The sharp ridge formed when two glaciers erode opposite sides is called an arête. It is a narrow, knife-edged ridge of rock separating two glacial valleys or two cirques.
Glaciers erode by scraping bedrock with embedded rocks and by pulling loosened blocks from valley walls. When glaciers occupy valleys on opposite sides of a mountain ridge, their erosion progressively steepens and narrows the intervening rock. Freeze–thaw weathering and rockfalls can sharpen the ridge even further.
If erosion continues from several surrounding cirques, the remaining summit may become a pyramidal peak, also called a glacial horn. When two cirques erode toward one another and leave a lower saddle-shaped gap, the feature is generally called a col rather than an arête.
The word arête comes from French for “edge” or “ridge.” Famous examples include the Garden Wall in Montana’s Glacier National Park, Crib Goch in Wales, and Striding Edge in England’s Lake District. These ridges can offer spectacular routes but are often exposed and technically demanding for climbers.