Geology
Fforest Fach is formed from sandstones and mudstones of the St. Maughans Formation of the Old Red Sandstone laid down during the Devonian period. Some of the sandstone beds on the north-eastern spur of the hill have been quarried in the past for building stone or possibly for use as roof tiles. The now disused Pantymaes Quarry is a major working of the sandstone on the eastern side of the hill which is readily seen from the road.
A legacy of the glaciation of this area during the last ice age is the presence of glacial till around the flanks of the hill. The sides of Cwm Clydach which cuts into the eastern side of the hill show evidence of landslips having occurred, probably in early post-glacial times.
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