U字谷
Lugnaquilla is described as a "bulky mountain", with a large plateau-type grassy summit (known as "Percy's Table"), bounded on two sides by steep glacial corries called the "North Prison" (looking into the Glen of Imaal), and the "South Prison" (looking into the glen of the River Ow). On Lugnaquilla's eastern side is the cliff-lined hanging valley of Fraughan Rock Glen, which falls into the glacial U-shaped valley of Glenmalure. Lugnaquilla is the source of the River Slaney.