Where have the Brecon Beacons come from?

Pen y Fan to Cribyn

A cirque between Pen y Fan and Cribyn

The main Brecon Beacons ridge has a very unusual shape, kind of resembling a lizzard with many short legs. When I went there for a weekend trip at the end of January I couldn’t stop wondering about how it had been formed.

As for the lizzard with many short legs …the main Brecon Beacons ridge undulates from West to East, while a number of smaller ridges or spurs come off the main ridge on the sides. This gives rise to a series of parallel U-shaped valleys running North/ South away from the main ridge.

It turns out these U shaped valleys are called cirques or cwms and were formed by glacial erosion. This is what Wikipedia has to say about cirques:

The concave amphitheatre shape is open on the downhill side corresponding to the flatter area of the stage, while the cupped seating section is generally steep cliff-like slopes down which ice and glaciated debris combine and converge from the three or more higher sides. The floor of the cirque ends up bowl shaped as it is the complex convergence zone of combining ice flows from multiple directions and their accompanying rock burdens, hence experiences somewhat greater erosion forces, and is most often scooped out somewhat below the level of cirque’s low-side outlet (stage) and its down slope (backstage) valley.

Cirques only arise above the snow line, in conditions where snow can accumulate at the side of a mountain and gradually be transformed into a glacier. Once a glacier is formed freeze-thaw weathering and glacial erosion start working on the mountain, eating into it until they eat away most of the mountain side and create a cirque.

The current shape of the Brecon Beacons ridge emerged at the end of the last ice age about 14,000 years ago, during which it must have been above the snow line.

More info:

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