A U-shaped valley is formed by:

Concept: The cross-sectional shape of a valley reveals which agent of erosion carved it. A river, being a narrow, flexible stream, erodes mainly downward (vertical erosion), cutting a deep, narrow channel with a V-shaped cross-section. A glacier, by contrast, is a wide, rigid mass of ice that fills the entire valley floor and touches both side-walls as it moves; it erodes the floor AND the walls simultaneously (plucking and abrasion), so it widens and deepens the valley into a broad, flat-floored trough with steep sides — a U-shaped cross-section.

Application: Applying this principle here, a valley described simply as "U-shaped" is the signature landform of glacial erosion, since only a glacier's mass is broad enough to scour both walls and the floor together into that rounded-bottom, steep-sided profile.

Cross-check — why the other agents don't fit:

• Winds erode by deflation and abrasion, shaping desert landforms such as yardangs and mushroom rocks — this action does not carve a valley cross-section at all.

• A river's narrow current erodes mainly downward, producing a deep, narrow, V-shaped valley — the opposite cross-section from the one described.

• Sea waves erode along the coastline, producing cliffs, caves, arches and stacks — coastal landforms, not inland valley troughs.

Since only glacial erosion widens and deepens a valley on both walls and floor together, the U-shaped valley is formed by a glacier.