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. Geography 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 land was originally covered with native forest mostly consisting of birch and oak. This was cleared over the millennia by the Neolithic, Bronze Age and Iron Age farmers that settled here and now there is little tree cover. Groups of feral goats can still be found on the Glyderau, probably the remnants of the herds that were farmed here a thousand years ago. The large number of sheep that now graze the common land were introduced in the 18th century with the