Later Lake Drainage
As the climate improved about 11,500 years ago, the glacier damming the lakes retreated and the lakes drained by glacier bursts (see explanation on page 15). Effectively, the buoyancy of the water floated the glacier tongue off its bed. This happened suddenly, allowing the lake waters to drain in great floods under the ice. The biggest glacier burst took place after the ice dam had retreated to Spean Bridge, and the lake was at the level of the 260 metres Parallel Road; at this stage some 5 cubic kilometres of water are believed to have escaped under the ice to Loch Ness and into the sea at Inverness. As the ice surface declined, lower lakes were dammed; these also seem to have drained by glacier bursts directed either northeastwards to Loch Ness or southwestwards to Fort William.
The glacier burst that drained the 260 metres lake seems to have taken place under the ice along the Spean Gorge. Some of the later, smaller glacier bursts were directed underneath the ice into the gorge of the River Lundy. Most of the later lakes were probably short-lived, but one at 113 metres above sea level formed a locally-preserved shoreline and was probably longer lived.