Rugos', a name found in other parts of Wales (also as Y Grugos). Geography and natural history "The south Wales Valleys support a treasure trove of biodiversity. Like all good treasure troves it had been lost and long forgotten and has only now been re-found, and like some giant archaeological dig, one discovery has led to another; one find has spurred on the finding of the next." Rhigos lies at the apex of the Cynon Valley and the Vale of Neath. Craig y Llyn, the mountain summit above it, is the highest peak in the traditional county of Glamorgan. A glacial cirque excavated into the Pennant Sandstone scarp of Craig y Llyn holds the glacial lake of Llyn Fawr. The glaciers sculpted and over-deepened the valleys into characteristic U-shaped profiles. All the deposits of the Welsh ice were locally derived. Certain parts of the local landscape surrounding Rhigos have been designated as Sites of Special Scientific Interest including the wooded gorges of Waterfall Country, Cwm Cadlan and nearby Blaencynon SAC. The Joint Nature Conservation Committee which advises the government and devolved administrations on UK-wide and international nature conservation, states that the damp grassland and heath of the Blaencynon site has been noted as an area that supports the marsh fritillary butterfly. This butterfly is threatened not only in the UK, but across Europe. which makes this area of importance in an international conservation effort. The Rhigos Mountain since Mid-August 2022 has been closed until further notice, due to a significant mountain fire, which caused the metal wiring that secures the boulders down, to melt away, posing a significant threat to the A4061 Rhigos Road, which plays a big part in connecting communities such as Treherbert & Hirwaun. Specialist access rope contractors are currently on site investigating the mountainside, and a date for reopening the road has not, and will not be announced for some time as of yet. History An important archaeological find was made at Llyn Fawr between 1909 and 1913 which was called 'The Llyn Fawr Hoard'. According to the National Museum of Wales, the Llyn Fawr hoard is extremely important because it illustrates the cross over between the Bronze and Iron Ages. It is unusual because of the mixed styles of objects that suggest a wide range of origins. Indeed, on the British scale the name Llyn Fawr is given to the period of time dating from