The twin upper reaches of the River Taf – the great and little Taf rivers – reach into the uplands of the Geopark from their confluence at Cefn-coed-y-cymmer.
The Taf Fawr rises on the western slopes of Corn Du at a height of around 600m/2000ft above sea level. Passing under the A470 road at Pont ar Daf, it heads southwards then slightly east down Cwm Taf towards Merthyr Tydfil. The valley is now home to three reservoirs – Beacons, Cantref and Llwyn-onn, which supply water to the thirsty towns to its south.
The valley has been eroded along the line of the Merthyr Church Fault. This zone of shattered rock would have been readily eroded by both ice and water over a long period of time.
The Taf Fechan rises as Blaen Taf Fechan beneath the peaks of Pen y Fan and Corn Du, heading south-eastwards then south before turning sharply westwards south of Pontsticill. Its last few miles before merging with its larger twin are cut into the Carboniferous Limestone along the southern edge of the Geopark to form a gorge which is followed by the Taff Trail.
The river has been dammed to form the Upper Neuadd, Lower Neuadd and Pontsticill Reservoirs.
The upper reaches of both rivers follow the southerly dip of the Old Red Sandstone of the Brecon Beacons as the rocks – the sandstones of the Brownstones Formation – dive southwards beneath the Carboniferous Limestone, the Millstone Grit and the Coal Measures of the South Wales Coalfield.