Tregynon waterfall is one of the highest waterfalls in Mynydd Preseli, and in Pembrokeshire more widely. In contrast to more mountainous regions of Wales, where high waterfalls can often be found in the steep reaches of upland streams, or marking the switch between uplands and lowlands, waterfalls in Pembrokeshire tend to be smaller, or located on the edge of coastal cliffs. Tregynon’s striking form is due, in part, to the impact of ice. The Tregynon stream flows into Cwm Gwaun, the Gwaun valley, and during past ice ages this valley was over-deepened. The smaller streams also flowing into Cwm Gwaun were unable to deepen their valleys as quickly and left as ‘hanging valleys’ high above, forming several waterfalls, such as at Tregynon.
The stream, waterfall, and valley provide a home for wildlife. Plants thrive in the damp environment created by the waterfall’s spray, and in the marshy environments along the stream’s banks. Insects, such as dragonflies, caddisflies, mayflies, and stoneflies live among the river gravels and the decaying woody pieces that fall from the plant covered slopes. Much birdsong can be heard among the trees. Near the head of the waterfall, and overlooking Cwm Gwaun below, the outline of Castell Tregynon, a defended iron age fort, now overgrown with trees and other vegetation, can also be seen.