Old Red Sandstone is the dominant rock within the Brecon Beacons National Park, forming peaks such as Pen y Fan, Sugar Loaf and the Carmarthen Fans but to the south of these Devonian rocks, a narrow band of Carboniferous Limestone stretches east-west through the park. It gives rise to characteristically karstic landscapes and hosts Britain's deepest (Ogof Ffynnon Ddu at 274.5m) and several of its longest caves such as Ogof Draenen and Ogof Agen Allwedd. The Brecon Beacons were glaciated during the last ice age and cirque lakes such as those at Llyn Cwm Llwch and Llyn y Fan Fach are amongst the most popular destinations for visitors.