Ystrad Mynach

Morphology: ystrad+mynach or possibly ystrad+min+ach
Phonetic: us-trahd mun-ah-ch

There are many Welsh words for a valley, depending on the nature and shape of the valley being discussed. "Ystrad" is a wide, flat-bottomed valley and "mynach" is the Welsh word for "monk". The reason for the name is not actually known, though this may be the now-unused name of a river that flowed into the Rhymney as currently there is no evidence of a monastery existing in the area. One possible derivation is that rather than "mynach", the name comes from "min" and "ach" – "min" meaning edge or border and "ach" being an old word for a fen or bog therefore the name here would mean "valley at the edge of the fen".
One charming folktale about the place name relating to monks does exist however (and relates to Llanbradach also). According to the story, a monk was forced to flee from Caerphilly Castle after it became known that he was having an "unsavoury" relationship with the young daughter of the French family in residence at the Castle at the time. He was captured, sentenced to death and hung from a tree in the area, so it became known as Monk’s Valley or in Welsh – Ystrad Mynach.