Geology of the Brecon Beacons National Park

The geological foundations of the Brecon Beacons National Park are of global significance. In October 2005, the western half of the National Park was accepted into the prestigious European and Global Geoparks Network as Fforest Fawr Geopark. The Brecon Beacons remains the only British national park to encompass a geopark.

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Sandstone hills

Two thirds of the area of the Park is comprised of Old Red Sandstone rocks. They form four distinct blocks of hills which are cut through by major river valleys, such as the Honddu, Grwynne Fawr and Fechan, Usk, Taf Fawr and Fechan, Hepste, Mellte, Nedd Fechan, Tawe, Twrch and Sawdde.

In the east of the Park are the Black Mountains, with a high point of 810m at Waun Fach. South of Brecon are the Central Beacons, which rise to 886m at Pen y Fan, the highest point in southern Britain.


Further west lies the sandstone massif of Fforest Fawr, comprising a series of hills all named as 'fans', with Fan Fawr the highest point at 734m. Water rushing southwards from this area has formed steep river valleys with spectacular waterfalls.

The most westerly block of sandstone is Y Mynydd Du, The Black Mountain, which culminates in the summit of Fan Brycheiniog at 802m and contains the two enchanting glacial lakes of Llyn y Fan Fach and Llyn y Fan Fawr.

Limestone edges, pavements and caves


Along the southern edge of the National Park the geology alters, and outcrops of limestone and millstone grit predominate. The limestone scenery contains many edges and screes and in some areas the land is pockmarked with hollows known as shakeholes.

When ice carved the landscape more than 10,000 years ago it exposed and ground smooth areas of limestone. The limestone has now been partly dissolved by rainwater creating a pattern of blocks and fissures. These fissures are called grykes and support communities of plants forming the habitat known as limestone pavement. It is one of the most scarce habitats in Wales.

There is only a thin band of limestone running east-west across the south of the Park. In places this limestone is exposed at the surface, particularly on higher ground. This forms a network of small, scattered pavements totalling only about 20 hectares. Many of these pavements have been damaged by the removal of the stone blocks known as clints. However, others have not been so disturbed or at least were damaged long ago.

The thin fissures provide shelter and protection for plants like limestone fern and herb robert. The flowers on the pavements and surrounding grasslands attract a number of flying insects while the rocks themselves catch the sun, attracting common lizards and slow-worms.

Where pavements have been left ungrazed, woodland has developed over them. These wooded pavements are also important for small mammals, while the shade and bare rock encourage the growth of lichens and mosses. 

Beneath the surface there are magnificent caves and passages, often adorned with stalagmites and stalactites.

The millstone grit and Coal Measures rocks produce a tough, rather sombre landscape which is often waterlogged. It is these rocks which are which is largely responsible for the formation of the National Park's most famous waterfalls.