Arêtes and pyramidal peaks

A view of the Himalayas showing an arête and pyramidal peakA view of the Himalayas showing an arête and pyramidal peak

An arête is a knife-edge ridge. It is formed when two neighbouring corries run back to back. As each glacier erodes either side of the ridge, the edge becomes steeper and the ridge becomes narrower.

U-shaped valley in Hola Valley, Norway
U-shaped valley in Hola Valley, Norway

A pyramidal peak is formed where three or more corries and arêtes meet. Glaciers erode backwards towards each other, carving out the rocks by plucking and abrasion. Freeze thaw weathers the top of the mountain, creating a sharply pointed summit.

U-shaped valleys

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.

Valley floor landforms

Ribbon lakes and misfit streams/rivers

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.