Ribbon lakes are long, narrow lakes which fill the floor of many U-shaped valleys which were glaciated during the last ice age. They occur where a terminal moraine or rock barrier stretches across the valley floor, forming a dam.
The sequence of their formation begins with the processes of glacial growth and erosion during the last ice (before 20 000 years ago), which excavated large glacial troughs and deposited terminal moraines at the glacier snout. Where glacial valleys cross rock bands of different hardness, softer rocks may have been eroded more deeply than harder rocks. Ribbon lakes formed after the ice age as outflowing streams were dammed by the moraine or filled the overdeepened parts of the valley floor.
Windermere in the Lake District is a good example. 10.5 miles long and 1 mile wide, it is England's largest lake, fed by the River Brathay.