Glaciers cut distinctive U-shaped valleys, or troughs, with a flat floor and steep sides. The glacier uses the processes of plucking and abrasion to widen, steepen, deepen and smooth 'V'-shaped river valleys into a 'U' shape.
The interlocking spurs in the narrow V-shaped river valley are cut-off by the ice, creating truncated spurs. After glaciation, a misfit stream/river or ribbon lake can sometimes occupy the floor of the U-shaped valley.
A ribbon lake is a large, narrow lake occupying a U-shaped valley. It forms in a hollow where a glacier has more deeply eroded less resistant rock or it may fill up a valley behind a wall of moraine across the valley.
Misfit streams/rivers meander through the flat, wide U-shaped floor. They have not eroded the valley, as they formed there after glaciation had carved out the much larger U-shaped valley.