The 260 m ice–dammed lake in glens Spean and Roy finally attained a volume of 5 km3. It is suggested that it was initially drained subglacially and catastrophically through the Spean gorge and Loch Ness. Maximal flow of water may have been about 22,500 m3s−1. In Glen Spean the ice dam, 7 km long and up to 200 m high, collapsed. Near Fort Augustus a vast deposit of sand and gravel was laid down in relation to a Loch Ness suddenly raised in level. Subsequently, the level of the Spean–Roy lake varied constantly as it was intermittently emptied by other sudden floods of lesser volume, some of which followed the Lundy gorge and one of which produced a now abandoned waterfall site near Loch Lochy. Some of these events are related to the formation of end moraines and fluvial terraces in the area around Spean Bridge and Gairlochy.