An arête is a thin, almost knife-like, ridge of rock which is typically formed when two glaciers erode parallel U-shaped valleys. The arête is a thin ridge of rock that is left separating the two valleys. Arêtes can also form when two glacial cirques erode headwards towards one another, although frequently this results in a saddle-shaped pass, called a col. The edge is then sharpened by freeze-thaw weathering, and the slope on either side of the arete steepened through mass wasting events and the erosion of exposed, unstable rock. The word "arête" is actually French for edge or ridge; similar features in the Alps are described with the German equivalent term Grat or Kamm (comb).
Where three or more cirques meet, a pyramidal peak is created.
A cleaver is a type of arête that separates a unified flow of glacial ice from its uphill side into two glaciers flanking, and flowing parallel to, the ridge. Cleaver gets its name from the way it resembles a meat cleaver slicing meat into two parts. A cleaver may be thought of as analogous to an island in a river. A common situation has the two flanking glaciers melting to their respective ends before their courses can bring them back together; the exceedingly rare analogy is a situation of the two branches of a river drying up, before the downstream tip of the island, by evaporation or absorption into the ground.
The location of a cleaver is often an important factor in the choice among routes for glacier flow. For example, following a cleaver up or down a mountain may avoid travelling on or under an unstable glacial, snow, or rock area. This is usually the case on those summer routes to the summit whose lower portions are on the south face of Mount Rainier: climbers traverse the "flats" of Ingraham Glacier, but ascend Disappointment Cleaver and follow its ridgeline rather than ascending the headwall either of that glacier or (on the other side of the cleaver) of Emmons Glacier.
Notable examples of arêtes include:
- Knife Edge, on Mount Katahdin, Maine
- Clouds Rest, in the Sierra Nevada, California
- The Minarets, in the Sierra Nevada, California
- The Garden Wall, in Glacier National Park, Montana (image to right)
- The Sawtooth, in the Colorado Rockies
- La Peineta, in the Andes of Los Ríos Region, Chile
- Crib Goch, in Snowdonia National Park, Wales
- Striding Edge in the English Lake District
- The Catwalk, in the Olympic National Park, state of Washington
- The Carn Mor Dearg arête on to Ben Nevis, Scotland.
- BBC bitesize
- Orlove, Ben. "Glacier Retreat: Reviewing the Limits of Human Adaptation to Climate Change". Environment Magazine. Retrieved September 2011.