Nant-y-Ffrith refers to a stream and the wooded valley through which it flows on the border between Flintshire and Wrexham County Borough in Wales. The stream begins in moorland to the east of Llandegla. It passes Bwlchgwyn village before entering a rather steep-sided, rocky valley. It passes under the viaduct of a disused railway line before joining the River Cegidog at Ffrith. There are several abandoned silica quarries and lead mines in the area. Lead mining may have begun as early as Roman times and has left a number of caves.