It is formed from sedimentary rocks from the mid Ordovician through to the late Carboniferous though it is the Devonian Old Red Sandstone which is the rock most identified with the park, since it forms the larger part of the different mountain massifs including South Wales' highest point Pen y Fan at 886m. Like many other upland national parks it is glacial activity during the Quaternary ice ages which is responsible for many of the well-known landforms. The west of the park is also designated as Fforest Fawr Geopark in recognition of its geological interest, and includes Waterfall Country.