Below Pontsticill the river forms the boundary of the Brecon Beacons National Park and descends into a wooded gorge, where it is crossed by the seven-arched Pontsarn Viaduct built for the Brecon and Merthyr Railway in 1866. It is high and long. Just to the south of the viaduct is Pont-sarn, a stone bridge built in 1864 after the previous wooden one collapsed. This occurred because the parishes of Merthyr and Vaynor were supposed to maintain it jointly, but could not agree, and so it became dilapidated. There were proposals to widen it in 1955, but the plans were withdrawn when there was stiff opposition from local people. Below the bridge, the river plunges into Pwll Glas or the Blue Pool. The river continues to flow south-westwards around the hamlet of Trefechan, and as it approaches Cefn-coed-y-cymmer, there is a tramway and leat on the east bank. The tramway was built in 1792, and was used to carry limestone to the Cyfarthfa Ironworks from Gurnos quarries, while the leat was constructed in 1825. Its main function was to provide the ironworks with a good supply of water, but it also fed the ornamental lake at Cyfarthfa Castle through a syphon. After passing under the A465 and the A4054 roads, the Taf Fechan merges with the Taf Fawr at the wooded confluence that gives Cefn-coed-y-cymmer its name.