The summit of Craig y Llyn lies within the borough of Neath Port Talbot whilst its eastern slopes are within Rhondda Cynon Taf, the boundary running in a north - south direction 200 m (660 ft) to the east of the summit.

The hill is covered by forestry, except for the steep north and northeast faces, and is crowned with a trig point.

The first habitat feature is standing water, supporting aquatic plants typical of lakes with low nutrient content.

"[1] The hill is composed of a thick sequence of sandstones and mudstones assigned to the South Wales Coal Measures.

In fact, the two lakes are the southernmost glacial tarns to occupy cwms in Great Britain.