The hill is composed of a variety of different rock formations all tilted steeply to the southeast in a structure known as the Myddfai Steep Belt. The summit ridge is formed from sandstones and mudstones of the Cae'r Mynach Formation. Immediately southeast of these beds is the narrow band of the Tilestones Formation along which are a line of grassed over diggings left after these flaggy micaceous sandstones were extracted for use as roof tiles. Southeast again is the thick sequence of the Raglan Mudstone Formation. These last two formations are considered to be the lowermost of the Old Red Sandstone sequence though assigned like the rest of the rocks forming Mynydd Myddfai to the Silurian period. Northwest of the summit ridge are the sandstones and siltstones of the Hafod Fawr Formation and beneath them more sandstones, mudstones and siltstones of the Cwm Craig Ddu and Halfway Farm Formations. Northwest to southeast oriented faults cross the ridge in places and can be seen where they offset the line of the Tilestones workings.