How were the caves in the Brecon Beacons National Park created?

The formation of the limestone caves beneath the Brecon Beacons National Park began over 300 million years ago.

The carboniferous limestone of South Wales was formed in shallow tropical seas in the Paleozoic era, over 300 million years ago. Much of it is of organic origin, being the shells and skeletons of sea creatures, large and small. Amongst the most spectacular fossils to be seen in the National Park are Lithostrotion corals. Their intricate internal detail is often beautifully preserved.

A band of carboniferous limestone stretches across the National Park from Blorenge in the east to Carreg Cennen in the west. It’s in this thin belt of limestone country, 45 miles long but rarely more than one mile in width, that some of the most spectacular caves in Britain are to be found.

The first caves

Limestone is soluble in mildly acidic water and the water running off the peaty ground of the National Park's hills is just that. Finding its way into small cracks in the rock, it widens them over the years creating a network of open fissures and tubes. When eventually these interconnecting passages reach a certain scale, we think of them as a cave network. 

The caves form both along bedding planes, layers of rock, and along vertical fractures present in the rock from the times millions of years ago when South Wales was successively stretched and squeezed as continents collided and split apart. Roof collapse also plays a part in the growth of a cave over many thousands of years. 

Water pours down the southern slopes of the familiar old red sandstone hills to the north and on meeting the limestone, disappears underground. Because most of the National Park's rocks slope gently towards the South Wales Coalfield, many caves follow this southward dip but they also extend east-west across it until they emerge in one of the major valleys carved through the limestone.

The karst landscape

This countryside with its limestone pavements, caves, dry valleys and shakeholes is known as a karst landscape after the classic region of that name in Slovenia. Our area differs from other karst areas of Britain in having fewer limestone pavements but many more shakeholes (depressions formed where surface water washes the boulder clay that covers the limestone down into cracks or fissures in the limestone). These are present particularly in areas that, at first sight, are not limestone areas.

Although such areas as Mynydd Llangynidr and Mynydd Llangatwg are gritstone plateaus, the limestone isn’t far below the surface and the collapse of sections of cave within the limestone leads to craters appearing at the ground surface. Some are truly impressive at 60m across and 20m deep!