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Mountain group in Wales

The Glyderau (a Welsh plural form, also known in English as the Glyders) are a mountain group in Snowdonia, North Wales. The name derives from the highest peaks in the range, Glyder Fawr and Glyder Fach. According to Sir Ifor Williams, the word "Glyder" derives from the Welsh word "Cludair", meaning a heap of stones. The Glyderau stretch from Mynydd Llandegai to Capel Curig, and include five of Wales' fourteen or fifteen summits over 3000 feet; these include Tryfan, considered one of the finest mountains in Wales and one of the few mountains on the British mainland requiring scrambling to reach the summit. The eastern half of the range in particular, including Glyder Fawr, Glyder Fach and Tryfan, is very popular with walkers and climbers. Dinorwig Power Station, a hydroelectric pump storage system, is located in a man made cavern within Elidir Fawr. The slopes of the Glyderau also include the lake Llyn Idwal, and a number of classic climbing areas such as Rhiwiau Caws.

The Glyderau were formed in the Ordovician period about 500 million years ago as the result of two land masses moving together and causing the Snowdonia massif to rise up. Since then, erosion and the advance and retreat of glaciers during the Ice Ages has worn down the mountains to their present proportions. The underlying rock is a mixture of sedimentary and volcanic material. The last ice sheet retreated about 10,000 years ago and Cwm Idwal is a good example of a cirque formed by the ice. The main glacier flowed down the adjoining Nant Ffrancon Valley, a route now followed by the A5 road, and Cwm Idwal housed a side glacier. The ice scarred the surrounding cliffs, hollowed out the bed of Llyn Idwal and dumped rocks and other material that formed moraines at its foot. Massive boulders and shattered rocks crashed down from above to form the boulder fields and screes.

The north facing amphitheatre shaped valley of Cwm Idwal with its dark cliffs has a unique flora, and some plants here are the most southerly remnants in Britain of the Arctic/Alpine flora. The Snowdon lily (Lloydia serotina) is found here, high on mountain ledges, the only place in Britain where it is found. Other Alpine species include the purple saxifrage (Saxifraga oppositifolia), tufted saxifrage (Saxifraga cespitosa), Alpine meadow rue (Thalictrum alpinum) and mountain sorrel (Oxyria digyna). Because of the scientific interest of this valley, sheep have been excluded from it to allow the native plants to flourish and it has been made into a national nature reserve. The lake itself is shallow and fringed with rushes, reeds and bottle sedge (Carex rostrata). Plants growing in the water or on the damp ground nearby include awlwort, pillwort, waterwort and spring quillwort (Isoetes echinospora). Other plants growing on damp shady ledges include the Welsh poppy (Meconopsis cambrica), the wood rush (Lazula sylvatica), water avens (Geum rivale), wood anemone (Anemone nemorosa) and goldenrod (Solidago virgaurea). Animals that live here include badgers, foxes and polecats as well as buzzards (Buteo buteo), peregrine falcons (Falco peregrinus), choughs (Pyrrhocorax pyrrhocorax), skylarks (Alauda arvensis), black grouse (Tetrao tetrix) and red grouse (Lagopus lagopus).

The National Trust took over the management of the Glyderau and the Carneddau in 1951 in lieu of death duties on the Penrhyn Estate. The total area is about 7,000 hectares, half of which is common land with registered grazing rights for 45,000 sheep and 741 ponies. There are eight tenanted farms on the estate and the National Trust is responsible for the maintenance of footpaths and drystone walls, some of which date back many hundreds of years. The two mountain ranges form part of the Snowdonia National Park.