A pyramidal peak, or sometimes in its most extreme form called a glacial horn, is a mountaintop that has been modified by the action of ice during glaciation and frost weathering. If the use is unambiguous within a mountain context, then the simple terms peak or horn may be used.
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Ice fields at the head of glaciers develop bowl-shaped basins in the sides of a mountain called cirques (sometimes called corries or cwms). Cirque glaciers have specific characteristics. They have rotational sliding that abrades the floor of the basin but not the edge and that causes the bowl shape to form and also contributes to the development of the morainal or rock thresholds that separate them from downvalley slopes as well as hold in glacial lakes called tarns. As cirques are formed by glaciation in an alpine environment, the headwall and ridges known as arêtes become more steep and defined. This occurs due to freeze/thaw and mass wasting beneath the ice surface. It is widely held that a common cause for headwall steepening and extension headward is the crevasses known as bergschrund that occur between the moving ice and the headwall. This is a location where plucking and shattering has been seen by observers that explore the crevasses. A completed cirque is exposed as the glacier that was in the cirque melts away, after the periods of glaciation in the Pleistocene. When there are three or more of these cirque headwalls and arêtes joined together to form a single peak it creates a pyramid shape with very steep walls and it is called a horn or glacial horn. They are a common shape for mountain tops in well glaciated areas. The number of faces that make up the sides of a horn are dependent on the number of cirques involved in the formation of the peak, but the most common number of faces for a horn to have is three or four.
A glacial horn takes the process to its limits, producing near vertical faces on all sides. In the Alps, "Horn" is also the name of very exposed peaks with slope inclinations of 45-60° (e.g. Kitzbüheler Horn).
Examples of horns include:
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