An arête is a narrow, serrated ridge of rock separating two parallel glacial valleys or bowl-shaped depressions. It is a direct result of intense erosion by ice and weathering processes that sculpt the mountainous landscape over long periods.
Defining the Feature
The physical appearance of an arête is characterized by its distinct, knife-edge profile, forming a high, thin wall of rock. It often presents a jagged or serrated crest with incredibly steep slopes, sometimes giving the impression of a saw blade against the skyline. The term itself is derived from the French word for “edge” or “ridge.”
The arête’s survival depends on the underlying geology, as the ridge is composed of highly resistant rock that is slow to break down. This structural strength allows the narrow ridge to persist even as the surrounding ice carves away the mountain’s flanks. The ridge may also contain low points, known as cols, which represent saddles or passes where glacial action eroded the rock sufficiently to create a dip.
The Glacial Sculpting Process
The formation of an arête requires the action of at least two adjacent glaciers operating on opposite sides of a mountain divide. Glaciers begin their carving process high on the mountainside within bowl-shaped depressions known as cirques. As these cirque glaciers grow, they erode the mountain backward and downward through a mechanism called headward erosion.
This process involves the ice plucking rock from the headwall and using abrasive materials embedded in the ice to scour the basin floor. When two glaciers occupy adjacent cirques or parallel valleys, their headward erosion works toward the same dividing ridge. The rock separating the two deepening depressions is steadily narrowed from both sides as the ice continues to cut backward into the mountain mass.
The erosion is amplified by the intense freeze-thaw cycle. Water seeps into cracks in the rock high above the glacier, where it freezes and expands, forcing rock fragments to break off in a process called frost shattering. This action, also known as glacial sapping, continually steepens the upper slopes of the ridge. The loosened material falls onto the glacier, which then carries it away. This two-sided attack of plucking, abrasion, and weathering eventually reduces the original broad mountain divide to the thin, serrated arête.
Related Alpine Landforms
Arêtes are connected to other features shaped by the glacial environment, primarily the cirques and the horns. The cirque is the semi-circular, amphitheater-like basin that sits at the head of a glacial valley. It serves as the starting point for the glaciers that sculpted the ridge.
After the ice has melted, the cirque floor is often over-deepened, sometimes holding a small mountain lake known as a tarn. The expansion of neighboring cirques directly produces the sharp ridges between them.
The arête often culminates in a peak called a horn, which is a pyramidal summit formed when three or more arêtes converge. The Matterhorn in the European Alps stands as a widely recognized example of this convergence of glacial erosion.