The valley of Glendalough was formed during the last ice age by a glacier which left a moraine across the valley mouth. The Poulanass River, which plunges into the valley from the south, via the Poulanass Waterfall, created a delta, which eventually divided the original lake in two. Surrounding Glendalough are the mountains of Camaderry , the hydroelectric station at Turlough Hill , and large massif of Conavalla which dominates the head of the valley, and the peaks of Lugduff , and Mullacor .