A U-shaped valley, also known as a glacial trough, is a distinctive landform characterized by a broad, relatively flat floor and steep, often near-vertical sides. This cross-sectional profile is the direct result of the movement and immense power of massive ice sheets, or glaciers. Unlike other valleys shaped by running water, the U-shape is created exclusively by glacial erosion, which carves a unique signature onto the landscape.
The Pre-Glacial Starting Point
The starting point for a glacial trough is typically a valley already carved by a river. River erosion creates a V-shaped valley, defined by steep, converging sides that meet at a narrow channel floor. The river’s erosive power is focused narrowly on its bed, causing it to wind around protruding ridges of rock called interlocking spurs. When a glacier forms and begins to flow downhill, it occupies this pre-existing V-shaped depression. The volume and thickness of the ice, which can be thousands of feet deep, completely fills the original valley. The ice mass then begins its work of widening and deepening the channel, transforming the narrow V into a much broader U.
How Glaciers Reshape the Valley Floor
Glacial ice reshapes the valley through two primary erosional processes: abrasion and plucking. Abrasion occurs as the glacier moves, dragging rock debris embedded in its base across the bedrock like gigantic sandpaper. This grinding action effectively smooths and polishes the valley floor and lower walls, deepening the valley over a vast area. Plucking is more destructive and often focuses on weaknesses in the rock. Meltwater penetrates cracks in the underlying bedrock, where it freezes and expands, dislodging blocks of rock. As the glacier continues to move, it pulls these loosened blocks away, incorporating them into the ice mass. This quarrying action is effective at steepening the valley walls and deepening the floor.
Signature Features of Glacial Troughs
Glacial erosion carves out distinctive features along the valley sides. One prominent feature is the truncated spur, which results from the glacier cutting straight through the ends of the original interlocking spurs. Where the river once meandered around these ridges, the glacier flows with a straighter, more direct path, shearing off the ends to leave behind steep, triangular cliff faces. Another sign of past glaciation is the presence of hanging valleys. These occur where smaller tributary glaciers flowed into the main, larger glacier. Because the main glacier was thicker and heavier, it had a greater erosional capacity, deepening its valley much faster than the smaller tributary could. When the ice melts, the floor of the tributary valley is left “hanging” high above the main valley floor, often with a waterfall cascading over the steep drop.
What Happens When the Ice Melts
When the ice mass retreats, the deep, wide U-shaped valley remains, often filled with water or sediment. The floor of the trough may be littered with a mixture of unsorted material called glacial till, which was deposited directly by the melting ice. This till often forms ridges, or moraines, which can act as natural dams. Uneven erosion of the valley floor, caused by variations in bedrock resistance, can create a series of over-deepened depressions. These hollows fill with meltwater and rainwater to form long, narrow bodies of water known as ribbon lakes. In coastal mountainous regions, if the U-shaped valley was carved out below sea level, the subsequent rise in sea level floods the valley. This creates a fjord, a deep, steep-sided inlet of the sea that represents a submerged glacial trough.