The waterfall below the Pontsarn viaduct.
From Cardiff, take the A470 to the end of Merthyr Tydfil. From Merthyr Tydfil, take the A465 towards Abergavenny, then almost immediately take the first exit. At the staggered junction with the A4054, continue ahead-right towards Pontsticill. After 2 miles, just past an extremely tight hairpin bend, take a lane down to the right. Park on the road side, just before it crosses a bridge, or in a parking area some way past it. SO 0450 0976.
At the bridge, head downstream on the right bank, following the path, to reach the Pontsarn Blue Pool. The path can be followed as far as desired beyond here, through the nature reserve. After some distance, there are a couple of paths leading up to the Taff Trail, which runs onwards through Merthyr, and back over the viaduct to Pontsticill.
On the other side of the road, the path heads upstream on the left bank, passing beneath the impressive Pontsarn viaduct. Beyond there, it eventually reaches a footbridge over the river, where a path leads up to the Taff Trail. It should be possible to use this to make a looping route, though I have not tried it.
Whilst every care has been taken in the compilation of this description, neither the author nor his fellow walkers can accept responsibility for loss or injury arising from any errors, omissions or inaccuracies in this description. This description is intended as a guide only, and you should select a route to suit your own abilities, at your own discretion. Ability requirements are a guide only. You follow this description at your own risk.