Glacial troughs are dramatic geographical features resulting from massive glaciers flowing through mountainous regions over geological timescales. These long, deep valleys stand as a record of past ice ages, specifically the Quaternary glaciations, which reshaped the continents.
Defining the Glacial Trough
A glacial trough is defined by its distinctive “U” cross-sectional shape. This morphology features a broad, relatively flat valley floor flanked by steep, near-vertical walls. This characteristic parabolic profile distinguishes it from a river valley, which typically exhibits a sharper, V-shaped cross-section.
These troughs often begin as existing river valleys that the glacier occupies and reshapes. The weight and erosive action of the ice straighten the valley course and remove the interlocking spurs common in river-cut terrain. This process creates a valley that is significantly deeper and wider than the original fluvial pathway, with a much smoother longitudinal profile.
The Mechanisms of Glacial Erosion
The transformation of a V-shaped river valley into a glacial trough occurs through two primary processes: plucking and abrasion. These mechanisms work in concert, driven by the mass and movement of the valley glacier. The ice can be thousands of feet thick, exerting pressure on the underlying bedrock.
Glacial plucking, sometimes called quarrying, occurs when the glacier freezes onto the jointed bedrock. Meltwater seeps into cracks, and as it refreezes, it expands, fracturing and loosening large chunks of material. As the glacier advances, it pulls or “plucks” these pieces of rock away, incorporating them into the base of the ice.
Abrasion involves the rock fragments and debris embedded in the base of the glacier being dragged across the bedrock. This grinding action smooths, polishes, and carves out the valley. Abrasion deepens the valley floor and actively widens it, creating the signature steep sides and broad base of the trough.
Secondary Landforms Associated with Glacial Troughs
The erosive action that forms the glacial trough gives rise to several distinctive landforms.
Hanging Valleys
A Hanging Valley is a tributary trough carved by a smaller glacier. Since the tributary glacier had less erosive power, its valley floor was not deepened as much as the main trough, leaving it “hanging” high above the main valley floor.
Truncated Spurs
As the main glacier flowed down the valley, it cut straight through the rocky ridges known as interlocking spurs. These truncated ridge ends are now visible as steep, triangular rock faces called Truncated Spurs.
Ribbon Lakes
The trough floor often features a series of depressions and rock basins that were over-deepened by the glacier. These depressions frequently fill with water after the ice melts, forming long, deep lakes known as Ribbon Lakes.
Fjords
If a glacial trough extends to the coast and is subsequently flooded by rising sea levels, it becomes a Fjord. These are submerged glacial troughs, characterized by deep waters, steep sides, and an over-deepened profile, often separated from the open sea by a shallow submerged ridge called a sill.