The name "Nedern" is largely used for the lower reaches of the stream, while the upper reaches are known as the "Cas Troggy Brook". It's not clear when or why the river became known as the Nedern. "Dern" [Dyrne] is Anglo-Saxon for "Secret", though Anglo-Saxon terms starting neádun- tend to relate to "force" or "violence"*. Although it is not entirely certain that "Cas" has this meaning here, the current Welsh meaning is "hateful" or "unpleasant". None of these adjectives could really be applied to the current stream, which is a peaceful beck, but they may point to a darker past. In the earliest definite reference c.895 CE* it is simply known as the Taroci (Book of Llan Dâv : See Other Works).