Geological Survey of Ireland (GSI) regard the valley as a stunning example of a glaciated U-shaped valley, with oversteepened cliff sides and a flat floor. The valley dates from the last glacial period called the quaternary glaciation or Ice Age, and contains moraines marking the positions of the ice-front as it retreated back towards the source area higher up in the centre of the Wicklow mountains. The GSI highlight an impressive moraine about halfway between the waterfall and Laragh and can be seen as a boulder-strewn mound running right across the valley.