuary which became a mass of ice during the Ice Age, the weight of which depressed the landscape. Then, following the thaw, the huge ice cap and ice boulders headed southwards creating great U shaped valleys.

Throughout the Beacons is a profusion of water – heavy rainfall feeds rivers and streams and, where these meet the limestone borders, produces dramatically different scenery. Sandstone is hard-wearing whereas limestone is less so and, over the years, limestone has worn away producing sudden drops in the level of river beds which has produced stunning waterfalls – with the best being around Ystradfellte, Porth yr Ogof and, just outside the Brecon Beacons National Park, the stunning Henryhd Falls.

Rainfall absorbs carbon monoxide and produces carbonic acid which is mildly acidic. As a result, in addition to the waterfalls created over hundreds of years, the rivers have gradually eroded the natural cracks and fissures of the limestone to create large pot-holes and underground caves and caverns. Huge underground cave complexes can be found at Dan yr Ogof where some caverns are open to visitors, complete with underground waterfall while other caves are suitable for exploration only by fully-equipped, experienced cavers – eg Ogof Ffynnon Ddu at Penwyllt which is 300m deep and hides over 50km of passageways, and Ogof Draenen beneath the Blorenge mountain near Abergavenny.

Chains of reservoirs have been constructed to capitalise on the high rainfall with several clustered together each having an outflow feeding the next in line as seen by the chain of reservoirs adjacent to the A470 where the Beacons Reservoir’s outflow feeds into Cantref Reservoir which has an outflow into Llywn-On. To the east, mountain streams emerge on Corn Du and Pen y Fan which run into the Neuadd Reservoirs which, in turn feed the linked Pentwyn and Pontsticill Reservoirs.