A U shaped valley is a broad valley with a relatively flat or gently curved floor and steep sides that together form a cross section resembling the letter U. It is a characteristic landform of glacial erosion and commonly develops when a valley glacier occupies and modifies a pre existing river valley. Rivers typically carve V shaped valleys because flowing water concentrates erosion along a relatively narrow channel. Glaciers fill much more of the valley cross section and can erode both the floor and sides, transforming the original V shaped profile into a wider trough. U shaped valleys are therefore also called glacial troughs.
Glacial transformation of a valley involves several erosional processes. Moving ice exerts pressure against the valley sides and floor, while rock fragments embedded in the glacier abrade the underlying bedrock. Plucking removes blocks where ice interacts with fractured rock. The glacier also transports debris that can scour the valley walls and bed. Because the ice occupies a broad cross section rather than a narrow channel, erosion is distributed across a much wider area than in a river valley. Over repeated glacial cycles, the valley becomes wider, deeper, and straighter in places, with steep walls and a broad floor.
U shaped valleys commonly contain additional glacial landforms. Truncated spurs are steep valley side remnants where interlocking river spurs have been cut off by glacial erosion. Hanging valleys may enter the main trough above its floor. Moraines may occur along the valley sides or floor, while glacial deposits can form terraces, outwash plains, or sediment filled basins. Roche moutonnee and striated bedrock may occur on the valley floor. The combination of these features can provide stronger evidence of glacial origin than valley shape alone, because other processes can occasionally create broad valleys with steep sides.
The longitudinal profile of a glacial trough differs from that of a typical river valley. Glacial valleys may contain steps, overdeepened basins, and rock thresholds produced by differential erosion. A sequence of basins and steps can develop where bedrock resistance, ice thickness, subglacial water, and glacier dynamics vary along the valley. After glacial retreat, these overdeepened sections can fill with lakes or sediment. Some large glacial troughs continue beneath modern sea level and become fjords when flooded by the ocean. Thus a U shaped valley on land and a fjord in a coastal setting can represent related stages of glacial valley evolution.
U shaped valleys occur throughout formerly glaciated mountain and high latitude regions. The Lake District and Scottish Highlands contain many glacial troughs in Britain. Large examples occur in Norway, the Alps, the Rockies, the Andes, Alaska, Greenland, Iceland, and New Zealand. In some places, glaciers have repeatedly occupied the same valley during several glacial periods, producing extensive widening and deepening. The resulting valleys can become major transportation corridors and settlement areas after deglaciation because their broad floors provide relatively accessible routes through mountainous terrain.
A U shaped valley is different from a V shaped river valley primarily because of its cross sectional morphology and erosional history. A V shaped valley generally has steep sides converging toward a narrow river channel, whereas a glacial trough has steep sides rising from a broad floor. However, real landscapes may contain transitional forms because glaciers can modify only part of an existing valley, rivers continue to erode after deglaciation, and rock structure affects valley shape. A valley should therefore not be classified solely by visual appearance without considering evidence such as glacial deposits, striations, moraines, hanging valleys, and truncated spurs.
U shaped valleys are important records of past climate because large valley glaciers require sustained periods in which accumulation exceeds ablation. Their dimensions can provide clues about the former thickness and extent of glaciers, particularly when combined with trimlines, moraines, erratics, and polished bedrock. The valley itself may survive for many thousands of years after the glacier disappears. Subsequent rivers, weathering, landslides, vegetation, and human development modify the trough but often do not erase its basic geometry.
Modern U shaped valleys can have major ecological and human significance. Their floors may contain rivers, lakes, wetlands, forests, farmland, roads, settlements, and infrastructure, while their steep sides can be susceptible to rockfall and landslides. Valleys containing glaciers continue to evolve as ice retreats or advances. In coastal regions, flooding by the sea can convert the lower parts of glacial troughs into fjords. In inland regions, glacial overdeepening can create lakes such as those found in many mountain areas. The U shaped valley is therefore both a diagnostic glacial landform and a framework within which later hydrological, ecological, and human processes operate.
