Geology of the Brecon Beacons National Park

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The central feature of the Brecon Beacons are the mountain ranges that extend across the full width of the National Park. The National Park is divided into three distinct collections of mountain escarpments named the Black Mountains (eastern side); the Brecon Beacons (the central area) and sometimes confusingly in the singular the Black Mountain or Carmarthen Fan (Western area). The interconnected ridgelines of the Black Mountains are separated from the Brecon Beacons by the Usk Valley. The Brecon Beacons from the Black Mountain by the Afon Tawe north of Abercrave. The Brecon Beacons area is itself in two parts (Brecon Beacons and Fforest Fawr) the interconnections of the ridgelines interrupted by the Storey Arms mountain pass.

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. Confusingly Old Red Sandstone is not always red though the underlying presence of its rich reddish hue is often a backcloth beneath the green vegetation in more than two thirds of the 519 sq miles of the National Parks area. In this old red sandstone geological system has a number of subdivisions including red marls (a deep rich red colour) , Senni beds (greenish hue), Brownstones, Plateau beds, and Grey Grits. Old Red Sandstone may therefore be thought of as a generic term. The brownstones are the building blocks of the main mountain ridges. However, 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 surface of the limestone area of the national Park can also be identified by the presence of funnel shaped depressions varying in depth and diameter and variously known as sink-holes, swallow holes or shake-holes. They are formed when the surface limestone collapses into an underground stream.

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. Within the carboniferous system on the southern edge of the National Park can be found Millstone Grit. Sometimes referred to as Farewell Rock so called because once a coal miner encountered this rock no coal is found worth working beyond this strata. It is popular with climbers and has been used for the hearths of iron furnaces being resistant to intense heat.

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. glacial theory postulates but there have been several ice ages stretching back hundreds of millions of years. Interestingly one such ice age occurred during the late Ordovician and Silurian period approximately 450 million years ago when the aforementioned rock systems were established in the north-west corner of the Brecon Beacons National Park. But the ice age that wrought the most recognizable features in the relationship between geology and scenery in the Brecon Beacons is the most recent one that ended between 10000 and 20,000 years ago. It is during this period that some of the distinctive features of the Brecon Beacons landscape were formed. Large masses of ice forming glaziers Bernard moved during warmer periods over extended periods of time produce the U shaped glacial valleys such as Llanthony, Grwyne Fawr, Grwyne Fechan, Taf Fawr and Taf Fechan. Fledgling river routes may well have existed in such locations prior to this ice age but it is a profound impact of glacial movement that created the distinctive fertile river valleys we recognize today. Similarly the gathering of a glacier in a hollow at the head of a valley or Cwm will have commenced a process eventually result in in the creation of Llyn Y Fawr and Llyn Y Fan Fach lakes in the Carmarthen Fan's and the smaller but no less spectacular Llyn Y Cwm Llwch below Corn ddu. Essentially what happened was that a large mass of ice slid downwards into the existing hollow. They are explored with it loose debris of rock and stone known as moraine which settled on the other edge of the existing hollow eventually and forming grass covered mounds usually on the edge of a steepish descent. At the end of the ice age was a rise in temperature the ice disappeared on the enhanced hollow became a lake filled initially with melt waters.

The Brecon Beacons National Park contains the Fforest Fawr Geopark one of seven in the United Kingdom. It was the first Geopark to be established in Wales in October 2005. UNESCO defines a Geopark as "A territory encompassing one or more sites of scientific importance, not only for geological reasons but also by virtue of its archaeological, ecological or cultural value". The Fforest Fawr Geopark covers some 300 sq miles of the 519 sq miles of the Brecon Beacons National Park. The tourist information office at Pontneddfechan has an exhibition about the Geopark. Presumably Geopark designation will serve to provide a focus for developing an understanding of the relationship between geology and landscape. The Brecon Beacons is an excellent landform classroom in the wild.

Swallow hole, sink hole, shake-hole or doline four different names for a steep sided depression commonly found in certain limestone locales in the Brecon Beacons. There are two kinds of doline a collapsed doline caused as its name would suggest by the surface ground dropping away into a space most probably vacated by the action of water on soluble limestone. secondly a solution doline surface streams may disappear from the base of swallow or sink holes into underground water courses running through a cave system.