The nature reserve is a 2.5km stretch of dramatic wooded limestone gorge cut by the River Taf Fechan, between the bridges of Pontsarn and Cefn Coed to the North of Merthyr Tydfil.

It lies adjacent to the eastern side of the popular Taff Trail.  and the reserve incorporates much of the area notified as Cwm Taf Fechan Woodlands Site of Special Scientific Interest.

Ecology

The reserve has high nature conservation value and supports uncommon flora and fauna.

On the western bank of the river you can see a good population of broad leaved helleborine, as well as small-leaved lime. In addition, there are large areas of grazed limestone grassland, some with bracken, with plentiful violets providing a suitable habitiat for silver-washed fritillary butterflies.

The Eastern bank sees a number of springs, with Tufa deposits, and the Gurnos quarry face is well developed, with facies and caves likely to be home to bats.

The reserve has a wide diversity of habitats, fast and slow flowing fresh water, open and dense deciduous woodland; calcereous and acidic grassland; exposed limestone cliff faces and old walls and tufa springs.

Signs of otters are regularly spotted, and the whole river stretch supports birds such as dipper and grey wagtail.  Goosander are regular in Winter, and evidence of badgers are often seen.

Geology

The gorge was created by the young Taf Fechan river cutting into the northern outcrop of carboniferous limestone, forming the northern border of the south Wales coalfield.

A particularly impressive section is to the east of the Trefechan estate. The geology is especially obvious in the Gurnos quarry.

South of the quarry, millstone grit is the underlying rock. At the site of the Heads of the Valleys road bridge there are fallen blocks of honeycomb sandstone, the marker horizon which occurs at the S zone / D zone boundary in the carboniferous limestone over a wide area of the north crop of the coalfield.

The sandstone outcrops at the top of the vertical face south of the Gurnos quarry cliff. The majority of the site, to the north of the road bridge, consists of the Cil yr Ychen series of carboniferous limestone. This is richly fossiliferous, with brachiopods (typically seminula), productids, corals and gastropods (including Euomphalus).

Historical Interest

The Pont Sarn bridge by the Blue Pool (Pont-sarn-hir - the bridge of the long paved road) is the site of the roman road crossing, travelling from Y Gaer at Brecon to the coastal fort at Cardiff. From the bridge it went through Gurnos, towards Penydarren Park and on to Gelligaer.

There are the remains of an eighteenth century corn mill on the eastern bank of the northernmost section of the reserve, one of four grinding mills that were located in the Taf Fechan valley. The original millstone grit millstone is nearby.

Further down the river are the remains of a fullers mill (a pandy) on western bank just north of timber bridge. Well-known local weavers, the Harris family, owned both these mills.

The Gurnos quarry supplied limestone for the furnaces of the Cyfarthfa Ironworks. Following the closure of the Ironworks, the quarry was planted with a selection of pine trees, with some still surviving on top of the cliff.

The tramway runs south from the quarry alongside the Taff, to beyond the Cefn Coed bridge. It was extended to reach the new Glamorganshire canal in 1792. It supplied the Cyfarthfa Ironworks with the all important limestone needed for the ironworking process, and the stones forming the base of the tramway rails are a scheduled ancient monument, as is the Cyfarthfa leat which runs alongside it.