Silurian rocks are found in the northwest of the park where a north-east to south-west aligned tract of country running (within the park) from Halfway southwest to Trap is known to geologists as the Myddfai Steep Belt and formed from a succession of sandstones and mudstones of Wenlockian, Ludlovian and Pridoli age. Beginning with the Tirabad Formation, the Wenlockian part comprises the Sawdde Sandstone, Ffinnant Sandstone and Halfway Farm formations. The Ludlow age part comprises the Hafod Fawr, Mynydd Myddfai Sandstone, Trichrug and Cae'r Mynach formations. There are occasional siltstone, ironstone and limestone units within this succession.