Which process is responsible for the formation of V-shaped valleys?

V-shaped valleys are typically formed by the erosive action of rivers and streams over a long period. The flowing water cuts down into the bedrock, creating a valley with a characteristic V-shape. This process of vertical erosion is most effective in the upper course of a river, where the gradient is steep and the water flow is fast.

References:

Integrated Physical Sciences materials on erosion and landform development.

Geomorphological studies on river valley formation.