Grwyne Fawr

A section of it forms the administrative border between Powys and Monmouthshire and also of the historic counties of Brecon and Monmouth.

The river now heads west along a valley whose alignment is guided by the presence of a geological line of weakness known as the Neath Disturbance.

It rises on the southern slopes of Waun Fach and gathers a number of tributary streams from the broad ridge which separates it from the Grwyne Fawr.

The lower reaches of the river run through an area blanketed with glacial till, a legacy of the last ice age.

[3] The valley of the Grwyne Fawr, then known as "Coed Grano", was the site of the murder in 1136 of the Norman Marcher lord Richard de Clare, 1st Earl of Hereford, by the Welsh under Iorwerth ab Owain and his brother Morgan, grandsons of Caradog ap Gruffydd.