The Boys' Brigade Mountaineering Club

Elidir Fawr is the 13th highest peak in Wales. Inside Elidir Fawr is the Dinorwig Power Station with 16 km of tunnels hewn out of the mountain. It is a pumped-storage power station that generates hydro-electricity when demand would otherwise exceed supply. It pumps water up from Llyn Peris in the valley to Marchlyn Mawr, a reservoir in a high cwm on the mountain, during the night and releases water to turn the turbines and generate electricity when the National Grid needs it.

See its entries on Walkhighlands or Wikipedia or view it from above on Google Earth.

Elidir Fawr is probably named after Prince Elidir Mwynfawr (the Wealthy) of Welsh-speaking Strathclyde who, in the mid 6th century, married a princess of Gwynedd and died in battle near Caernarfon. It also possibly could be named after Elidir or Eliffer Gosgorddfawr (Eliffer of the Great Army) who was a legendary warrior prince of the same period.

Fawr (Welsh with the M soft-mutated to F) means Great, Large or Big. The normal rule for soft mutation is that an adjective only mutates if the noun it qualifies is feminine and singular. But there is an exception to this rule, when the noun is a proper name. E.g. Llywelyn Fawr for Llewelyn the Great.

Another theory is that Elidir derives from Leitir (Old Irish), meaning Hillside and the definite article Y was, over time, misinterpreted as an E and the name of the mountain was corrupted to Elidir.

So, Elidir Fawr probably means Great Elidir or possibly The Large Hillside.