The upper ORS is formed by deposition of materials associated with the start of the Variscan orogenic cycle.[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. The former provide the flat tops of peaks like Pen y Fan and Corn Du as well as other of the higher summits. 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. It forms an isolated outlier at the very summit of the Sugar Loaf. The park sits at the northern edge of the South Wales Coalfield basin which was active throughout the Carboniferous period. 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. A narrow outcrop of Carboniferous Limestone extends east-west across the southern part of the park from Blorenge in the east through Gilwern Hill, Mynydd Llangatwg, Mynydd Llangynidr, Cefn yr Ystrad, Cefn Cil Sanws and Moel Penderyn to Dinas Rock. West of here its outcrop is offset to the north by the Neath Disturbance as it wraps around the north of Waterfall Country through the Ogof Ffynnon Ddu national nature reserve to Cribarth and the Black Mountains. Both the thickness and details of the sequence vary from east to west with dolomites and palaeokarsts more common in the east. 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 limestone sequence here is notably thinner than that evident a few miles to the south in the main ‘north crop’. There are further outliers around Cwm Taf Fechan associated with faulting along the Neath Disturbance.