A U-shaped valley, also known as a glacial trough, is a distinctive landform created by the powerful erosive action of moving glaciers. These valleys possess a unique cross-section characterized by a broad, flat floor and high, steep side walls. The resulting shape is a wide, parabolic curve that strongly resembles the letter “U” when viewed in profile. This specific shape serves as a clear indicator of past glacial activity.
Pre-Glacial Landscape
The formation of a glacial trough begins with a pre-existing river valley carved by flowing water. This original landscape typically features a “V” shape, which is the signature of fluvial erosion. Rivers primarily cut downward into the land through vertical erosion, creating narrow, steep-sided channels. The valley sides are further shaped by weathering and mass movement, leading to a tapered, V-like cross-section.
The river’s course is often forced to meander around projections of rock extending from the valley walls, which are known as interlocking spurs. The comparatively low erosive power of a river means it cannot cut straight through these rock obstacles. This initial V-shaped valley provides the basin that will ultimately be occupied and dramatically reshaped by the advancing glacier.
The Mechanics of Glacial Carving
When a massive glacier descends into this pre-existing V-shaped valley, it begins a slow but relentless process of erosion. The sheer weight and volume of the ice, which can be thousands of feet thick, allow it to scour the landscape on a scale unmatched by water. This transformation from a narrow V-shape to a broad U-shape is accomplished through two primary physical processes: plucking and abrasion.
Plucking, or quarrying, occurs when meltwater seeps into cracks and fractures in the bedrock at the base and sides of the glacier. This water freezes and expands, dislodging blocks of rock from the valley walls and floor. As the glacier continues its slow movement, it pulls these loosened fragments away, incorporating them into the ice mass. This action is particularly effective at steepening the valley sides.
The second process is abrasion, often described as the “sandpaper effect.” Rock fragments and sediment embedded within the glacier act as a powerful grinding tool against the valley’s bedrock. As the ice flows, this debris scrapes and smooths the valley floor and sides, deepening and widening the entire trough. This continuous grinding action contributes significantly to the flat-bottomed nature and characteristic parabolic profile of the U-shaped valley.
Resulting Glacial Landforms
The retreat of the glacier leaves behind a suite of distinctive landforms that confirm its role in shaping the landscape. One primary indicator is the presence of truncated spurs, which are the remnants of the interlocking spurs from the original V-shaped river valley. The immense power of the ice flow sliced off the ends of these spurs, leaving behind steep, blunted rock faces.
Smaller tributary valleys that once joined the main valley are often left suspended high above the newly deepened glacial trough. These features are known as hanging valleys because the smaller tributary glaciers could not erode their valleys to the same depth as the much larger main glacier. Water flowing from these elevated side valleys often plunges into the main valley as a waterfall.
In areas where the glacier’s erosive power was concentrated, the valley floor was unevenly deepened, creating depressions called rock basins. When the ice melts, these hollows fill with water, forming long, narrow bodies of water known as ribbon lakes. If the U-shaped valley extends all the way to the coast and is subsequently flooded by a rise in sea level, it forms a fjord, a dramatic, steep-sided, and deep glacial trough inundated by seawater.