Truncated spurs are distinct landforms found in glaciated valleys, characterized by their steep, triangular faces that have been cut off or 'truncated' by the erosive action of glaciers. Before glaciation, river valleys typically exhibit interlocking spurs, where the river winds around projections of land extending from the valley sides. However, when a glacier moves through such a valley, its immense size, weight, and erosive power modify these features significantly.

Formation: Truncated spurs are formed primarily through two glacial processes: abrasion and plucking.

1. Abrasion: As a glacier flows down a valley, it carries a vast amount of rock debris (till) embedded within its ice. This debris acts like sandpaper, grinding and scraping against the valley sides and floor. The sheer force of the moving ice, laden with rock fragments, effectively 'sands down' the sides of the interlocking spurs, removing their projecting ends and straightening the valley walls.

2. Plucking (or Quarrying): This process involves the glacier freezing onto existing rock outcrops, particularly along joints and weaknesses. As the glacier continues to move, it 'plucks' away blocks of rock from the valley sides. This is especially effective on the down-valley side of obstacles and along the sides of spurs, where the ice exerts significant pressure and then pulls away loosened rock.

Together, abrasion and plucking work to remove the ends of the interlocking spurs, creating the characteristic steep, often triangular faces. The valley itself is widened and deepened, transforming from a V-shaped river valley into a U-shaped glacial trough. The truncated spurs are thus remnants of the original valley sides, reshaped and steepened by the powerful glacial erosion. They are typically found in mountainous regions that have experienced past glaciation, such as the Alps, the Himalayas, and parts of North America and Scandinavia.