The last major geological event to shape the Wicklow Mountains was the Quaternary glaciation during the Pleistocene epoch (2.6 million to 11,700 years ago). The ice deepened and moulded the valleys into the U-shape that characterises the Wicklow Glens, such as Glendalough and Glenmacnass. As the ice melted, small glaciers were left in corries where moraines now dam lakes such as at Loughs Bray and Nahanagan. Corries without lakes also occur, such as the North Prison and South Prison of Lugnaquilla. Escaping meltwater cut narrow rocky gorges at several locations including the Glen of the Downs, the Devil's Glen and The Scalp. Ribbon lakes, such as Lough Dan and the lakes of Glendalough, also formed.