Its headwaters rise on the southern and western slopes of Fan Hir at the eastern edge of the Black Mountain (Y Mynydd Du) and the river joins the River Tawe at Glyntawe after a short and steep south-eastward descent of 3 km / 2 miles.
Cwm Haffes is a trench which has been incised into the glacial till left by the glaciers of the last ice age along the geological boundary between the Old Red Sandstone rocks to the north and the Carboniferous Limestone rocks to the south. At the head of the incised valley is Sgwd Ddu (meaning 'black fall').
- Ordnance Survey Explorer map OL12 'Brecon Beacons National Park: western area'
- British Geological Survey 1:50,000 shett 231 'Merthyr Tydfil' & accompanying memoir
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