glacial trough

(geology) A deep U-shaped valley with steep sides that leads down from a cirque and was excavated by a glacier.

Once termed a U-shaped valley, this is a wide valley floor with steep sides formed by glacial erosion. Glacial troughs tend to have a straighter course than river valleys. The harder the rock which the glacier has cut through, the steeper the valley walls. The shape of a glacial trough more resembles a parabola than the letter U.

The long profile of a glacial trough is frequently irregular and marked by basins and steps. Explanations for this over-deepening and reverse flow vary: that deeper sections are formed where two glaciers meet, that plucking is more effective in the weaker or closely jointed rocks, that flow alternates between compressive and extensive, or that glaciers may pass through naturally occurring narrows. In this last case, the power of the glacier would be intensified as it pushes up against the valley walls.