Geology
Pen y Fan is formed from various units of the Old Red Sandstone which were laid down during the Devonian period. The lower slopes of the mountain are formed from the sandstones and mudstones of the Senni Beds Formation whilst the upper slopes are formed from those of the Brownstones Formation. The very summit, like that of neighbouring Corn Du, is composed of hard-wearing sandstones of the Plateau Beds Formation. The beds of sandstone dip gently southwards towards the South Wales Coalfield basin.
The Brecon Beacons nurtured their own glaciers during the last ice age and both Cwm Llwch and Cwm Sere were excavated by glaciers which grew in the lee of the high ground. Llyn Cwm Llwch occupies a glacially excavated rock hollow and the lake is partly dammed by a late-glacial moraine. The mountain is the highest point within the Fforest Fawr Geopark which was established in 2005 to promote economic development, particularly sustainable tourism, based on the area's geological heritage.
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