The Brecon Beacon Mountains to the north of the area are formed by thick, strong, southward-dipping sandstone strata of Devonian age. To the south initially thick strong limestone strata of Carboniferous age and further south, mudstones, siltstones and sandstones of Carboniferous Coal Measures strata are exposed. In the southern part of the area there are areas of much younger deposits (Quaternary Age) that cover a large proportion of the bedrock formations. In the river valleys these comprise alluvium composed of varying amounts of clay, silt, sand and gravel. In between the valleys and draped over much of the area is a cover of glacial till. To the north there is a less continuous cover of superficial material which is largely restricted to alluvium in river and stream valleys and a cover of peat on part of the sandstone upland.
Significantly compressible material is found mainly in the river alluvium, which may contain layers of peat or interbedded clay, silt and sand, and where there is significant thickness of peat on the upland to the north of the area. Deposits containing sand which may become mobilised by flowing water (running sand) may be present in sandy alluvial deposits associated the rivers and streams within the area.
The formations present in the area do not generally contain highly plastic clays that would cause a significant hazard due to shrink/swell clay subsidence. The greatest potential is in the alluvium that may have some weathered clay deposits within it of medium plasticity.
The greatest potential for landslide activity is on the steep valley sides in the more mountainous areas and, to a lesser extent the northern slopes of the Brecon Beacons.
The limestone area that extends east to west across the area to the north of Merthyr Tydfil is well-know for its limestone caves and there is a significant potential for hazards associated with them. Although the sudden collapse of caves causing subsidence at the surface is very rare the potential for material to subside into collapse features as their infilling material settles or for loose material on the surface to be washed into subterranean voids is a common cause of surface subsidence.