Geology
The area is almost exclusively underlain by sandstones and mudstones of the Old Red Sandstone, the more resistant sandstone beds of the Brownstones Formation give rise to the major peaks.
To the south of the main peaks are two more broken north-facing scarps, that of the Carboniferous Limestone and, to its south, that of the Millstone Grit. These three suites of rock all dip southwards into the regionally important South Wales Coalfield basin.
The area is criss-crossed by faults which fall into two groups. Two major faults which are closely associated with tight folding of the rocks through which they pass, constitute one group. These are known as the Cribarth and Neath Disturbances. They run north-east to south-west through the area and are usually associated with the Variscan orogeny though the origins of these weaknesses in the Earth's crust can perhaps be ascribed to the earlier Caledonian Orogeny. The second set of faults form something of a swarm which run north-northwest to south-southeast and are most apparent within the limestone and Millstone Grit outcrop.
The area was glaciated during the ice ages and at least three glacial cirques are evident, of which Craig Cerrig-gleisiad is the most dramatic. At both this location and on the eastern slopes of Fan Dringarth are spectacular landslips.
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