A glacial trough is a valley that has been modified by the erosive action of a large, flowing glacier. They typically form where ice masses descended through pre-existing mountain valleys. The resulting landform is a deep, elongated depression that provides clear evidence of past glaciation. Glacial troughs can be thousands of feet deep and extend for many miles, marking the path of the former ice flow.
The Defining U-Shape Profile
The glacial trough is defined by its U-shaped or parabolic cross-section. This geometry consists of steep, often near-vertical side walls that transition into a wide, relatively flat valley floor. This shape offers the least frictional resistance to the movement of the glacier, which flows differently than water.
This profile contrasts sharply with the cross-section of a fluvial, or river, valley. Valleys carved primarily by rivers are typically V-shaped, with steep sides meeting at a narrow bottom. The glacier’s ability to erode both vertically and laterally, as opposed to the river’s focused downward cutting, creates the broader, U-shaped form. Glacial valleys can be significantly wider than their fluvial counterparts, even with similar drainage areas.
The Glacial Erosion Processes That Create the Shape
The transformation of a V-shaped river valley into a U-shaped glacial trough is achieved through two primary erosional processes: glacial plucking and glacial abrasion. Glacial plucking occurs when meltwater beneath the glacier penetrates cracks and joints in the bedrock. As this water refreezes, it bonds to the rock, and the movement of the overriding ice “plucks” large blocks of rock away from the valley floor and sides.
These detached rock fragments become entrained within the ice and cause glacial abrasion. Abrasion acts like sandpaper, where the embedded rocks grind and scour the bedrock as the glacier slides forward. This grinding action smooths the valley floor, leaving behind fine sediment known as rock flour. Plucking and abrasion widen the valley floor and steepen the walls, creating the signature U-shape.
Associated Landforms of Glacial Troughs
The erosive power that forms the glacial trough also sculpts several related landforms along the valley’s edges and floor. As the main glacier widens the valley, it shears off the ends of the original interlocking spurs—the rocky ridges that jutted into the pre-glacial river valley. These sheared-off rock faces are known as truncated spurs.
A second feature is the hanging valley, which forms where a smaller tributary glacier met the main glacier. Since the smaller glacier had less mass, it did not cut its valley floor as deeply as the main trough. When the ice retreats, the floor of the tributary valley is left “hanging” high above the main trough, often resulting in waterfalls where the tributary stream descends to the main valley floor. Over-deepened sections of the trough floor can fill with water after the ice melts, forming long, narrow bodies of water called ribbon lakes.