is a mountain in Snowdonia, Wales, the highest peak in the Glyderau range at just over 1,000 metres, having had its height recalculated in 2010 using GPS. It is the fifth-highest mountain in Wales and has several walking and scrambling routes leading to its summit. According to Sir Ifor Williams, the word "" derives from the Welsh word "", meaning a heap of stones. Geography Glyder Fawr is part of the Glyderau mountain range which lies to the northeast of the Snowdon massif, separated from that mountain by the Llanberis Pass and the Nant Peris Valley. Going from west to east, the Glyderau range consists of Elidir Fawr (), Y Garn (), Glyder Fawr (), Glyder Fach () and Tryfan (). To the north of the range lie the Carneddau, separated from the Glyderau by the Ogwen Valley and the Nant Ffrancon Valley. The Glyderau are separated from the surrounding mountain ranges making them stand out clearly from afar and giving them commanding views of the surrounding peaks and valleys. Until late 2010, most maps showed the height of Glyder Fawr to be ; however, the mountain was resurveyed with accurate GPS equipment and its height was found to be . A spokesman from the Snowdonia National Park Authority was pleased by the new height, and suggested that now the mountain exceeded 1,000 m, it was sure to attract more walkers to the area. The Glyderau range was formed about five hundred million years ago, when two land masses collided, causing the Snowdonia massif to rise up. Since then, wind, water and frost, and the advance and retreat of glaciers during the ice ages have gradually worn down the mountains to their present proportions. The underlying rock is a mixture of volcanic and sedimentary material. The ice sheet of the most recent ice age retreated about ten thousand years ago leaving Cwm Idwal as an example of a cirque. 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 visible today. Routes to the summit The summit can be accessed from several directions: one route starts at Ogwen Cottage on the shores of Llyn Idwal and approaches the mountain from the north; another starts at Pen-y-Pass and approaches it