The range stretches approximately from Ammanford in the south-west to Sennybridge in the north-east. The larger part of these hills is formed from Old Red Sandstone, though bands of Carboniferous Limestone and of Twrch Sandstone (or Millstone Grit) are important landscape-forming rocks in the south and west of the range owing to their greater resistance to erosion. The most resistant of all are the Plateau beds, which form a protective cap over most of the range. They also form steep cliffs just below the edge of the escarpment. The rocks are of the Devonian, in common with their companion peaks of the Brecon Beacons to the east. The area was glaciated during the ice ages and a number of fresh moraines are to be found beneath the spectacular north and east facing sandstone scarps in the north-east of the range, especially below Fan Hir. There are smaller moraines lying immediately below the cliffs of Waun Lefrith and Picws Du.