The area was subject to glaciation during the Quaternary ice ages and periglacial processes and landslips have contributed further to the character of the modern landscape.
Quarrying for limestone and sandstone were significant extractive industries in the past whilst tourism based in large part upon the perceived quality of the area's landscape is important for the modern economy.
Also in this tract of country between Llandeilo and Llangadog are the hard and often pebbly sandstones of the overlying Ffairfach Grit Formation which form the low hill of Y Garn Goch.
Above these are the thinly laminated sandstones of the Llandeilo Flags Formation succeeded in turn by the slumped beds of the Ashgillian Nantmel Mudstones, seen at Myddfai for example.
The Ludlow age part comprises the Hafod Fawr, Mynydd Myddfai Sandstone, Trichrug and Cae’r Mynach formations.
[1] In the east of the park, the formation is completed by a calcrete locally up to 10m thick and representing a long period of sub-aerial weathering at the end of the Pridoli.
[2] The lower ORS is essentially detrital material eroded from the Caledonian Mountains to the north and west and deposited in this basin south of the orogen.
Besides calcretes, other horizons which appear markedly different are the intraformational conglomerates which often stand out as steps within stream profiles due to their being relatively hard-wearing.
[pdf, p15] In the west and centre of the park, the Upper Old Red Sandstone consists of the Plateau Beds and the unconformably overlying Grey Grits Formation.
Going east from Talybont-on-Usk the Plateau Beds are increasingly overlain by the sandstones of the Quartz Conglomerate Group until, as the Plateau Beds are cut out south of Llangynidr, the Quartz Conglomerate constitutes, in its outcrop along the southern side of the Usk valley, the entire upper ORS sequence in the east.
During much of this time, world sea levels were fluctuating as South polar icecaps grew and shrank in the course of the long drawn out Karoo glaciation.
Outliers of the Carboniferous Limestone occur at Carreg Cennen where the castle is built atop a 100m cliff of the rock in what is otherwise ORS territory, and again at Pen Cerrig-calch north of Crickhowell in the east of the park.
The Marros Group is the modern name for the South Walian rock sequence traditionally referred to as the Millstone Grit Series.
The Twrch Sandstone forms large expanses of often rough and poorly drained moorland from the southern slopes of the Black Mountain in the west to those of Cefn yr Ystrad and Mynydd Llangynidr further east.
[3] To the southeast of this zone is the Carreg Cennen Disturbance which runs on Caledonoid lines from Pembrokeshire to the English border, passing through the northwest of the park and around the castle from which it derives its name.
At Cribarth, Variscan movement on this Caledonoid feature has given rise to a couple of folds in the limestone and overlying Twrch Sandstone from which the present landform has subsequently been carved, in part by glacial action.
The interstratal karst of Mynydd Llangynidr has been recognised as being of importance and designated as a geological site of special scientific interest.
The former is situated at the rear of the great embayment at the Craig y Cilau national nature reserve and the latter is high on Mynydd Llangynidr.
Glacial ice is thought to have covered the majority of the park at the height of the last glaciation around 22,000 years ago though some areas remained unglaciated including the larger part of the Black Mountains.
Some 'rucking' of slipped masses of bedrock appears on the hillside to the north of this feature, the result of a separate bedding plane slide.
Narrow strips of alluvium i.e. accumulations of clay, silt, sand and gravel, occur along the valley floors Fragmentary rock material, the product of weathering, known as head is recorded.
[13] Certain units within the Twrch Sandstone (formerly known as the Basal Grit) were found in the nineteenth century to be of sufficient purity to make the rock suitable for the manufacture of refractory bricks for lining furnaces.
[18] The world heritage site at Blaenavon straddles the national park's southeastern boundary; its focus is on the exploitation of the area's diverse geological resources and the economic and social changes wrought.